Vitamin B6: Biochemical Overview

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Table of Contents

Introduction
Vitamin B6: Overview
Conclusion
References

Introduction

A vitamin that naturally occurs in many types of food, Vitamin B6 is a collective name for six different vitamers, all with vitamin 6 activity¹. Though it may be not as commonly known as other vitamins, it still is essential for the proper functioning of human body. In order to maintain the proper percentage of Vitamin B6 in the patient’s body, it is imperative that the dietary allowances of the vitamin should be in direct proportion to the patient’s age; more to the point, pregnant women will need a greater quantity of the vitamin than other patients, which means that the emphasis must be put on fruits (banaa), vegetables, fish (tuna) and cereals.

Vitamin B6: Overview

The dietary reference intakes (DRIs) established by the FNB are the recommended reference values for the intake of the vitamin¹. The recommended dietary allowance (RDA) is the most important DRIs¹. For infants aged between 0 and 6 months, the RDAs are set at 0.1mg for both genders¹. Between 7 and 12 months, infants should be given 0.3mg while those aged 1 to 3 years should be given 0.5 mg¹. For children aged 4 to 8 and 9 to 13, the RDA for both genders are 0.6mg and 1.0 mg respectively¹.

Between 14 and 18 years, the RDAs for males is 1.3 while that for females is 1.2 mg. Between 19 and 50 years, both genders’ RDA is 1.3mg while that of people aged 50 and above is 1.7 mg (males) and 1.5 mg (females)¹. Pregnant and lactating females aged between 14 and 50, the RDA is 1.9 mg and 2.0 mg respectively¹.

This biosynthetic route is well defined in E. coli. In this process, the substrate 3-hydroxyl-1-aminoacetone phosphate undergoes a four step synthetic pathway from D-erythrose 4-phosphate². In the first step, the compound is oxidated to D-erythronate 4-phosphate by its respective dehydrogenase enzyme (GapB), which uses NAD redox factor².

In the second step, the D-erythronoate 4-phosphte undergoes a process of oxidation under the action of its respective enzyme PdxB and the NAD cofactor to produce (3R)-3-hydroxy-2-oxo-4-phosphonooxybutanoate². Then, the enzyme PdxF catalyzes the PLP-dependent transmission from the product of the second oxidation to glutamate, which results into the formation of 4-hydroxy-L-threonine phosphate (HTP)².

Then, HTP undergoes an oxidative decarboxylation under the influence of enzyme PdxA to form 3-hydroxy-1-aminocetne phosphate. It is believed that the process takes place in two steps². First, alcohol undergoes a nicotinamide-dependent oxidation at the α-position. Secondly, the resulting α-ketoacid is decarboxylated to form the 3-hydroxy-1-aminocetne phosphate².

Then, the unstable 3-hydroxy-1-aminocetne phosphate is changed to PNP by PNP synthase. The enzyme PNP oxidase oxidizes PNP to form PLP, the active form of vitamin B². In this pathway, glutamine is first hydrolyzed into ammonia under the catalytic action of Pdx2 using the Glu-His-Cys triad of catalysis. The formed ammonia diffuses to the active site of Pdx1 through the hydrophobic channel². Here, PLP is synthesized from glyceraldehyde 3-phosphate and D-ribose 5-phosphate, ending the process.

Two pathways, A and B, are involved³. Pathway A has 8 steps for degrading pyridoxine. First, pyridoxine is oxidized to pyridoxal by enzyme pyridoxine 4’-oxidase². NAD-dependent pyridoxal dehydrogenase enzyme oxidizes pyridoxal to produce 4-pyridoxolactone, which also hydrolyzed². This product undergoes oxidation to form 2-methyl-3-hydroxypyridine-4-carboxylic acid in two steps, the first of which is catalyzed by FAD dependent pyridoxic acid 4-dehydrogenase enzyme and he second by a NAD depedent-5-formaly-3-hydroxyl-2-methylpyridine-4-carboxylic acid dehydrogenase².

Then, the acid is decarboxylated to form 2-methyl-3-hydroxypyridine-5-carboxylic acid under the catalytic action of an enzyme that depends on the presence of magnesium ions². The product then undergoes an oxidative opening of the ring (2-(N-acetamidomethylese) succinic acid). A FAD dependent 2-methyl-3-hydroxypyridine-5-carboxylic acid oxygenase catalyzes the succinic acid to form acetate, carbon dioxide, ammonia and succinic semialdehyde, which completes the pathway².

On the other hand, the B pathway has only five steps. First, pyridoxine is oxidized under the catalytic action of FAD-dependent pyridoxine-5-dehydrogenase enzyme to form isopyridoxal, which is then oxidized to form 5-pyridoxolactone. This product is further oxidized to form 5-pyridoxic acid under the enzyme 5’-lactonase action². The 5-pyridoxic acid undergoes a process of oxidative ring opening under the catalytic action of FAD dependent 5-pyridoxic acid oxegenase, resulting into 2-hydroxymethyl-(N-acetomidomethylene) succinic acid. The final step involves the hydrolysis of the succinct acid to form acetate, carbon dioxide and ammonia².

Conclusion

Because of the need to maintain the rates of the Vitamin B6 in the patient’s body in accordance with the patient’s age and gender (the older the patient is, the greater amount of the vitamin must be consumed; pregnant women must be provided with 2.0 mg as opposed to the rest of the patients (1.9 maximum)), it is crucial that the patient’s diet should include vegetables, fruits (specifically bananas), fish (especially tuna and salmon) and cereals.

As the vitamin in question facilitates the biosynthesis of several essential neurotransmitters, including adrenalin, noradrenalin, gamma-aminobutyric acid GABA, etc., its rates in the human body must be maintained at a constant level, which is calculated based on the patient’s age. With the choice of a diet based on vegetables, fish, chicken meat and fruit, the patient will be able to regain the proper rates of Vitamin B6 within a relatively short amount of time.

References

Vitamin B6: Dietary supplement fact sheet. The National Institutes of Health. 2014. Web.
Mukherjee T, Hanes J, Tews I, Ealick S E, Begley T P. Pyridoxal phosphate: Biosynthesis and catabolism. Biochimica et Biophysica Acta 2011; 1814(11): 1585–1596.
Salvo M L d, Contestabile R, Safo M K. Pyridoxal phosphate: Biosynthesis and catabolism. Biochimica et Biophysica Acta 2011; 1814(11): 1597–1608.

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Vitamin B6: Biochemical Overview

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