Vitamin B6 in the form of pyridoxal phosphate (PLP) is a co-factor for over 60 enzymatic reactions involved in amino acid, carbohydrate and fatty acid metabolism. Pyridoxal phosphate functions in practically all reactions involved in amino acid metabolism, including transamination, decarboxylation, deamination and desulfhydration, as well as the hydrolysis and synthesis of amino acids.
Vitamin B6 participates in enzymatic functions (Marks, 1975; Driskell, 1984; Merrill and Burnham, 1990; Leklem, 1991) that include:
- Deaminases: serine, threonine and cystathionine.
- Desulfhydrases and transulfurases.
- Synthesis of niacin from tryptophan.
- Formation of alpha-aminolevulinic acid from succinyl CoA and glycine — the first step in porphyrin synthesis.
- Conversion of linoleic to arachidonic acid in the metabolism of essential fatty acids (a function also requiring biotin); this function is also related to brain sphingolipid production.
- Glycogen phosphorylase: catalyzes glycogen breakdown to glucose-1-phosphate. Pyridoxal phosphate appears not to be a coenzyme for this enzyme but rather to affect the enzyme conformation.
- Synthesis of epinephrine and norepinephrine from either phenylalanine or tyrosine; and therefore effects on neural function.
- Racemases: PLP-dependent racemases enable certain microorganisms to utilize D amino acids. Alanine racemase has been detected in mammalian tissues.
- Transmethylation reactions involving methionine.
- Incorporation of iron in hemoglobin synthesis, and therefore effects on erythrocyte function.
- Antibody response to antigens and lymphocyte formation.
Vitamin B6 is required by numerous microorganisms and appears to play a role in rumen metabolism. Vitamin B6 enhanced the in vitro production of phenylalanine from its precursors phenylpyruvic acid and phenylacetic acid in mixed rumen bacteria and protozoa (Ruhul and Onodera, 1998). The saccharolytic rumen bacterium, Butyrivibrio fibrisolvens, requires vitamin B6, along with biotin and folic acid (Baldwin and Allison, 1983).