OXIDATIVE DEAMINATION OF AMINO-ACIDS BY MOLECULAR-OXYGEN WITH PYRIDOXAL DERIVATIVES AND METAL-IONS AS CATALYSTS

The oxidative deamination of (p-sulfophenyl)glycine by Cu2+ and vitamin B6 coenzymes pyridoxal 5'-phosphate or 5'-deoxypyridoxal to give (p-sulfophenyl)glyoxylic acid is described. The reaction conditions for kinetic measurements and catalysis were selected on the basis of previous equilibrium measurements of complex species present in these reaction systems. The reaction occurs in two modes, stoichiometric and catalytic. The stoichiometric route involves the reaction of the amino acid Schiff base of pyridoxal 5'-phosphate or 5'-deoxypyridoxal to form the oxime of the coenzyme and the keto acid. The stoichiometric reactions, which occur moderately rapidly at room temperature in aqueous solution, become part of an overall catalytic system in which the amino acid and dioxygen are converted to keto acid and ammonia in the presence of low concentrations of coenzyme and metal ion. Spectrophotometric studies demonstrate the relatively rapid conversion of the Schiff base metal chelate to the oxime complex, which is slowly converted to additional Schiff base complex by reaction with additional amino acid. Hydroxylamine, which is presumably formed in the displacement reaction, is converted to ammonia under the reaction conditions employed. The concentration of the intermediate oxime complex remains relatively constant when the reaction is run in the catalytic mode. Conversion to the final products was followed by isolation of the ammonia and keto acid formed, and by the quantitative estimation of ammonia. A dioxygen- 18 tracer study demonstrated that dioxygen is the source of the oxime oxygen, while the oxygen of the keto acid comes from the solvent. Mechanisms previously proposed for vitamin B6 and metal ion catalyzed amino acid oxidation are discussed, and a selection is made on the basis of O-18(2) labeling of the hydroxylation product.