Pyridoxal 5'-phosphate hydrate (PLP)(1:x), the active form of vitamin B6, is a cofactor for many different enzymes involved in transamination reactions, including mitochondrial cysteine desulfurase, cystathionine γ-synthase, various aminotransferases, and a d-serine dehydratase.

Pyridoxal phosphate (Vitamin B6 phosphate) is the active form of vitamin B6 and a coenzyme for many pyridoxal phosphate (PLP)-dependent enzymes. PLP is involved in numerous enzymatic transamination, decarboxylation and deamination reactions; it is necessary for the synthesis of amino acids and amino acid metabolites, and for the synthesis and/or catabolism of certain neurotransmitters, including the conversion of glutamate into gamma-aminobutyric acid (GABA) and levodopa into dopamine. PLP can be used as a dietary supplement in cases of vitamin B6 deficiency. Reduced levels of PLP in the brain can cause neurological dysfunction.

Quinaldic acid is an endogenous metabolite.Quinaldic acid is a metal chelator.