Effect of dietary fat on lipids and enzyme activities of rat hepatic membranes

The effect of consumption of omega-3 fatty acids on membrane lipid composition and specific activities of some membrane-bound enzymes was investigated in rats fed a semisynthetic diet containing 14% by weight of either menhaden oil (MO), safflower oil (SO), or beef fat (BF). These diets also contained 2% corn oil to assure an adequate supply of essential fatty acids. The diets were fed to the experimental animals ab libitum for 2 or 7 weeks. The data obtained on lipid analysis indicated a significant increase in phospholipid: cholesterol ratio in membranes of animals fed the MO diet than those fed the SO diet, but was lower than that of animals fed the BF diet. These changes were due to the significantly higher phospholipid content in membranes of animals fed the MO diet when compared with the other diets, and to the significantly lower cholesterol content of animals fed the BF diet. These findings in lipid composition were observed after both 2 and 7 weeks of feeding the experimental diets. Significant effects from dietary MO were observed in the contents of two major phospholipids: phosphatidyl choline and phosphatidyl ethanolamine. No significant effect from the diet was observed in membrane phosphatidyl inositol content. There were significant differences in the fatty acid composition that were reflected on the unsaturation index (UI) of the membranes. Membranes of animals fed the MO diet had a 36% increase in membrane UI as compared with those fed either SO or BF. Hepatic alkaline phosphatase was 33% higher in membranes of animals fed the MO diet as compared to the other diets. No significant effect of MO feeding on hepatic (Na+, K+)-ATPase was observed. There was no significant difference between membrane glucose 6-phosphatase activity of animals fed MO and BF diets, which were, in turn, significantly higher than that of animals fed the SO diet. All the above enzyme activities were found to be uninfluenced by the length of the feeding period. The observed alterations in the lipid composition and specific activities of membrane-bound enzymes in addition to the increase in liver weight in animals fed the MO diet indicate significant effects on the liver and therefore warrant further studies on the effect of MO feeding on liver function.