B6 Pyridoxine

Question 1

B6 is required by Enzymes that catalyze which type of reaction?

Answer 1

Transamination

PLP is associated with >100 enzymes the majority of which are involved in amino acid metabolism

reactions involving enzymes catalyzed by PLP include

• transaminiation

Question 2

Match the micro nutrients to their biological function

B6
Folic Acid
B12

Purine and pyrimidine synthesis
Transamination
conversion of methymalonyl CoA to a Kreb’s cycle intermediate

Answer 2

B6 - transaminiation
Folate - purine and pyrimidine synthesis
B12 - conversion of methymalonyl CoA to a Kreb’s cycle intermediate (succinyl CoA)

Question 3

What B-vitamin deficiency is associated with microcytic anemia?

Answer 3

B6

B6 is required as a coenzyme in the first step in the synthesis of heme

A hypochromic microcytic anemia may also result from vit B6 deficiency due to impaired heme synthesis

Question 4

Name a functional test used for assessing vitamin B6.

Answer 4

xanthurenic acid excretion following tryptophan loading

Question 5

The coenzyme form of B6 (PLP) is involved in which metabolic reactions?

Answer 5

1. amino acid metabolism
   - cysteine is synthesized from methionine
   - glycine synthesis
   - serine synthesis
2. decreasing effect of steriod hormones
3. gene expression nucleic acid synthesis
4. convertion of tryptophan to niacin
5. heme synthesis
6. synthesis of neurotransmitters
   - GABA
   - dopamine
   - serotonin

Question 6

Name good food sources of vitamins B6

Answer 6

pyridoxine found in plant foods
pyridoxal phosphate and pyridoxamine phosphate found in animal products

good sources:
meat whole grain products
vegetables
bananas
nuts
fortified cereals

Question 7

List the active forms of vitamin B6.

Answer 7

vitamin B6 exists as serveral vitamers which are interchangable and comparably active

pyridoxine
pyridoxal
pyridoxamine

and each has a 5’-phosphate deriviative

PLP pyridoxal 5’ phosphate is the most important in human metabolism

Question 8

Name all of the B vitamins that are involved in homocysteine metabolism.

Answer 8

folate (as THF)
B12
B6 (cystathionine synthase is PLP dependent)

betaine
(also called trimethyl glycine. Folic acid supplementation appears to increase betaine concentrations and betaine appears to be able to reduce plasma homocysteine concentrations in those with elevated blood levels

Question 9

What are the 6 vitamers of vitamin B6?

Answer 9

pyridoxine PN (alcohol form)
pyridoxal PL (aldehyde form)
pyridoxamine PM (amine form)

each also has a 5’-phosphate derivatives

all forms are interchangeable and comparably active

Question 10

What form of B6 is the coenzyme for >100 enzymes?

Answer 10

PLP (pyroxidine 5’phosphate)

Question 11

What types of food processing can damage vitamin B6?

Answer 11

prolonged heating (especially sterilization and canning)
and
milling and refining of grains
and
prolonged storage

can result in loss of vitamin B6

B6 is fairly stable with cooking

Question 12

What B vitamin is required for the conversion of PNP and PMP to PLP in the liver?

Answer 12

Riboflavin

The oxidase that cataylzes the reaction to convert PNP or PMP to PLP is dependent on adequate riboflavin. This enzyme is found mainly in the liver and intestine.

Question 13

Where is vitamin B6 stored in the body?

Answer 13

Muscles store 75-80% (most as PLP bound to glycogen phosphorylase)

Liver stores 5-10%

Question 14

In its coenzyme role in reactions involving amino acids what role does PLP play?

Answer 14

PLP forms a Schiff base (product of an amino group from the amino acid and the aldehyde group of PLP.

This labilizes (destabilizes) the bonds around the amino acids alpha-carbon so further reactions can take place more easily.

Question 15

What two enzymes PLP as a cofactor for transamination reactions?

Answer 15

aspartic amino transferase (AST)

alanine amino transferase (ALT)

transamination - the transfer of an amino group (-NH2) from one amino acid to an alpha-keto acid. This is important for the synthesis of nonessential amino acids and so amino acid skeletons can be used for energy or glucose production.

Question 16

What are the key reactions that require PLP as a cofactor for amino acid metabolism?

Answer 16

1. transamination of amino acids
2. transsulfhydration (key reaction synthesis of cystathionine from homocysteine)
3. cleavage (key reaction hydroxymethyl group moved from serine to form 5-methyl THF which is needed to regenerate methionine from homocysteine)
4. decarboxylation - removal of carboxy (COO-) group as in the reaction forming GABA from glutamate
5. transelenation - selenomethionine may be converted by transelenation to selenocysteine

Question 17

What other reactions (non amino acid, non-synthetic) is B6 required for?

Answer 17

1. heme synthesis - vitamin B6 is required for the first step in the synthesis of heme
2. glycogen degradation - B6 is required for glycogen phosphorylase activity. This role is though to account for more than 50% of the body’s vitamin B6.
3. niacin synthesis from tryptophan
4. synthesis of carnitine
5. synthesis of taurine
6. gene expression - the vitamin has been shown to bind to DNA and may modulate steroid hormone or transcription factor binding to regulatory regions of DNA.

Question 18

B6 is required as a cofactor for what synthesis reactions?

Answer 18

1. heme synthesis - vitamin B6 is required for the first step in the synthesis of heme
2. niacin synthesis from tryptophan
3. synthesis of carnitine
4. synthesis of taurine
5. synthesis of histamine from histidine
6. synthesis of dopamine from tyrosine
7. synthesis of GABA from glutamate

Question 19

Why can B6 deficiency result in microcytic anemia?

Answer 19

Vitamin B6 is required for the first step in the synthesis of heme.

deficiency of B6 results in inadequate production of heme and microcytic hypochromatic anemia.

Question 20

What are signs and symptoms of B6 deficiency?

skin
GI
blood
neurological
other nutrients and substances

Answer 20

Vitamin B6 deficiency is relatively rare in the US.

Skin
- seborrheic (oozing) rash on face, neck, shoulders, buttocks

Blood

• microcytic anemia
• weakness
• fatigue

GI tract

• cheilosis (sores on lips)
• angular stomatitis (sores in corners of mouth)
• glossitis (swollen tongue)

Neurological

• confusion
• peripheral neuropathy
• seizures and convulsions (especially in infants and children)

Other nutrients

• impairs niacin synthesis from tryptophan
• may inhibit metabolism of homocysteine

Question 21

What groups are at increased risk of B6 deficiency?

Answer 21

1. the elderly - may have poor intake of the vitamin
2. alcoholics
3. people taking certain drugs
   - birth control pills (oral contraceptives)
   - isoniazid used to treat tuberculosis
   - penicillamine used to treat some autoimmune diseases
   - corticosteriods
   - anticonvulsants

Question 22

What tests are used to assess B6 status?

Answer 22

1. plasma PLP concentrations - thought to be best indicator tissue stores of B6.
2. xanthuranic acid excretion following load of tyrptophan.
3. Erythrocyte transaminase index - these assays have limitations and are better used as an adjunct to other tests.