HYDROXYL RADICAL MODIFICATION OF FISH MUSCLE PROTEINS

A xanthine oxidase-based system was used to generate hydroxyl free rad icals in washed, minced cod muscle. Oxidation of protein was measured by increase in protein carbonyl content; the system used produced appr oximately 0.1 mol of carbonyl groups per 10(5) g of protein. This degr ee of oxidation had only minor effects on the SDS-PAGE patterns of the muscle proteins. The solubility of the proteins was not affected by t his amount of oxidation unless they were also subjected to a freeze/th aw cycle. With a freeze/thaw cycle, a highly significant decrease in p rotein solubility occurred compared to that which took place in a samp le not exposed to the free radical system. Lowering the pH from 6.8 or 6.5 to 6.0 or 5.5 had a strong negative impact on protein solubility. Protein oxidation appeared in two phases in washed cod mince, an init ial rapid increase followed by a second phase that may have been linke d to oxidation of the small amount of lipid in the sample. Comparison of protein carbonyl formation in stored mackerel fillets or mince indi cated that the range of oxidation studied in the cod model system was similar to what occurs in stored mackerel muscle postmortem.