The protection of long chain polyunsaturated fatty acids by melatonin during nonenzymatic lipid peroxidation of rat liver microsomes.

The in vivo and in vitro effects of melatonin (N-acetyl-5-methoxytryptamine) on lipid peroxidation of long chain polyunsaturated fatty acids (PUFA) located in rat liver microsomes were determined. The effect of intraperitoneal administration of melatonin (10 mg/kg weight/24 hr) on ascorbate-Fe++ induced lipid peroxidation of isolated rat liver microsomes was first examined. The ascorbate induced light emission in hepatic microsomes was inhibited by melatonin treatment [control group: 10.714 +/- 0.894; melatonin group: 3.162 +/- 0.515, counts per minute (cpm) x 10(-5)]. Significant differences in the content of arachidonic C20:4 n-6 and docosahexaenoic acid C22:6 n-3 were observed when control microsomes were lipid peroxidized in the presence of ascorbic acid. These changes were less pronounced in liver microsomes isolated from melatonin treated rats. In vitro assays showed that after incubation of rat liver microsomes in an ascorbate-Fe++ system, at 37 degrees C for 210 min, the total cpm originated from light emission (chemiluminescence) was found to be lower in those membranes incubated in the presence of melatonin. The fatty acid composition of total lipids isolated from rat liver microsomes was substantially modified when subjected to nonenzymatic lipid peroxidation with a considerable decrease of docosahexaenoic acid 22:6 n-3 and arachidonic acid 20:4 n-6. The inhibition of the lipid peroxidation process evaluated as chemiluminescence (total cpm at selected times) was melatonin concentration dependent. Melatonin, at a concentration 1.2 mm, inhibited almost completely the lipid peroxidation process. Arachidonic and docosahexaenoic acids were more affected than docosapentaenoic acid during the lipid peroxidation process. Not all fatty acids were equally protected after the addition of melatonin to the incubation medium. Our results indicate that melatonin may act in vivo and in vitro as an antioxidant protecting long chain PUFA present in rat liver microsomes from the deleterious effect by a selective mechanism that reduces the loss of docosahexaenoic and arachidonic acids.