Pyridoxal phosphate, the metabolically active form of vitamin B₆, is involved in many aspects of macronutrient metabolism, neurotransmitter synthesis, histamine synthesis, hemoglobin synthesis and function and gene expression. Pyridoxal phosphate generally serves as a coenzyme for many reactions and can help facilitate decarboxylation, transamination, racemization, elimination, replacement and beta-group interconversion reactions.^[4] The liver is the site for vitamin B₆ metabolism.

Amino acid metabolism

Pyridoxal phosphate (PLP) is a cofactor in transaminases that can catabolize amino acids. PLP is also an essential component of two enzymes that convert methionine to cysteine via two reactions. Low vitamin B₆ status will result in decreased activity of these enzymes. PLP is also an essential cofactor for enzymes involved in the metabolism of selenomethionine to selenohomocysteine and then from selenohomocysteine to hydrogen selenide. Vitamin B₆ is also required for the conversion of tryptophan to niacin and low vitamin B₆ status will impair this conversion.^[4] PLP is also used to create physiologically active amines by decarboxylation of amino acids. Some notable examples of this include: histidine to histamine, tryptophan to serotonin, glutamate to gamma-aminobutyric acid (GABA), and dihydroxyphenylalanine to dopamine.

Gluconeogenesis

Vitamin B₆ also plays a role in gluconeogenesis. Pyridoxal phosphate can catalyze transamination reactions that are essential for the providing amino acids as a substrate for gluconeogenesis. Also, vitamin B₆ is a required coenzyme of glycogen phosphorylase,^[4] the enzyme necessary for glycogenolysis to occur.

Lipid metabolism

Vitamin B₆ is an essential component of enzymes that facilitate the biosynthesis of sphingolipids.^[4] Particularly, the synthesis of ceramide requires PLP. In this reaction serine is decarboxylated and combined with palmitoyl-CoA to form sphinganine which is combined with a fatty acyl CoA to form dihydroceramide. Dihydroceramide is then further desaturated to form ceramide. In addition, the breakdown of sphingolipids is also dependent on vitamin B₆ since S1P lyase, the enzyme responsible for breaking down sphingosine-1-phosphate, is also PLP dependent.

Metabolic functions

The primary role of vitamin B₆ is to act as a coenzyme to many other enzymes in the body that are involved predominantly in metabolism. This role is performed by the active form, pyridoxal phosphate. This active form is converted from the other natural forms founds in food: pyridoxal, pyridoxine and pyridoxamine.^[5]

Vitamin B₆ is involved in the following metabolic processes:

• amino acid, glucose and lipid metabolism
• neurotransmitter synthesis
• histamine synthesis
• hemoglobin synthesis and function
• gene expression

• Clinical assessment of vitamin B₆

  The biochemical assessment of vitamin B₆ status is essential, as the clinical signs and symptoms of its deficiency are very nonspecific.^[15] The three biochemical tests most widely used are the activation coefficient for the erythrocyte enzyme aspartate aminotransferase, plasma pyridoxal phosphate (PLP) concentrations, and the urinary excretion of vitamin B₆ degradation products, specifically urinary pyridoxic acid. Of these, plasma PLP is probably the best single measure, because it reflects tissue stores.^[16] When plasma pyridoxal phosphate is less than 10nmol/L, it is indicative of vitamin B₆ deficiency.^[17] Urinary 4-pyridoxic acid is also an indicator of vitamin B₆ deficiency; levels of less than 3.0 mmol/day is suggestive of vitamin B₆ deficiency.^[18]