Inactivation of key metabolic enzymes by mixed-function oxidation reactions: Possible implication in protein turnover and ageing

Abstract

Several mixed-function oxidation systems catalyze the inactivation of Escherichia coli glutamine synthetase. Inactivation involves modification of a single histidine residue in each enzyme subunit and makes the enzyme susceptible to proteolytic degradation. We show here that 10 key enzymes in metabolism are inactivated by a bacterial NADH oxidase and by an oxidase system comprised of NADPH, cytochrome P-450 reductase, and cytochrome P-450 isozyme 2 from rabbit liver microsomes. Most of the inactivatable enzymes require a divalent cation for activity and all but one (enolase) possess a nucleotide binding site. Glutamine synthetase, pyruvate kinase, and phosphoglycerate kinase are protected from inactivation by their substrates; substrate protection of other enzymes was not tested. We propose that inactivation involves mixed-function oxidization system-catalyzed synthesis of H 2 O 2 and reduction of Fe(III) to Fe(II) followed by oxidation of enzyme-bound Fe(II) by H 2 O 2 to generate oxygen radicals that attack a histidine (or other oxidizable amino acid) at the metal binding site of the enzyme. This is supported by the following: ( i ) most of the inactivation reactions are inhibited by EDTA and by catalase, ( ii ) both mixed-function oxidation systems reduce Fe(III), and ( iii ) H 2 O 2 together with Fe(II) catalyzes nonenzymic inactivation of glutamine synthetase. In view of the fact that inactivation of glutamine synthetase makes it susceptible to proteolytic degradation, it is possible that mixed-function oxidation system-catalyzed inactivation of enzymes is a regulatory step in enzyme turn-over. In addition, the implication of oxidative inactivation reactions in ageing is suggested by the fact that many of the enzymes inactivated by mixed-function oxidation systems are known to accumulate as inactive forms during ageing.