Seven forms (vitamers) of vitamin B6 are known: Pyridoxine (PN), the form most commonly given as vitamin B6 supplement Pyridoxine 5'-phosphate (PNP) Pyridoxal (PL) Pyridoxal 5'-phosphate (PLP), the metabolically active form (sold as 'P-5-P' vitamin supplement) Pyridoxamine (PM) Pyridoxamine 5'-phosphate (PMP) 4-Pyridoxic acid (PA), the catabolite which is excreted in the urine All forms except PA cannot be interconverted. Absorbed pyridoxamine is converted to PMP by pyridoxal kinase, which is further converted to PLP by pyridoxamine-phosphate transaminase or pyridoxine 5'-phosphate oxidase which also catalyzes the conversion of PNP to PLP. Pyridoxine 5'-phosphate oxidase is dependent on flavin mononucleotide (FMN) as a cofactor which is produced from riboflavin (vitamin B2) i.e. in this biochemical pathway, dietary vitamin B6 cannot be used without vitamin B2. Functions PLP, 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. PLP generally serves as a coenzyme (cofactor) for many reactions including decarboxylation, transamination, racemization, elimination, replacement, and beta-group interconversion. The liver is the site for vitamin B6 metabolism.