Inhibition of enzymic and non-enzymic lipid peroxidation of flounder muscle sarcoplasmic reticulum by pretreatment with phospholipase A2.

Sarcoplasmic reticulum, isolated from the muscle of winter flounder (Pseudopleuronectes americanus), was preincubated with phospholipase A2 to determine the effects of enzymic and non-enzymic lipid-peroxidation systems. Eicosapentaenoic acid (20:5) was preferentially released by phospholipase A2, but the percentage of docosahexaenoic acid (22:6) in the free fatty acids was similar to the percentage in membrane total fatty acids. A decrease in the production of both thiobarbituric acid-reactive substances and lipid hydroperoxides was observed in both the enzymic and non-enzymic peroxidation systems upon preincubation with phospholipase A2. Addition of palmitic acid or lysophosphatidylcholine did not inhibit either peroxidation system. Non-enzymic peroxidation was inhibited by the addition of beta-glycerophosphate, but enzymic peroxidation was apparently unaffected. A model is proposed to explain the inhibitory effect of phospholipid hydrolysis on lipid peroxidation in both systems, suggesting that fatty acids are structurally realigned upon hydrolysis, leading to decreased free-radical chain propagation. Additional factors appear to contribute to inhibition of enzymic peroxidation.