Perhydroxyl radical (HOO.) initiated lipid peroxidation. The role of fatty acid hydroperoxides.

It is demonstrated that the perhydroxyl radical (HOO., the conjugate acid of superoxide (O2-], initiates fatty acid peroxidation (a model for biological lipid peroxidation) by two parallel pathways: fatty acid hydroperoxide (LOOH)-independent and LOOH-dependent. Previous workers (Gebicki, J. M., and Bielski, B. H. J. (1981) J. Am. Chem. Soc. 103, 7020-7025) demonstrated that HOO., generated by pulse radiolysis, initiates peroxidation in ethanol/water fatty acid dispersions by abstraction of the bis-allylic hydrogen atom from a polyunsaturated fatty acid. Addition of O2 to the fatty acid radicals forms peroxyl radicals (LOO.s), the chain-propagating species of lipid peroxidation. In this work it is demonstrated that HOO., generated either chemically (KO2) or enzymatically (xanthine oxidase), is a good initiator of fatty acid peroxidation in linoleic acid ethanol/water dispersions; O2- serves only as the source of HOO., and HOO. initiation can be observed at physiologically relevant pH values. In contrast to the previous results, the initiating effectiveness of HOO. is related directly to the initial concentrations of LOOHs in the lipids to be peroxidized. This defines a LOOH-dependent mechanism for fatty acid peroxidation initiation by HOO., which parallels the previously established LOOH-independent pathway. Since the LOOH-dependent pathway is much more facile than the LOOH-independent pathway, LOOH is the kinetically preferred site of HOO. attack in these systems. Experiments comparing HOO./LOOH-dependent fatty acid peroxidation with transition metal- and peroxyl radical-initiated peroxidation rule out the participation of the latter two species as initiators, which defines the HOO./LOOH initiation system as mechanistically unique. LOOH product studies are consistent with either a direct or indirect hydrogen atom transfer between LOOH and HOO. to yield LOO.s, which propagate peroxidation. The LOOH-dependent pathway of HOO.-initiated fatty acid peroxidation may be relevant to mechanisms of lipid peroxidation initiation in vivo.