Biochemistry - The Chemical Reactions of Living Cells, Volume 2 - D. Metzler 1980

Coenzymes - specialized natural reagents
Lipoic acid and oxidative decarboxylation of α-keto acids
Oxidative decarboxylation by hydrogen peroxide

The non-enzymatic Oxidative Decarboxylation of α-keto acids by hydrogen peroxide is a well-characterized reaction. The first step presumably involves The formation of an adduct—an organic peroxide—which decomposes according to equation (8-67):

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The transformation, which can be viewed as an enzymatic variant of this reaction, is catalyzed by lactate oxidase, a hexameric flavoprotein with a Molecular Weight of ~350,000 isolated from Mycobacterium. Under anaerobic conditions, the enzyme produces Pyruvate via simple dehydrogenation [142]. In the presence of oxygen, however, acetic acid is formed, with one of the oxygen atoms in the carboxyl group originating from O₂. Since hydrogen peroxide is a common product generated from oxygen by the action of Flavoproteins, it is possible that in the case of lactate oxidase, the generated hydrogen peroxide directly oxidizes pyruvate¹⁾ in accordance with equation (8-67).

Nevertheless, the precise MECHANISM OF ACTION of this enzyme remains elusive. Indeed, while the enzyme generates pyruvate in the absence of O₂, neither pyruvate- nor H₂O₂-trapping agents inhibit the reaction [143]. It is possible that H₂O₂ and pyruvate are generated in close proximity and react so rapidly that neither product can be intercepted. Alternatively, the formation of superoxide anion radicals instead of H₂O₂ cannot be ruled out [equation (8-60)]. Several other mechanisms have also been proposed [144, 145]. Certain other flavoprotein hydroxylases may operate in a similar fashion (Chapter 10, Section G.2).

¹⁾ α-Keto acids produced by the action of amino acid oxidases are likewise oxidized by the accumulating hydrogen peroxide. The addition of catalase (Chapter 10, Section B.6) results in the decomposition of H₂O₂ and the accumulation of keto acids.