On the oxidation of cytochrome c by hypohalous acids

Oxidation of cytochrome c, a key protein in mitochondrial electron transpor t and a mediator of apoptotic cell death, by reactive halogen species (HOX, X-2), i.e., metabolites of activated neutrophils, was investigated by stop ped-flow. The fast initial reactions between Fe(III)cytc and HOX species, w ith rate constants (at pH 7.6) of k > 3 x 10(6) M-1 s(-1) for HOBr, k > 3 x 10(5) M-1 s(-1) for HOCl, and k = (6.1 +/- 0.3) x 10(2) M-1 s(-1) for HOI, are followed by slower intramolecular processes. All HOX species lead to a blue shift of the Soret absorption band and loss of the 695-nm absorption band, which is an indicator for the intact iron to Met-80 bond, and of the reducibility of Fe(III)cytc. All HOX species do, in fact, persistently impa ir the ability of Fe(III)cytc to act as electron acceptor, e.g., in reactio n with ascorbate or O-2(.-). I-2 selectively oxidizes the iron center of Fe (III)cytc, with a stoichiometry of 2 per I-2, and with k(Fe(II)cytc + I-2) approximate to 4.6 x 10(4) M-1 s(-1) and k(Fe(II)cyte + I-2(.-)) = (2.9 +/- 0.4) x 10(8) M-1 s(-1). Oxidation of Fe(II)cytc by HOX species is not sele ctively directed toward the iron center; HOBr and HOCl are considered to re act primarily by N-halogenation of side chain amino groups, and HOI mainly by sulfoxidation. There is some evidence for the generation of HO. radicals upon reaction of HOCl with Fe(II)cytc. Chloramines (e.g., NH2Cl), bromamin e (NH2Br), and cyclo-Gly(2) chloramide oxidize Fe(II)cytc slowly and unsele ctively, but iodide efficiently catalyzes reactions of these N-halogens to yield fast selective oxidation of the iron center; this is due to generatio n of I-2 by reaction of I- with the N-halogen and recycling of I- by reacti on of I-2 with Fe(II)cytc. Iodide also catalyzes methionine sulfoxidation a nd thiol oxidation by NH2Cl. The possible biological relevance of these fin dings is discussed. (C) 2001 Academic Press.