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Spring 2014 > Metabolism - Exam #2, Part 2 > Flashcards

Metabolism - Exam #2, Part 2 Flashcards

1
Q

What makes up Pantothenic Acid?

A

Beta-alanine and pantoic acid linked by a PEPTIDE BOND/AMIDE LINKAGE

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2
Q

What is the history of Pantothenic Acid?

A
  • Originally B5;
  • R.J. Williams isolated in 1931; structure in 1939; also C.V. Elvehjem and T.H. Jukes;
  • Williams named “everywhere” as “pantos” in Greek → the vitamin is found widely in foods → Deficiencies are unlikely;
  • F. Lipmann won the Nobel prize in 1957 for his discoveries that coenzyme A facilitated biological acetylation reactions
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3
Q

What are the supplement forms of Pantothenic Acid?

A

Calcium pantothenate or as pantethenol (alcohol form)

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4
Q

What is Coenzyme A?

A

-Coenzyme, notable for its role in the synthesis and oxidation of fatty acids, and the oxidation of pyruvate in the citric acid cycle;
-Thio that can form thirsters with carboxylic acids and act as an acyl carrier;
CoA biosynthesis requires cysteamine, pantothenate, and adenosine triphosphate

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5
Q

What are the structural components of Coenzyme A?

A

-Beta-Alanine + Pantoic Acid = Pantothenic Acid;

…

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6
Q

hat are the FOOD sources of Pantothenic Acid?

A
  • ALMOST ALL plant and animal foods;
  • VERY GOOD sources are meats, egg yolk, yogurt, legumes, whole-grain cereals, potatoes, mushrooms, broccoli and avocados;
  • FREE or BOUND in food
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7
Q

How is majority of Pantothenic Acid found in food?

A

-85% as coenzyme A (CoA) → CoA DEGRADED to pantothenic acid by phosphatases and pyrophosphatases

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8
Q

How is Pantothenic Acid ABSORBED?

A

-Mainly in JEJUNUM by a sodium-dependent active multivitamin transporter (SMVT) with typical ingestion;
-50% of pantothenic acid is absorbed with NORMAL intakes
-HIGH dietary doses there is PASSIVE absorption;
(absorption DROPS when higher amounts are ingested – don’t need the excess)

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9
Q

Why is the transporter called “multivitamin”?

A
  • The transporter is SHARED with biotin and lipoic acid;

- Carrier competition!

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10
Q

What is the form of Pantothenic Acid in the blood?

A
  • FREE in BLOOD PLASMA;

- Higher amounts are in the RED BLOOD CELLS

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11
Q

How is Pantothenic Acid taken into TISSUES?

A

The uptake into TISSUES occurs by SMVT (sodium-dependent multivitamin transporter)

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12
Q

What happens to Pantothenic Acid once it enters the cells?

A
  • Found within CELLS and leads to CoA;

- Highest concentrations in liver, adrenal glands, kidney, brain, and heart = very metabolically tissues

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13
Q

What compounds inhibit the synthesis of CoA from Pantothenic Acid?

A

-Acetyl CoA;
-Malonyl CoA;
-Propionyl CoA;
and other acyl chain CoAs
(Products of rxn preventing the initial conversion b/c it is not needed)

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14
Q

What is the main function of CoA?

A
  • Functions as carriers of acetyl and acyl groups;

- •4’-phosphopantetheine is the prosthetic group responsible for its ability to act as acyl carrier protein

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15
Q

How does 4’-phosphopantetheine and CoA function as acyl carriers?

A
  • Forming THIO ESTERS (substitute S for an O) with carboxylic acids;
  • Thioester – compounds with the functional group C-S-CO-C; Product of esterification between a carboxylic acid and a thiol → the best-known derivative being acetyl-CoA.
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16
Q

How is CoA used in metabolism of CHO, Fat, and PRO?

A

-CoA as acetyl CoA holds the central position in the transformation of energy;
→ Key step to the entrance into the TCA cycle and glycolysis metabolism and NO MORE carb synthesis

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17
Q

Pantothenic acid, thiamin, riboflavin, and niacin work together in what reactions of the TCA cycle?

A

-Oxidative decarboxylation of pyruvate;
And
-Oxidative decarboxylation of alpha-ketoglutarate to succinic acid

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18
Q

What SYNTHESIS reaction utilize CoA?

A
  • Synthesis of cholesterol, bile acids, ketone bodies, fatty acids, and steroid hormones (used for fatty acid oxidation);
  • Phospholipid and sphingomyelin require CoA for synthesis from phosphatidic acid and sphingosine
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19
Q

What is the function of CoA relating to protein modification?

A
  • Posttranslational acetylation or acylation of PROTEINS (also some with sugars and drugs);
  • EX: Acetylation of histone proteins;
  • Can activate or inactivate;
  • Acetylation – adds and acetyl functional group = CH3CO
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20
Q

How is Pantothenic Acid EXCRETED?

A
  • Excreted in the URINE, small amounts in feces → NO METABOLITES in urine and feces have ever been identified;
  • Excretion ranges generally from between 1 to 8 mg/day
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21
Q

What is the AI for Pantothenic Acid?

A
  • AI = 5mg/day for all adults
  • This amount replaces losses in urinary excretion;
  • Excretion of less than 1 mg/day considered poor status;
  • No nationally recored of average intake
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22
Q

What is “Burning Feet Syndrome”?

A
  • Numbness of toes and burning of the feet; warmth makes worse and cold improves; other symptoms are vomiting, fatigue, weakness, restlessness, and irritability;
  • Any diseases that affect absorption put a person at risk of deficiency, usually deficiency occurs with multiple nutrients
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23
Q

What is the UL for Pantothenic Acid?

A
  • NO UL → no reports of adverse effects for oral intake of pantothenic acid;
  • Intakes up to 10 g/day for 6 weeks caused no problems, higher doses have been associated with intestinal distress including diarrhea
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24
Q

How was Folate and B12 discovered?

A
  • By Mitchell et al in 1941;
  • Resulted from the need to find a cure for macrocytic or megablastic anemia that was a problem in the late 1870s and 1880s → eating LIVER cured this condition as well as other vitamin deficiency disorders
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25
Q

What is the Folic Acid form?

A
  • SYNTHETIC OXIDE form (refers to REMOVAL of all but one glutamic acid residue) of the vitamin in fortified foods and supplements and is RARE IN NATURE;
  • Can only have ONE glutamic acid to make it Folic Acid
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26
Q

What is the Folate form?

A
  • Folate is the reduced form (means contains 2-10 glutamic acid residues) that is found NATURALLY in food;
  • Folate = generic name for this vitamin → folate and folic acid are NOT exactly interchangeable (Folate is reduced; Folic Acid is oxidized);
  • Latin folium means “leaf” and folate from Italian means “foliage”
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27
Q

What is the function of the Folate COENZYMES?

A

-Folate COENZYMES function in the acceptance and transfer of 1-carbon units in the synthesis, interconversion and modification of nucleotides, amino acids, and other key cellular components

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28
Q

What makes Folate a vitamin?

A
  • the ENZYME for coupling the pteridine (pterin) to the para-aminobenzoic acid (PABA) to form the pteroic acid component of folate is NOT present in human bodies;
  • LACK of synthetic ENZYME to make folate in the body makes it essential
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29
Q

What is included in the structure of Folate?

A
  • Folate is made up of PTERIDINE, which is conjugated by a methylene group (—CH2—) to PABA, forming Pteroic Acid;
  • The carboxy group (—CO—) of PABA is peptide bound to the amino group (—NH—) of glutamic acid to form the monoglutamate form of folate=
  • Pteridine + PABA + Glutamic Acid = Pteroylmonoglutamic Acid (FOLTATE)
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30
Q

What are the THF derivatives of Folate?

A
  • 5-and-10-formly THF (O=CH);
  • 5-formimino THF (-Hc=NH-);
  • 5,10-methenyl THF (=CH-);
  • 5,10-methylene THF (-CH2);
  • 5-methyl THF (-CH3)
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31
Q

What are the GOOD natural food sources of Folate?

A
  • Mushrooms, green vegetables (spinach, brussels sprouts, broccoli, asparagus, turnip and collard greens, okra), peanuts, legumes (lima, pinto, kidney), lentils, fruits (strawberries and oranges) and their juices, and liver;
  • Like other water soluble vitamins can LOSE the vitamins with COOKING as destroyed by heat and oxidation
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32
Q

What is the fortification of Folate?

A

-FORTIFICATION of flours, grains and cereals with folic acid began in 1998 as 140 micrograms folic acid per 100 g of products!!
-Some juices are also fortified
→ Greatly improved folate nutrition and lessened Neural Tube Defects

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33
Q

What are the forms of Folate within Foods?

A

Food has multiple glutamic acid residues (REDUCED forms)=

  • 5-methyl tetrahydrofolate (THF);
  • 10-formyl THF;
  • but over 150 forms of folate have been reported
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34
Q

What is the bioavailability of Folate?

A
  • Folate bioavailability from foods varies from 10 to 98% → 50% is usually assumed for a mixed diet;
  • Folic acid (the most oxidized form) as a supplement is ~100% absorbed on an empty stomach, in foods ~85% available
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35
Q

What form of Folate is easiest to absorb?

A

MONOGLUTAMATE form;

-Two folypoly γ-glutamyl carboxypeptidases (FGCP) that REMOVE glutamates = Enzymes

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36
Q

What are the enzymes that allow Folate to be absorbed as Monoglutamate?

A
  • Enzymes in the JEJUNUM;
  • Pteroylpolyglutamate hydrolases (hydrolysis) or conjugases (deconjugation);
  • One is free and the other in the membrane of mucosal cells (brush border) and latter one is zinc-dependent;
  • Found in pancreatic juice and bile
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37
Q

What can reduce the absorption of Monoglutamate Folate?

A
  • Chronic alcohol ingestion and conjugase inhibitors in several foods REDUCE absorption;
  • Conjugase inhibitors DO NOT affect synthetic folic acid
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38
Q

How does forms of folate enter the intestinal CELLS?

A

-Monoglutamate form of folate and 5-methyl tetrahydrofolate (THF) are transported into intestinal cells in DUODENUM and UPPER JEJUNUM by the proton-coupled folate transporter (PCFT)

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39
Q

What occurs when there is a defect of PCFT?

A

Hereditary folate malabsorption

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40
Q

What happens to folate in CELLS before it enters the portal blood?

A

REDUCED from dihydrofolate (DHF) to THF by cytosolic NADPH-dihydrofolate reductase

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41
Q

How does Folate enter the portal blood?

A
  • Folate binding proteins;
  • Transported across the cell and the THF is transported across the basolateral membrane into PORTAL blood by carrier-dependent active transport probably by multidrug resistance protein (MRP)
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42
Q

What happens to Folate when absorbed by the LIVER?

A
  • Taken in by PCFT also;
  • ONLY Monoglutamates can cross cells;
  • GLUTAMATES are added to trap folates by folypoly-glutamate synthetase (Polyglutamyl form);
  • To leave cells, hydolases remove the glutamates
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43
Q

What are the forms of folate found in the LIVER?

A

THF, 5-methylTHF, and 5- or 10-formyl THF:

  • About half body folate is found in liver that supplies actively proliferating cells;
  • Less metabolically active tissues return folate monoglutamates back to the liver for redistribution
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44
Q

What forms of folate are found in Systemic Plasma?

A
  • THF or
  • 5-methylTHF and 10-formylTHF as monoglutamates;
  • They are FREE (1/3) in equilibrium or (2/3) BOUND;
  • Bound to either albumin, alpha-2 macroglobulin or folate binding protein (a soluble form of the membrane folate receptor);
  • Reduced folate carriers may also transport folate in systemic plasma and may be the folate binding protein
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45
Q

What folate is found in red blood cells?

A
  • RBCs have MORE of folate forms than plasma;
  • All is taken up before they mature in their development;
  • B/c mature RBCs cannot take up folate forms;
  • Folate in RBCs is a sign long term folate status (2 to 3 months)
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46
Q

How does folate enter other tissues besides the liver?

A
  • Transport into CELLS is by PCVT and by folate receptor α, β, or ϒ;
  • Inside cells, glutamates are added and make polyglutamyl form by folypolyglutamyl synthetase to trap;
  • To leave HYDROLASES similar to those found in intestine convert back to monoglutamyl form;
  • Addition of glutamates is ATP-dependent
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47
Q

What other purpose does the addition of glutamates to folate in the cell/tissues serve?

A
  • POLYglutamate form is better substrate for metabolic enzymes;
  • Found in both the cytoplasm mitochondria for reactions
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48
Q

What is the main function of Folate?

A

Accept and donate ONE CARBON UNIT

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49
Q

What is the metabolic role of 10-formyl THF?

A

Folate transfers formate as 10-formyl THF for PURINE synthesis

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50
Q

What is the metabolic role of 5,10-methylene THF?

A
  • Folate transfers formaldehyde as 5,10-methylene for PYRIMIDINE synthesis;
  • Folate receives formaldehyde for SERINE degradation/GLYCINE synthesis;
  • Folate receives formaldehyde for GLYCINE degradation;
  • Folate receives formaldehyde for GLYCINE synthesis
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51
Q

What is the metabolic role of 5-methyl THF?

A

-Folate provides a METHYL group (1 carbon) for METHIONINE synthesis

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52
Q

What is the metabolic role of 5-formimino THF?

A

-Folate receives a formimino group in HISTIDINE degradation

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53
Q

What is the “methyl trap” of folate interconversion?

A

5-methyl THF cannot be converted to 5,10-methylene THF due to Vitamin B12 DEFICIENCY;

  • MTHFR is B12-dependent for the remethylation of homocysteine to methionine;
  • Leads to MEGALOBLASTIC Anemia and due to impaired folate metabolism
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54
Q

What other condition can cause a “methyl trap” beside B12 deficiency?

A

-Mutation in MTHFR associated with birth defects and hyperhomocysteinemia = eleveated Homocysteine in the blood

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55
Q

How can HIGH can high folate levels overcome B12 deficiency and prevent megaloblastic anemia?

A

–

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56
Q

What are the red blood cell characteristics of megaloblastic anemia?

A
  • Immature stage of development;

- Still have nuclei and are slightly larger than normal red blood cels

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57
Q

What causes Microcytic hypochromic anemia?

A

-Iron deficiency

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58
Q

Folate is involved in the metabolism of what AMINO ACIDS?

A
  • Histidine;
  • Serine;
  • Glycine;
  • Methionine
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59
Q

What reaction in Histidine catabolism can be used to measure a Folate deficiency?

A
  • Intermediate interconversion of Formiminoglutamate to Glutamate;
  • Changes THF to 5-formimino THF;
  • Gives a Histidine Load and measures FIGLU in the Urine (will have high concentrations)
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60
Q

How is Glycine synthesized from Serine in the body?

A
  • Reversible rxn;
  • Enzyme = serine hydroxymethyltransferase catalyses transformation using THF;
  • Generates methylene THF and glycine.
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61
Q

How is Glycine primarily degraded?

A
  • Glycine Cleavage System;

- Leads to the degradation of glycine into ammonia and CO2 with the use of NAD+

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62
Q

How is Glycine synthesized from Choline degradation?

A
  • Requires folate and goes through Betaine (trimethylglycine);
  • Betaine is a NEUTRAL compound due to having both a + and - functional group
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63
Q

What other benefit has been show of Betaine?

A
  • Reduction of the levels of homocysteine and folate supplementation appears to increase betaine levels;
  • Involved in the alternative conversion of homocysteine to methionine with homocysteine transferase
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64
Q

Why is Folate essential for cell division?

A
  • Essential for purine and pyrimidine synthesis;
  • Acts as a substrate/reactant in the formation of dTMP (nucleotide monomer for DNA synthesis);
  • dTMP is absolutely necessary for DNA synthesis
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65
Q

What are the two enzymes that are responsible for the formation of dTMP from 5,10-methylene THF?

A

1.Thymidylate synthetase = 5,10-methylene THF to DHF
and
2. Dihydrofolate reductase = DHF to THF

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66
Q

What is the overall reaction for the generation of dTMP?

A

5,10-methylenetetrahydrofolate + dUMP ← → dihydrofolate + dTMP

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67
Q

What is Methotrexate?

A
  • ANTIFOLATE drug → binds to active site of dihydrofolate reductase and is used as chemotherapeutic drug, to treat rheumatoid arthritis, psoriasis, Crohn’s to prevent synthesis of actively dividing cells;
  • Prevents METABOLISM of Folic Acid;
  • Side affects mimic that of folate deficiency;
  • High intakes or supps with Methotrexate can calm side effects w/o altering drug effects
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68
Q

What tissues are affected by Folate deficiency?

A
  • All tissues that turn over regularly;
  • May be related to cancer as DNA repair is impaired;
  • Will reflect different functions in cytoplasm and mitochondria
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69
Q

What is Cobalamin (B12)

A

Generic term for a group of compounds called CORRINOIDS with a CORRIN nucleus

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70
Q

What makes up a Corrin Nucleus?

A
  • Macrocyclic ring (BIG ring) made of FOUR reduced pyrrole rings linked together;
  • In the center of the corrin is an atom of COBALT;
  • Attached to the cobalt is a group defining each type of Cobalamin
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71
Q

What are the various groups attached to Cobalt to make different Cobalamin forms?

A
  • CN = Cyanocobalalmin;
  • OH = Hydroxycobalamin;
  • H2O = Aquocobalamin;
  • NO2 = Nitrotocobalamin;
  • 5’deoxyadenosyl = 5’deoxadenosylcobalamin;
  • CH3 = Methylcobalamin
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72
Q

What are the ACTIVE COENZYME forms Cobalalmin?

A

-5’-deoxyadenosyl-cobalamin (adenosyl);
OR
-Methylcobalamin;
-Other forms can be converted to coenzyme forms

73
Q

What is Vitamin B12?

A

-Cyanocobalamin – the most common and widely produced of the chemical compounds that have vitamin activity as vitamin B12;
Vitamer of B12 because body can convert cyanocobalamin to ANY one of the active coenzyme forms of B12

74
Q

When was B12 discovered?

A
  • LAST discovered;
  • 1948 isolated by Smith and Riskes et al;
  • Structure by Hodgkin;
  • 1926, Minot and Murphy discovered that consumption of LIVER could help correct pernicious anemia
75
Q

What is Pernicious Anemia?

A
  • B12 deficiency;
  • Body can’t make enough healthy red blood cells because it doesn’t have enough vitamin B12;
  • Due to Lack of intrinsic factor for B12 metabolism;
  • Causes Megolobalastic anemia
76
Q

What are the FOOD sources of B12?

A
  • ONLY sources are animal products;
  • vegans at risk for vitamin B12 deficiency;
  • Only microbes can synthesize;
  • Meat and meat products, poultry, fish, shellfish, eggs (yolk) mainly in form of adenosyl- and hydroxocobalamin;
  • Plants DO NOT use, would only be from nitrogen-fixing bacteria
77
Q

What are the Supplemental forms of B12?

A
  • Cyanocobalamin;

- Hydroxocabalamin

78
Q

What is the FIRST step of B12 digestion?

A
  • Ingested cobalamins must be RELEASED from proteins/polypeptides in foods;
  • Pepsin and HCl perform this release in the STOMACH denature and release
79
Q

What then binds B12 after the action of HCl?

A
  • R protein (protects vit from acid):
  • Found in saliva and gastric juice, binds to the vitamin B12 prior to or after its release from food;
  • Complex leaves stomach into the duodenum
80
Q

What happens with B12 digestion in the DUODENUM?

A

-The R protein is digested
by pancreatic proteases;
-FREE vitamin (all forms) then binds to the intrinsic factor (IF) that was produced by gastric parietal cells, but is not catabolized by proteases

81
Q

How is B12 finally absorbed?

A

-Receptors in the ILEUM, especially the distal third, (called cubulin, IF factor or cubam) bind the
IF-vitamin B12 complex and take in the whole thing

82
Q

What are the absorption receptors for the IF-B12 complex?

A
  • Ileum receptors = cubulin, IF factor or cubam;
  • Amnionless (protein) helps cubulin bind to membrane to bind to IF-B12;
  • Megalin (protein) might help transport this complex into the cell
83
Q

What are R proteins?

A

-Cobalophilins or haptocorrins (HCs)

84
Q

What results from Pancreatic Insufficiency with B12 digestion?

A
  • Prevents release of B12 from R protein;
  • Reduced bicarbonate release into duodenum, so acids aren’t neutralized;
  • oAcidic environment FAVORS BINDING of R protein to B12 rather than IF
85
Q

What is Zollinger-Ellison Syndrome?

A
  • Increased gastric acid secretion;

- Causes a LOWER pH in the small intestine and this PREVENTS release of the vitamin from R protein

86
Q

Why are older people recommended to consume B12 fortified foods or supplements?

A
  • Cyanocobalamin does NOT decrease with age;
  • 10% to 30% of older people have GI changes that limit absorption of food-bound forms of the vitamin (release from food hampered)
87
Q

What limits the release from food with age?

A

-Decrease in gastric acidity so not released from binding in food;
-Atrophic gastritis treatable with antibiotics or achlorhydria;
or
-Overgrowth of bacteria that compete for uptake of vitamin B12;
-Might be increased autoimmune in aging and destruction of cells that produce IF (intrinsic factor)

88
Q

How is B12 absorbed in to cells?

A
  • IF-mediated transport is saturated at 1.5 to 2.0 micrograms of vitamin B12;
  • 1% to 3% is absorbed by diffusion especially with large doses;
  • Absorption ranges from 11% to 65%, decreasing as intake increases;
  • 50% absorption is generally assumed;
  • Enterohepatic circulation (already within the body) is important as this contributes from 3-8 micrograms daily
89
Q

What happens to B12 once in the cells?

A

Within the mucosal cells, vitamin B12 is RELEASED from IF

90
Q

What are the forms of coenzyme forms of B12?

A
  • 60% to 80% – methylcobalamin;

- 20% –adenosylcobalamin

91
Q

What is B12 bound to in the blood?

A
  • Bound in blood to either TCI, TCII or TCIII;

- When bound = HOLOTRANSCOBALAMINS

92
Q

What are the Holotranscobalamins?

A

-Form of R proteins:
•TCII is the main one initially;
•Later 80% is bound to TCI (haptocorrin) and may be circulating storage and cells take on by non specific receptors
•TCIII may return the vitamin to the liver

93
Q

How is B12 taken into TISSUES?

A
  • Receptor dependent as ALL tissues have receptors for TCII;
  • TCII-cobalamin complex is taken up by receptor-mediated endocytosis;
  • Then fuses with lysosomes to DEGRADE TCII and free the vitamin
94
Q

What is the mutation TC766C>G?

A
  • Genetic mutations in TCII exist;
  • Arginine to proline and reduces protein’s ability to bind the vitamin;
  • 20% of population is homozygous for GG with elevated homocysteine;
  • Women appear most vulnerable
95
Q

How much B12 is stored in the body?

A
  • About 2 to 4 mg of the vitamin is STORED mainly (~50%) in LIVER and can be stored for even years;
  • More like a fat soluble vitamin
96
Q

What are the 2 enzymatic reactions require the coenzyme forms of B12?

A
  • One requires methylcobalamin and the other adenosylcobalamin;
    1. Resynthesis of methionine from homocysteine (with Folate)
    2. Oxidation of L-methylmalonyl-CoA.
97
Q

How does B12 deficiency cause Megaloblastic Macrocytic Anemia?

A
  • Like folate deficiency;
  • Occurs when vitamin B12 is deficient causes methyl-folate trap;
  • Folate stays in the 5-methylTHF form and homocysteine increases;
  • Leads to a higher risk for atherosclerosis;
  • Can be MASKED by folate supps.
98
Q

What neurological problems come from B12 deficiency?

A
  • 75-90% deficient people;
  • Tingling and numbness (paresthesia) in extremities, abnormal gait, loss of concentration, loss of coordination, memory loss, disorientation, swelling of myelinated (insulated) nerve fibers, psychosis and possibly dementia
99
Q

What causes the neurological problems?

A
  • DEMYLINATION of nerves in the central nervous system - nerves coated in MYELIN;
  • May be from lack of SAM needed for methylation reactions for maintenance of myelin, or an imbalance in cytokines and growth factors
100
Q

What is Pernicious Anemia?

A
  • B12 deficient;
  • Autoimmune disease in which PARIETAL CELLS of the stomach responsible for secreting intrinsic factor are DESTROYED;
  • IF is crucial for the normal absorption of B12, so a lack of intrinsic factor, causes a vitamin B12 deficiency;
  • Nothing to do with Folate;
  • Pernicious refers to the neurological damage
101
Q

How is Folate excreted?

A
  • Intact or metabolized to N-acetyl paraaminobenzoyl glutamate is excreted in the urine;
  • Also enterohepatic recirculation of folate;
  • Folate of microbial origin can be high in feces (colonic microbes)
102
Q

How is B12 excreted?

A
  • ~0.1% loss/day bound to R protein in bile;

- Only excreted in urine if injected amount is so much that exceeds binding capacity of blood

103
Q

What is the RDA for Folate?

A
  • Adults – 400 micrograms per day of Dietary Folate Equivalents (DFEs) = fortification/supps and foods;
  • Reduce the risk of Neural Tube Defects for women of childbearing age;
  • Too much fortification could mask a B12 deficiency, especially in the elderly
104
Q

What are DFEs?

A

-One DFE is equal to 1 microgram of food folate or 0.6 microgram of folic acid from a supplement or fortified food consumed with a meal or 0.5 micrograms of folic acid from a supplement taken without food

105
Q

What is the RDA for B12?

A
  • Adults – 2.4 micrograms per day;

- People older than 50 are told to consume most of their requirement from FORTIFIED foods or SUPPS

106
Q

What is the UL/toxicity for Folate?

A
  • Concern of masking B12 deficiency;

- UL = 1,000 micrograms/day from fortified foods and supps. (not natural)

107
Q

What is the UL/toxicity for B12?

A
  • NO toxicity;
  • No benefits of large doses for those with adequate Vitamin status;
  • Large doses are effective for people that CANNOT ABSORBD the vitamin by the normal process → 1 to 3% absorbed by passive diffusion especially with pharmacological doses
108
Q

How is Folate status assessed?

A

3 indicators:

  • Erythrocyte folate (<6.8 ng/ml)
  • Plasma homocysteine
  • Remember low vitamin B12 can affect these measures
109
Q

How is B12 status assessed?

A
  • Based on normal hematological status and serum levels → concentration in serum or plasma reflects BOTH intake and stores;
  • Lower limit of adequate is 170-250 pg/ml
  • In the short-term serum levels may be maintained at the expense of tissue stores, low serum levels reflects long-term;
  • High serum and urine methylmalonic acid is favored, but not enough data at this time
110
Q

Who discovered B6?

A
  • Gyorgy showed role in curing rat acrodynia (skin lesions);
  • 1930s distinguished from other B vits;
  • Lepkovsky obtained crystalline structure from plants 1938;
  • Strucutre by Harris and Folkers, 1939;
  • Snell synthesized pyridoxal (PL) and pyridoxamine (PM) as other natural forms of the free vitamin
111
Q

What does B6 deficient Infants develop?

A
  • Epileptiform seizures and dermatitic lesions;

- Occured in 1950s due to heat treatment of formula and formation of pyridoxine-lysine (not active)

112
Q

What is the functional COENZYME form of B6?

A
  • Pyridoxal 5’-phosphate (PLP);

- Umbreit discovered in 1944

113
Q

What are the derivatives of B6?

A
  1. Pyridoxine (PN)
  2. Pyridoxial (PL)
  3. Pyridoxamine (PM)
  4. Pyridoxine phosphate (PNP)
  5. Pyridoxial phosphate (PLP)
  6. Pyridoxamine phosphate (PMP
114
Q

What are the 3 vitamins of B6 found in foods?

A
  • All interchangleable and can make coenzyme;
    1. PN/PNP is in PLANTS and can be conjugated as pyridoxine glucoside;
    2. PL/PLP and PM/PMP in ANIMAL products
115
Q

What are the best sources of B6?

A
  • EXCELLENT sources are meats, whole-grains, vegetables, some fruits (bananas), and nuts;
  • Fortified cereals are a good source;
  • Supps = pyridoxine hydrochloride
116
Q

How can B6 be LOST from foods?

A
  • Food matrix affects bioavailability;
  • Can be lost with processing;
  • Prolonged high heating as in sterilization (sterilization of infant formula) and canning can destroy;
  • Lost with milling and refining of grains
117
Q

How is B6 absorbed into MUCOSAL CELLS?

A
  • DEPHOSPHORYLATED form very rapidly;
  • Range ~ 61% to 92% or avg. 72%;
  • Alkaline phosphatase and other phosphatases at the intestinal brush border hydrolyze the phosphorylated forms of PN, PL, and PM = ALLOW ABSORPTION;
  • High doses phosphorylated and pyridoxine glucoside will be absorbed
118
Q

How do CELLS utilize B6?

A
  • PN, PL and PM are mainly NOT METABOLIZED by the intestinal CELLS;
  • Released into the portal blood
119
Q

How is are the forms of B6 utilized in the LIVER?

A
  • Absorbed by Passive diffusion;
  • Majority is converted to PLP;
  • Major form in systemic blood and bound to ALBUMIN;
  • Binding proteins in the liver protect PLP from hydrolysis
120
Q

How do do B6 derivatives enter TISSUES?

A
  • RBCs take up the PLP and it is bound to HEMOGLOBIN;

- Other tissues take up only NON-phosphorylated forms so alkaline phosphatase converts PLP to PL

121
Q

How is PL converted to PLP in CELLS?

A

-Pyridoxine kinase phosphorylates PL to PLP → this enzyme is found in most tissues; most tissues LACK the PMP and PNP FMN-dependent oxidase;
The oxidase is found mainly in LIVER and INTESTINE and to lesser extent in some other tissues

122
Q

What is the storage of B6?

A
  • 5 to 10% in liver
  • 75 to 80% in muscle
  • Storage in body ~40-185 mg
123
Q

What is the main functions of B6?

A
  1. PLP, the COENZYME form, is associated with >100 enzymes = AMINO ACID metabolism;
  2. NON-coenzyme role in GENE EXPRESSION;
  3. Glycogen metabolism
124
Q

What does PLP form as a coenzyme during amino acid metabolism?

A

-Schiff base = has a functional group with a C-N double bond with the N attached to an aryl or alkyl, NOT hydrogen

125
Q

What effect does the PLP have on the amino acid during metabolism?

A
  • Covalent bonds of an alpha amino acid are made MORE REACTIVE (labile) by its binding to PLP containing enzymes;
  • All depends on which enzyme as far as which group is attacked
126
Q

What types of reactions take place with the alpha amino acids?

A
  • Transamination;
  • Decarboxylation
  • Transulfhydration;
  • Desulfhydration;
  • Dehydration (elimination)/deamination;
  • Cleavage;
  • Racemization,
  • Synthesis
127
Q

B6 and Transamination?

A

Step 1. Transfers from enzyme to amino acid and then back to enzyme;
Step 2. Formation of a new amino acid and reformation of PLP-enzyme

128
Q

B6 and Dehydration/Deamination

A
  • Dehydratase enzyme removes water and amino group;

- Requires PLP

129
Q

B6 and Decarboxylation

A
  • Removal of the carboxyl group (COO-) from an amino acid or other compound;
  • EX: GABA synthesis from the amino acid glutamate= Glutamate decarboxylase is PLP-dependent;
  • EX: Serotonin synthesis (from tryptophan) and degradation = decarboxylase is PLP-dependent;
  • EX: Arginine, proline, histidine, and glutamate metabolism.;
  • EX: Selenium metabolism
130
Q

B6 and Cleavage Reaction

A
  • Removal of a hydroxymethyl group from serine by hydroxymethyl-transferase;
  • Hydroxymethyl group of serine is transferred to THF and GLYCINE is formed
131
Q

How do bacteria utilize B6?

A
  • Bacteria need PLP for the interconversion of D- and L-amino acids;
  • Enzymes = Racemases (isomerase enzymes that alter stereochemistry);
  • EX: Serine Racemase – an enzyme which generates D-serine from L-serine.
132
Q

What are some other reactions that require PLP?

A
  • First step of synthesis of heme by (delta) δ-aminolevulinic acid synthetase = combines Glycine and Succinyl-CoA;
  • Condensation reaction to form sphingolipid;
  • Niacin synthesis from tryptophan;
  • Carnitine and taurine synthesis
133
Q

How does B6 function in Glycogen metabolism?

A
  • PLP-dependent glycogen phosphorylase DEGRADES glycogen by one glucose to produce glucose-1-phosphate;
  • PLP in MUSCLE is bound to glycogen phosphorylase
134
Q

What is the NON-coenzyme role of B6?

A

-Bind to DNA and interact with steroid hormone receptors and other transcription factors to modulate their binding to regulatory regions of DNA = Gene Expression

135
Q

What is the metabolite of B6 that is excreted?

A
  • 4-pyridoxic acid (PIC);
  • Produced by PL acted on by either NAD-dependent aldehyde dehydrogenase or FAD-dependent aldehyde oxidases found in LIVER and KIDNEY;
  • Excreted in the URINE and indicates RECENT INTAKE and not stores;
  • Ingestion of large doses (100 mg) of PN results in urinary excretion of PN and 5-pyridoxic acid
136
Q

What is the RDA for B6?

A

-Adult men and women ages 19 to 50 = 1.3 mg/day
•Men older than 51 = 1.7 mg/day
•Women older than 51 = 1.5 mg/day
*Based on plasma PLP of at least 20 nmol/L;
**Elderly higher because of possible acceleration of PLP hydrolysis and oxidation of PL to 4-pyridoxic acid → also at risk of deficiency because of low intakes

137
Q

What are the symptoms of B6 deficiency?

A
  • RARE;
  • Symptoms from lack of enzyme functions → hypochromic, microcytic anemia or hyperhomocysteinemia
  • Presence of acetaldehyde from alcohol metabolism enhance the vitamin’s DEGRADATION
138
Q

What vitamins INCREASE the dietary need for B6?

A
  • Corticosteroids
  • Oral contraceptives
  • Penicillamine (autoimmune diseases and Wilson’s disease [copper not excreted into bile]);
  • Isoniazid (tuberculosis)
139
Q

What is the UL for B6?

A
  • UL = 100 mg to minimize the risk of NEUROPATHY;
  • Causes sensory and peripheral neuropathy with symptoms of unsteady gait, paresthesia, impaired tendon reflexes;
  • Also may get degeneration of dorsal root ganglia in the spinal cord, loss of myelination, and degeneration of sensory fibers in peripheral nerves → Severe nerve damage with B6 toxicity!!
140
Q

What have pharmacological doses of B6 been used to treat?

A

Hyperhomocysteinemia, carpal tunnel syndrome, premenstrual syndrome, depression, muscular fatigue, and paresthesia (tingling or numbness of the feet and hands)

141
Q

How is B6 status assessed?

A

-Plasma PLP concentrations are thought to be the BEST indicator of TISSUE stores
oDEFICIENCY = Levels 30 nmol/L

142
Q

When was Biotin discovered?

A
  • 1931, “egg white injury”, which consisted of hair loss, dermatitis, and neuromuscular problems;
  • 1940s structure determined and chemically synthesized;
  • once called vitamin H for the German word “haut” means “skin”;
  • Has been referred to as vitamin B7
143
Q

What is “Egg White Injury”?

A
  • Caused by AVIDIN;
  • A protein in egg whites that forms one of the strongest covalent bonds in nature;
  • Cooking DENATURES avidin and so cooked egg whites are safe for biotin nutrition;
  • Denaturing prevents the Avidin from binding biotin and making unavailable
144
Q

What are the food sources of Biotin?

A

Widely distributed in foods = liver, soybeans, egg yolk, cereals, legumes, and nuts are good sources;
-In many foods biotin is BOUND to lysine in proteins or = Biocytin (AKA: biotinyllysine)

145
Q

What makes up the structure of Biotin?

A

-TWO rings;
-A Ureido ring;
and;
-Thiophene ring connected to a valeric acid side chain;
-Valeric Acid – pentanoic acid, is a straight-chain alkyl carboxylic acid with the chemical formula C5H10O2

146
Q

What is “Egg White Injury”?

A
  • Caused by AVIDIN;
  • A protein in egg whites that forms one of the strongest covalent bonds in nature;
  • Cooking DENATURES avidin and so cooked egg whites are safe for biotin nutrition;
  • Denaturing prevents the Avidin from binding biotin and making unavailable
147
Q

What are the food sources of Biotin?

A

Widely distributed in foods = liver, soybeans, egg yolk, cereals, legumes, and nuts are good sources;
-In many foods biotin is BOUND to lysine in proteins or = Biocytin (AKA: biotinyllysine)

148
Q

What makes up the structure of Biotin?

A

-TWO rings;
-A Ureido ring;
and;
-Thiophene ring connected to a valeric acid side chain;
-Valeric Acid – pentanoic acid, is a straight-chain alkyl carboxylic acid with the chemical formula C5H10O2

149
Q

What is the hydrolysis of proteins and protein-bound biotin?

A

proteins broken down into component amino acids

150
Q

How are Biotinyl peptides digested?

A

-HYDROLYZED by proteases and peptidases

151
Q

How is Biocytin further digested?

A
  • HYDROLYZED by biotinidase which is found on the intestinal brush border and secreted in pancreatic and intestinal juices;
  • Some biocytin can be absorbed by peptide carriers and HYDROLYZED by biotinidase in plasma and in most body cells
152
Q

What is the hydrolysis of proteins and protein-bound biotin?

A

proteins broken down into component amino acids

153
Q

What is BIOTINIDASE?

A
  • Enzyme that hydrolyzes protein-bound biotin and frees the vitamin;
  • Present in most body cells;
  • At ACIDIC pH it separates biotin from lysine or peptides
  • At ALKALINE pH, the enzyme gets biotinylated and generates FREE lysine;
  • Enzyme also removes biotin from histone proteins
154
Q

What is Biotinylation?

A

-Covalently attaching biotin to a protein → This is how it binds to AVIDIN to cause “egg white injury”

155
Q

What occurs with a Biotinidase (enzyme) efficiency?

A
  • DEFICIENCY discovered in 1983;
  • Autosomal recessive inborn error of metabolism;
  • Without biotinidase (enzyme) biotin deficiency develops (secondary deficiency);
  • Symptoms = seizures, ataxia, skin rash, alpopecia, and acidosis;
  • *CAN’T metabolize and free biotin for use
156
Q

How is Biotin absorbed into the mucosal cells?

A
  • FREE biotin is absorbed in the JEJUNUM, but some is absorbed in the ileum → Greater number of carriers in the jejunum;
  • Physiological (normal) doses uses carrier mediated and sodium dependent;
  • Pharmacologic (HIGH) doses use passive diffusion;
157
Q

What are the Biotin carrier proteins into TISSUES?

A
  1. INTESTINE and LIVER = Sodium-dependent multivitamin transporter (SMVT) → Also transports pantothenic acid and lipoic acid;
  2. Most tissues is solute carrier gene family 19 (SLC19A3) that also transports thiamin
  3. Leukocytes, in addition to SLC19A3, have monocarboylate transporter 1 (MCT1)
158
Q

What is the form of Biotin found in the PLASMA?

A

~80% is found FREE in PLASMA;

  • Smaller amounts are bound to alpha and beta globulins;
  • And also biotinidase
159
Q

What are the functions of Biotin?

A
  • Coenzyme covalently bound to enzymes, especially for caboxylase runs;
  • Also functions in non-coenzyme roles possibly in cell proliferation and gene expression
160
Q

What is Pyruvate Carboxylase (Biotin-dependent coenzyme)?

A
  • Converts pyruvate to oxaloacetate (OAA)
  • Activated by Acetyl CoA;
  • If the cell has a SURPLUS of ATP along with acetyl CoA, the OAA is used for gluconeogenesis;
  • If the cell is DEFICIENT in ATP, the OAA enters TCA cycle and condenses with acetyl CoA for catabolism of the acetyl CoA and energy generation
161
Q

What are the Biotin-dependent enzymes of Holocarboxylases?

A
  1. Pyruvate carboxylase;
  2. Acetyl-CoA carboxylase
  3. Propionyl-CoA carboxylase;
  4. Beta-methylcrotonyl-CoA carboxylase;
    * Holocarboxylase synthetase attaches to Biotin in 2 steps
162
Q

What occurs with a MUTATION to Holocarboxylase Synthetase?

A

*Enzyme that attaches biotin to dependent enzymes;
-Metabolic problems;
-TWO active sites on the enzymes:
1. For generation of the carboxybiotin enzyme
2. Transfers the activated carbon dioxide to a reactive carbon on the substrate = carboxylation rxn to ADD. CO2 to a compound;
The long flexible chain (Valeric Acid) allows the swinging between the active sites

163
Q

What is Pyruvate Carboxylase (Biotin-dependent coenzyme)?

A
  • Converts pyruvate to oxaloacetate (OAA)
  • Activated by Acetyl CoA;
  • If the cell has a SURPLUS of ATP along with acetyl CoA, the OAA is used for gluconeogenesis;
  • If the cell is DEFICIENT in ATP, the OAA enters TCA cycle and condenses with acetyl CoA for catabolism of the acetyl CoA
164
Q

What happens in the beginning of the FED state containing carbs?

A
  • Glucose is catabolized to acetyl-CoA;
  • ATP will be low and some pyruvate is diverted to OAA to handle acetyl CoA from dietary glucose to produce ATP;
  • ATP increases, the OAA is diverted to a greater extent to gluconeogenesis and less of the acetyl CoA is catabolized and more of it is used for fatty acid synthesis;
  • Coupled with stimulation of acetyl CoA carboxylase
165
Q

What activates Acetyl-CoA carboxylase?

A
  • Activated by citrate/isocitrate;
  • Citrate increases in concentration in the well-fed state and is an indicator of a plentiful supply of acetyl CoA → The way to get ACETATE out of the mitochondria
166
Q

What inactivates Acetyl-CoA carboxylase?

A

-INACTIVATED by phosphorylation of the enzyme and long-chain acyl CoA;
-Glucagon, epinephrine and insulin regulates this enzyme via changes in phosphorylation state;
= Add -PO4

167
Q

What is Propionyl-CoA carboxylase?

A

-Important for the catabolism of isoleucine, threonine and methionine

168
Q

What is Beta-Methylcrotonyl CoA ?

A
  • Enzyme that catalyzes the metabolism of Leucine;
  • If biotin is DEFICIENT beta-methylcrotonyl CoA is shunted to another pathway that produces 3-hydroxyisovaleric acid (3-HIA);
  • Increased 3-HIA and decreased biotin in the urine are signs of biotin deficiency
169
Q

What are the NON-Coenzyme functions of Biotin?

A
  • Functions are mediated through biotin attaching to histones;
  • DNA is wrapped around histones as nucleosomes;
  • Attachment of biotin to histones is mediated by HOLOCARBOXYLASE SYNTHETASE and BIOTINIDASE;
  • Increased biotinylation correlates with increased cell proliferation compared to quiescent cells (dormant, inactive)
170
Q

What are the metabolites from Biotin degradation?

A
  1. Bisnorbiotin converted to Tetranorbiotin by Beta-oxidation of side-chain;
  2. Biotin sulfoxide converted to Biotin sulfone by Oxidation of Sulfur
    * *All excreted in the Urine
171
Q

How are Biotin Holocarboxylases broken-down?

A
  1. (Biotin + attached enzymes) are DEGRADED by PROTEASES to give biotin oligopeptides
  2. Biotin oliogopeptides are converted to biocytin (biotin + lysine);
  3. Then biocytin is converted to free biotin and lysine by biotinidase
172
Q

How is Biotin excreted?

A

-Some biotin is excreted in the urine as biotin, some reused, and some degraded;
-Usually there is NO DEGRADATION of the RING structure, but DEGRADATION of the VALERIC ACID side chain;
-CAN be oxidation of the sulfur in the biotin RING → Lose electrons and become a radical/reactive;
Women that SMOKE seem to have accelerated biotin catabolism (breakdown)

173
Q

What are the metabolites from Biotin degradation?

A
  1. Bisnorbiotin converted to Tetranorbiotin by Beta-oxidation of side-chain;
  2. Biotin sulfoxide converted to Biotin sulfone by Oxidation of Sulfur
    * *All excreted in the Urine
174
Q

What is the requirement for Biotin?

A
  • AI for adults = 30 micrograms/day
  • Some biotin SYNTHESIZED by bacteria is absorbed, but not enough
  • Biotin deficiency in humans is rare, but can happen especially with genetic problems
175
Q

What causes Biotin deficiency?

A 
1. Eating too many raw egg whites (Aviding binding to vitamin);
Most common=
2. Various GI problems
3. Chronic excessive alcohol consumption
4. Anticonvulsant drug therapy
176
Q

How are defects of Biotinidase and Holocarboxylase Synthetase treated?

A
  • Biotinidase defects can be helped with 5 to 10 mg/day;

- Holocarboxylase Synthetase defects can be helped with 40 to 100 mg/day

177
Q

Is Biotin considered toxic?

A
  • NO toxicity of biotin has been reported so no UL;
  • Use of 100 mg/day for genetic disorders of biotin enzymes and uses of biotin in hair and skin conditioning have not caused any problems
178
Q

How is Biotin status assessed?

A
  • Urinary assessment of biotin and other compounds are BEST assessment methods – better than plasma biotin
  • Normal urinary 3-hydroxyisovaleric acid are <0.2 micromoles/mg of creatinine

Spring 2014 (22 decks)

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