title: Myoglobin-H2O2 catalyzes the oxidation of β-ketoacids to α-dicarbonyls: mechanism and implications in ketosis.
abstract: Acetoacetate (AA) and 2-methylacetoacetate (MAA) are accumulated in metabolic disorders such as diabetes and isoleucinemia. Here we examine the mechanism of AA and MAA aerobic oxidation initiated by myoglobin (Mb)/H(2)O(2). We propose a chemiluminescent route involving a dioxetanone intermediate whose thermolysis yields triplet α-dicarbonyl species (methylglyoxal and diacetyl). The observed ultraweak chemiluminescence increased linearly on raising the concentration of either Mb (10-500 μM) or AA (10-100 mM). Oxygen uptake studies revealed that MAA is almost a 100-fold more reactive than AA. EPR spin-trapping studies with MNP/MAA revealed the intermediacy of an α-carbon-centered radical and acetyl radical. The latter radical, probably derived from triplet diacetyl, is totally suppressed by sorbate, a well-known quencher of triplet carbonyls. Furthermore, an EPR signal assignable to MNP-AA(•) adduct was observed and confirmed by isotope effects. Oxygen consumption and α-dicarbonyl yield were shown to be dependent on AA or MAA concentrations (1-50 mM) and on H(2)O(2) or tert-butOOH added to the Mb-containing reaction mixtures. That ferrylMb is involved in a peroxidase cycle acting on the substrates is suggested by the reaction pH profiles and immunospin-trapping experiments. The generation of radicals and triplet dicarbonyl products by Mb/H(2)O(2)/β-ketoacids may contribute to the adverse health effects of ketogenic unbalance.
authors: Departamento de Bioquímica, Instituto de Química, Universidade de São Paulo, São Paulo, SP, Brazil.
authors: Douglas
authors: D
authors: Ganini
authors: Marcelo
authors: M
authors: Christoff
authors: Marilyn
authors: M
authors: Ehrenshaft
authors: Maria B
authors: MB
authors: Kadiiska
authors: Ronald P
authors: RP
authors: Mason
authors: Etelvino J H
authors: EJ
authors: Bechara
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