Apparent ionization constants, spectral properties and metal chelation of the cotransaminases and related compounds

Abstract

1. 1. The apparent ionization constants of pyridoxal, pyridoxine, pyridoxamine, pyridoxal 5-phosphate, and pyridoxamine 5-phosphate have been determined. 2. 2. The compounds exist as zwitterions in neutral solution. 3. 3. The acid strength of the phenolic hydroxyl group follows the order pyridoxamine > pyridoxamine phosphate > pyridoxal phosphate > pyridoxal > pyridoxine. 4. 4. The base strength of the heterocyclic nitrogen atom follows the inverse sequence pyridoxine > pyridoxal > pyridoxal phosphate > pyridoxamine phosphate > pyridoxamine. 5. 5. The effect of ionization of the phenolic hydroxyl group and ring nitrogen atom on the ultraviolet absorption spectra have been recorded systematically. 6. 6. Titration of pyridoxal in the presence of excess ethanolamine, ethylalanate, or ethylenediamine gave a new partial equivalent of acid consumed in the pH range 5.0–6.5. This new partial equivalent probably represents the titration of a Schiff base derivative of the amines with pyridoxal. 7. 7. Titration of pyridoxamine in the presence of various metals showed that only copper, of the series tested, efficiently chelated with the vitamin in neutral solution. 8. 8. The emerald-green copper chelate of pyridoxamine contains two moles of pyridoxamine per mole of copper. Both the phenolic and amino groups of pyridoxamine enter into chelate formation with the metal.