The first naturally occurring form of vitamin B-6 was isolated in 1938. It has the structure, confirmed by chemical synthesis (1939), of 3-hydroxy-4,5-bis(hydroxymethyl)-2-methylpyridine (I; R = -CH2OH). The trivial name "pyridoxine", proposed for this compound by P. György, came into general use as a synonym for "vitamin B-6". Two other natural compounds possessing vitamin B-6 activity detected in 1944 and recognized as the aldehyde, or 4-formyl analogue (I; R = -CHO) of pyridoxine, and the corresponding amine, or 4-aminomethyl analogue (I; R = -CH2NH2), were designated "pyridoxal" and "pyridoxamine" respectively.
Note. The systematic name for I; R = -CH2OH, by IUPAC Organic Rule C-204.1, is 4,5-bis(hydroxymethyl)-2-methyl-3-pyridinol.
Within the next few years, I. C. Gunsalus, E. E. Snell, A. E. Braunstein and others demonstrated that a phosphoric derivative of pyridoxal, later identified as pyridoxal 5'-phosphate (II; R = -CHO), is the coenzyme of a large group of specific enzymes catalysing reactions of amino-group transfer, decarboxylation and other metabolic transformations of individual amino acids. In the course of enzymic transamination, pyridoxal 5'-phosphate undergoes reversible conversion into pyridoxamine 5'-phosphate (II; R = -CH2NH2), which has coenzyme activity for the aminotransferases (EC 2.6.1.-), but not for other types of vitamin B-6-dependent enzymes