Dynamics of lipid oxidation and antioxidant depletion in Mediterranean fish stored at different temperatures

Numerous changes occur post-mortem in fish, affecting its chemical composition and nutritional quality. In the present paper we describe the effect of storage on ice or at -30 degrees C or -80 degrees C on 10 species of Mediterranean fish. Water and lipid soluble antioxidants, lipid pattern and products of oxidative attack on lipids, proteins and DNA were quantified for 7 consecutive days on homogenates of fish light muscle. The earliest events were oxidation of ubiquinol and vitamin C, which disappeared almost completely within 48 hours. Ubiquinol oxidation gave rise to an initial increase of ubiquinone, which peaked at the second day: thereafter ubiquinone itslef decreased, more rapidly and to a greater extent than vitamin E. The decrease in antioxidants was accompanied by significant oxidative damage to lipids, proteins and DNA. TBARS significantly increased beginning from the third day of storage in all species and were linked to a significant reduction in the n-3 PUFA of triglycerides (TG) and phospholipid fractions (PL). A remarkable elevation of protein carbonyls and 8OHdG occurred approximately 24 hours later than PUFA oxidation. For SOD, GPX and GSH significant depletions occurred for all species only at 6th or 7th day, but the final values were always higher than 50% compared to the initial ones. Deep-freezing of the same species at -30 degrees C and -80 degrees C for up to 12 months did not significantly affect the levels of enzymatic antioxidants, the redox couple GSH/GS-SG, n-3 and n-6 PUFA of TG and PL fractions of the light muscle. The only antioxidants, which at -30 degrees C and -80 degrees C appeared to be degraded after 6 and 12 months were ubiquinol and vitamin C. As expected their degradation was higher at -30 degrees C than at -80 degrees C. In fact the average decrease for ubiquinol at -80 degrees C was 42% and 70% at 6 and 12 months respectively, whereas at -30 degrees C the decrease was 61% and 87%. For vitamin C the average decrease at -80 degrees C was 36% and 67% at 6 and 12 months respectively, and at -30 degrees C it was 61% and 82%. Vitamin E was considerably more stable than ubiquinol and vitamin C. The relative stability of the antioxidants, with the exceptions of ubiquionols, vitamin C and, to a certain extent, vitamin E, was accompanied by a very limited increase in oxidation products. In addition no significant hydrolysis of TG and PL fractions were observed throughout the storage time. The dynamics of lipid, protein and DNA oxidation is discussed in the light of depletion of the various antioxidant systems.