REACTIVITY OF METMYOGLOBIN TOWARDS PHOSPHOLIPID HYDROPEROXIDES

Ferrylmyoglobin, the high oxidation state of myoglobin analogous to co mpound II of peroxidases, promotes the peroxidation of palmitoyl-linol eyl-phosphatidylcholine (PLPC) large unilamellar vesicles. This was as sociated with oxygen consumption and a slow conversion of ferrylmyoglo bin to metmyoglobin. The time course of oxygen consumption was charact erized by the occurrence of a lag phase, which could be overcome by th e addition of sodium deoxycholate to the reaction mixture. The rate of conversion of ferrylmyoglobin to metmyoglobin was slower than that of oxygen consumption, and there was not stoichiometric correlation betw een both events. These findings suggest that the observed oxygen consu mption linked to lipid peroxidation is supported by a peroxidatic acti vity encompassed by the frrnylmyoglobin double left right arrow metmyo globin transition as well as free radical propagation reactions. Incub ation of metmyoglobin with PLPC vesicles containing 3% hydroperoxide r esulted in oxygen consumption, the time course of which was devoid of the lag phase observed with hydroperoxide-free unilamellar lipid vesic les. The incubation of metmyoglobin with peroxide-containing PLPC vesi cles or with equimolar amounts of lipid hydroperoxide was not associat ed with Soret or visible absorption spectral changes of metmyoglobin, which could be ascribed to its conversion to ferrylmyoglobin. Treatmen t of the metmyoglobin/lipid hydroperoxide mixtures with Na2S did not l ead to the formation of the sulfheme protein derivative, which can be considered as a fingerprint for the occurrence of ferrylmyoglobin. Des pite the lack of evidence for formation of the latter species, metmyog lobin catalyzed efficiently the conversion of palmitoyl-linoleyl-phosp hatidylcholine hydroperoxide (PLPCOOH) to various products, which have been tentatively identified as conjugated carbonyl compounds (with ab sorption maxima at 233 and 270 nm), and a set of lower molecular weigh t and/or higher polarity derivatives containing the conjugated diene m oiety (with differing intensity of absorption maxima at 233 nm). The p athophysiological implications for the phospholipid hydroperoxide/metm yoglobin interaction appear to broaden the range of oxidizable targets of this hemoprotein in a manner other than that involving the reactiv e oxoferryl complex of its high oxidation state.