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NUTR 302 > Thiamin > Flashcards

Thiamin Flashcards

1
Q

What is the history of the discovery of Thiamin?

A
  • First described water-soluble vitamin
  • 1884: Dr. Takaki hypothesized that beriberi was caused by dietary insufficiency
  • 1897: Christiaan Eijkman - paralysis in birds fed cooked, polished rice. Reversed when rice polishing stopped. (Nobel prize)
  • 1901: essential nutrient in the outer layer of the grain
  • ’20s and ’30s: isolation, structure and synthesis of thiamin: “sulfur-containing vitamin”
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2
Q

What is B1 called?

A

Thiamin

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3
Q

What are the properties of thiamin?

A
  • Colorless, water soluble
  • Not stable in UV light, moisture, alkaline/neutral solutions
  • Pyrimidine + thiazole
  • Active site on C2 of thiazole. Can accept and donate molecule
  • Active site important for what reactions it goes through and how it impacts metabolism
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4
Q

What is the active form of B1?

A
  • ThDP or TPP (thiamin pyro/diphosphate) is the active (coenzyme) form of thiamin (80%)
  • Can also exist as mono-and tri-phosphate forms
  • Thiaminylated adenines
  • Pyrophosphate coenzyme = active form. Enzyme activates the vitamin
  • Thiamin + 2 phosphates added
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5
Q

Explain how thiamin is activated

A
  • ThMP is the active form. Phosphatase can remove a phosphate to get a thiamin
  • ATP and magnesium can be added to thiamin to form free ThDP through thiamin pyrophosphokinase
  • thDP can be bound, converted to ThTP or AtHTP
  • ThDP converted to ThMP if phosphate is removed through thiamin diphosphatase
  • Adenylate kinsase and ATP and magnesium converts free ThDP to ThTP
  • ThTP converted to ThDP via thiamin triphosphatase (removal of P)
  • ThDP adenylyl transferase converts ThDP to AthTP with ADP
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6
Q

What does a phosphatase do? What does a kinase do?

A

Phosphatase: Removes phosphate
Kinase: Uses ATP to add a phosphate

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7
Q

What is the metabolic role of thiamin?

A
  • Coenzyme in >24 enzymes
  • Plays a role in:
    → Nervous system function: role in nerve conduction and in neural meembranes
    → Energy production
    → Biosynthesis of lipids
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8
Q

What are the coenzyme vitamins?

A
  • B vitamins, vitamin C, and vitamin K
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9
Q

What do coenzymes do?

A
  • Some vitamins form part of the coenzymes that enable enzymes either to synthesize compounds
  • Bind to proteins to help it do its activity
  • Without coenzymes protein does not work
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10
Q

Thiamin is involved in the oxidative decarboxylation of alpha-keto acids. What three examples are given in class of this?

A
  • Pyruvate dehydrogenase complex
  • Alpha-ketoglutarate dehydrogenase
  • Branched-chain alpha-keto acid dehydrogenase

All involve breaking down metabolites for energy

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11
Q

What three major impacts does thiamin have on the metabolism?

A
  1. Oxidative decarboxylation of alpha-keto acids (energy metabolism)
  2. Transketolation (Converting sugars)
  3. Alpha-oxidation of phytanic acid (found in meat, dairy, fish) (breaking down phytanic acid)
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12
Q

Thiamin is involved in transketolation. What is the example of this given in class?

A

Interconversions of sugar phosphates in pentose phosphate shunt

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13
Q

Thiamin is involved in alpha-oxidation of phytanic acid. Give examples of these given in class.

A
  • 3-methyl-substituted fatty acid = cannot undergo Beta oxidation
  • Genetix enzyme disorders can cause buildup of phytanic acid and lead to Refsum’s disease.
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14
Q

Where are B vitamins involved in glycolysis?

A
  • Involved in the conversion of pyruvate to acetyl-CoA (PDC) and the TCA cycle
  • B vitamins involved throughout the cycle
  • CoASH
  • FADH2
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15
Q

How is thiamin involved in the pyruvate dehydrogenase complex?

A
  • Green = B vitamins. CoA-SH, TPP, FAD, NAD
  • TPP: thiamin pyrophosphate binds to pyruvate and get CO2 produced
  • CHOH = acetyl compound that makes acetyl CoA later
  • Thiamin accepts and donates acetyl groups
  • Without thiamin would not be able to break down pyruvate
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16
Q

Explain how thiamin is involved in alpha-ketoglutarate dehydrogenase

A
  • Alpha ketoglutarate + NAD and CoA converted to succinyl CoA by alphaketoglutarate dehydrogenase
  • Involves the removal of carbon dioxide from alpha ketoglutarate
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17
Q

How is thiamin involved with branch chain amino acids?

A
  • Need thiamin for BCAA catabolism
  • If you do not have thiamin have a build up of BCAA and Keto acids and would not be able to break them down to get acetyl coA or succinyl CoA
  • BCAA deaminated to form ketoacids
  • Ketoacids then go through TPP dependent decarboxylation by alpha keto acid decarboxylase to form succinyl CoA and acetyl CoA
  • **TPP needed to decarboxylate deaminated BCAA to get energy from amino acids
18
Q

Explain how thiamin is involved in branched-chain alpha-keto acid dehydrogenase

A
  • BCAA convertd to keto acids
  • Keto acids enter into mitochondrial BCKD complex form CoA complexes using CoA-SH and NAD
  • Need thiamin for CoA-SH and NAD
  • This enzyme complex catalyzes the oxidative decarboxylation of branched, short-chain alpha-ketoacids
19
Q

Explain how thiamin is involved in the pentose phosphate pathway

A
  • one type of sugar is converted to a different type of sugar
  • Glucose converted to G6P
  • G6P enters PPP to form Ribose-5-Phosphate and NADPH
  • Ribose-5-Phosphate can be used for nucleic acids, complex sugars, and coenzymes
  • NADPH for coenzymes, steroids, fatty acids, amino acids, neurotransmitters, and glutathione
20
Q

What is the function of transketolase?

A
  • TPP involved with Transketolase
  • Causes conversion of xylose-5-phosphate to G3P and another byproduct
  • This by product can be converted into ribose-5-P and erythrose-4-P which can be furthur converted
  • A way of getting the sugars we need in our cells
  • TPP is the way in which the enzyme moves the carbons around!
  • Has a role in metabolism and a clinical function which should be monitored
21
Q

Summarize the metabolic role of thiamin

A
  • TPP Transketolase in the PPP to convert Ribose 5 phosphate to G3P
  • Conversion to pyruvate
  • Pyruvate converted to acetyl-CoA by TPP PDC
  • TPP alphaketoglutarate dehydrogenase in the CAC
  • BCAA broken down by TPP BCKDH to Branched cain acetyl CoA to be used in CAC
22
Q

What will occur to phytanic acid levels when there is a B1 deficiency?

A
  • No thiamin can have a build up of phytanic acid which can cause neurological disorders
  • Alpha oxidation of phytanic acid does not occur and leads to Refsum’s disease (nerves don’t work properly, neurodegenerative disease)
  • Genetic enzyme (phytanoyl-CoA hydrolase) disorder leads to build up of phytanic acid
23
Q

How is thiamin absorbed?

A
  • TPP and TMP in diet broken down to thiamin by pyrophosphatases
  • Thiamin in low levels will be transported into enterocyte by SLC19A2/3 (mainly through this way)
  • Thiamin in high levels >5mg/day diffuse directly into enterocyte
  • Once in cell converted to active TPP/TMP by phosphorylation via TPK (thiamin pyrophosphokinase)
  • Thiamin goes into portal circulation to liver where conversion can also occur. What it doesn’t need will go to muscle where most of it is stored
24
Q

What is metabolic trapping?

A
  • To keep nutrient within the cell will add phosphate charges and make thiamin bigger and more charged so it isn’t easily transported out
  • In plasma: free thiamin and TMP
  • In cellular compartments: TPP
25
Q

Where is most of the thiamin stored in our body?

A
  • Only small amount (~30mg) stored in body
    → 80% as TPP (metabolically trapped form), 10% TTP, rest as thiamin and TMP
  • Mostly in skeletal muscle (50% body thiamin), as well as liver, kidney, and nervous tissues
  • High metabolic rate + low storage amounts = daily intake necessary. See deficiency in weeks
26
Q

What form is thiamin in in our food?

A
  • Main form are coenzyme/cofactor form of TPP/TMP
  • Converted to thiamin in the body for transport
27
Q

How is thiamin excreted?

A
  • Reabsorbed in renal brush border membrane via saturable carrier-mediated transport system
  • Thiamin consumed in excess of tissue needs is rapidy excreted (water soluble)
  • Several metabolites arise from the action of gut microflora (not an important producer of thiamin for our bodies
  • Good at maintaining levels when we need it. Reabsorb thiamin by kidneys back into circulation
28
Q

What are the food sources of thiamin?

A
  • Thiamin synthesized by plants and some micro-organisms (not animals)
  • Humans require dietary thiamin - we do not make it ourselves
  • Plant sources:
    → Unrefined/whole grains, nuts, legumes
    → Enriched flour, grains, bread, cornmeal
  • Animal sources:
    →Mainly phosphorylated forms (because it is in our muscle)
    → Organ meat and (lean) pork
    → Milk and eggs
29
Q

How is thiamin reduced/destroyed in food?

A
  • Boiling/blanching reduces them
  • Thiamin can be destroyed by:
    → Baking soda
    → UV
    → Thiaminases (enzymes that breakdown thiamin) in raw freshwater fish, shellfish, and ferns
    → Heat-stable antagonist (caffeic acid and tannic acid; found in coffee, tea, Brussels sprouts, rice bran)
    → Sulfite (high in processed food - preservative)
30
Q

What are thiamin requirements altered by?

A
  • Related to total energy intake → involved in metabolism of CHO, lipids, and proteins
  • May be elevated in those with high caloric requirements. E.g. patients on dialysis, long-term IV-feeding and chronic infection
  • Requirements can be altered by:
    → Thiaminases
    → Thiamin antagonists
    → Vitamin C: may counteract anti-thiamin activity of tannic acid
31
Q

What is the thiamin recommendation?

A
  • 0.4mg/1000kcal a day (for 2000+ kcal diets) but no less than 1.0mg
  • No UL set due to lack of reported adverse effects
32
Q

How can you determine thiamin status?

A
  • Urinary excretion
  • Blood pyruvic acid or alpha-ketoglutarate
    → Increase in thiamin deficiency
    → Problem = not sensitive enough to ID early deficiency and is non-specific
    → %TPP effect (AC)
33
Q

What is the TPP effect?

A
  • Transketolase activity (blood or erythrocytes)
  • TPP effect: if activity increases with added TPP, blood is not saturated with TTP
  • Values: %TPP effect (AC):
    → Adequate: <15% (1-1.15)
    → Marginal: 16-20% (1.16-1.20)
    → Deficient: >20% (>1.20)

Activity coefficient (AC): +TPP/-TPP
% Stimulation = (AC*100)-100

34
Q

What are the populations at risk of thiamin deficiency?

A
  • Low intake
    → Developing world where polished rice is main staple (getting a lot of calories but not the main staple)
    → Elderly living at home (tea and toast)
    → Children who eat mostly snack food and have unbalanced diets
  • GI disorders
  • Periods where there is increased CHO metabolism (pregnancy, lactation, growth)
  • Genetic disorders (i.e. enzyme systems)
35
Q

What are the clinical signs of thiamin deficiency?

A
  • Decreased urinary excretion of thiamin (transporter in kidneys will try to retain as much as they can during starvation)
  • Decreased erythrocyte transketolase activity
  • Usually occurs along with deficiencies of other B vitamins
36
Q

In marginal and severe thiamin deficiency what will you see?

A
  • Marginal: see symptoms after 21-28 days of dietary inadequacy
    → tired, irritable, depression, weight loss
    → Loss of appetite, sometimes nausea and vomiting
  • Severe = Beriberi
37
Q

What are the symptoms of wet beriberi?

A
  • Cardiovascular effects
  • Lactic acidosis
  • Accumulation of lactic acid → edema → cardiac output increased → myocardial exhaustion
  • Results from severe physical exertion + high CHO intake and chronic thiamin deficiency
  • Have enough calories coming in and doing a lot of physical work but not getting enough micronutrients so do not have cofactor to break down pyruvate and build up lactate
38
Q

What are the symptoms of dry beriberi?

A
  • Neuritic (neurological disease)
  • Peripheral neuropathy; tingling, numbness, weakness
  • Myelin degeneration
  • Due to inactivity + caloric restriction and chronic thiamin deficiency
  • No calories coming in; starvation situation so no thiamin and main symptoms are neurological - mental issues
39
Q

Alcoholism can ead to what two things?

A
  • Wernicke and korsakoff
40
Q

What is wernicke?

A
  • similar symptoms to dry beri beri
  • Neurological disorder
  • Diplopia (double vision), nystagmus (rapid eye movement), ataxia (uncoordinated gait)
41
Q

What is Korsakoff?

A
  • Deficiency of thiamin has similar symptoms to dry beri beri
  • Psychosis/abnormal mental state
  • Memory loss, inability to learn new information, confusion
42
Q

What has research found about thiamin?

A
  • Diabetes might be a state of “relative” thiamin deficiency
    → current trials to see if thiamin or benfotiamine (lipophilic derivative) may counteract the damaging effects of hyperglycemia on vascular cells
  • Reports of severe thiamin deficiency after surgery for obesity = “bariatric beriberi”
    → frequent complication is Wernicke encephalopathy
    → Thiamin deficiency induced by → food intake and repeated vomiting

NUTR 302 (10 decks)

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