Myoglobin-H2O2 catalyzes the oxidation of beta-ketoacids to alpha-dicarbonyls: Mechanism and implications in ketosis
Kadiiska, Maria B.
Mason, Ronald P.
Bechara, Etelvino José Henriques [UNIFESP]
Is part ofFree Radical Biology and Medicine
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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)/H2O2. We propose a chemiluminescent route involving a dioxetanone intermediate whose thermolysis yields triplet alpha-dicarbonyl species (methylglyoxal and diacetyl). the observed ultraweak chemiluminescence increased linearly on raising the concentration of either Mb (10-500 mu M) 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 alpha-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(center dot) adduct was observed and confirmed by isotope effects. Oxygen consumption and a-dicarbonyl yield were shown to be dependent on AA or MAA concentrations (1-50 mM) and on H2O2 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/H2O2/beta-ketoacids may contribute to the adverse health effects of ketogenic unbalance. (C) 2011 Elsevier Inc. All rights reserved.
CitationFree Radical Biology and Medicine. New York: Elsevier B.V., v. 51, n. 3, p. 733-743, 2011.
SponsorshipFundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)
Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)
Instituto Nacional de Ciencia e Tecnologia (INCT) Redoxoma
National Institute of Environmental Health Sciences (NIEHS)
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