Free fatty acids enhance the oxidation of oxymyoglobin and inhibit the peroxidase activity of metmyoglobin

Abstract

The effect of low concentrations of sodium oleate on the oxidation of oxymyoglobin to metmyoglobin has been examined. This long chain fatty acid results in a tripling of the initial rate (1.5–4.3 h−1) at which oxymyoglobin is converted to metmyoglobin and more than doubling of the rate of the long-term reaction (0.12–0.33 h−1). Examination of rate constant enhancement over a range of oleate concentrations (0–0.215 mM) has allowed an estimate of association constants for both phases of the reaction system. The peroxidase activity expressed by metmyoglobin towards hydrogen peroxide is inhibited by the presence of sodium oleate by a fivefold increase in the apparent Km value (0.33–1.77 mM). The observed changes in oxymyoglobin concentration over time are discussed in terms of competition between metmyoglobin, which acts as a peroxidase decreasing in situ concentrations of H2O2, and oxymyoglobin, which also is oxidized by the peroxide. It is shown that oleate can bind to metmyoglobin and azidometmyoglobin, but not oxymyoglobin. Catalase reduces the oxidation rates of oxymyoglobin in the presence or in the absence of oleate, substantiating the involvement of H2O2. The results are discussed in relation to the potential increase in tissue peroxidations in the presence of ischaemically elevated fatty acid concentrations.Key words: myoglobin, fatty acid, peroxidation, autoxidation, peroxidase.