MYOGLOBIN CATALYSIS IN LIPID OXIDATION - ASSAY FOR ACTIVITY WITH LINOLEIC-ACID AS SUBSTRATE

An assay for the relative catalytic effect of myoglobin derivatives on lipid oxidation has been developed for use in prediction of oxidative stability of processed meat and meat products, combining electrochemical determination of O2 consumption and spectral analysis of oxidation products. Oxidation of linoleic acid solubilized (Tween 20) in air-saturated aqueous phosphate buffer is initiated by the myoglobin derivative, and the initial O2 consumption rate is determined by a Clark electrode, simultaneously with spectral determination at 280 nm of formation of ketone dienes and spectral determination at 410 nm (Soret Band) of myoglobin degradation. The assay has been optimized with respect to: (i) concentration of Tween 20 (oxidation rate increases linearly with concentration); (ii) solution pH (oxidation rate decreases with increasing pH for 5.3 < pH < 7.6); (iii) temperature (02 consumption rate increases in the temperature range 8-25-degrees-C and decreases at higher temperatures); and (iv) myoglobin concentration (maximal oxidation rate at a myoglobin concentration of approximately 10(-5) mol . L-1. Using the optimized assay, it was shown that: (i) metmyoglobin (MMb) is a more effective catalyst than ferrylmyoglobin (in the presence of catalase), which in tum is more effective than oxymyoglobin (MbO2); (ii) the presence of catalase or superoxide dismutase reduces the rate of MMb- and MbO2-catalysed linoleic acid oxidation; (iii) short-term heat denaturation of MMb reduces the catalytic activity, whereas longer heat treatment (70-degrees-C for more than 10 min or 60-degrees-C for more than 2 h) increases the catalytic activity by approximately 40%; (iv) exposure of MMb to UV light hardly changes the catalytic effect up to a accumulated exposure of 35 einstein . mol-1 (monochromatic light at 254 nm), which results in an almost complete loss of catalytic activity.