Lipid Composition and Metabolism in Liver Mitochondria and Microsomes of Rats Fed a Low Protein Diet

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Male, Wistar rats were fed a diet low in protein (4% casein) or a control diet high in protein (25% casein) for 6 weeks. The effect of these diets on the incorporation of 32P into liver mitochondrial and microsomal fractions and on the fatty acid and phospholipid composition of these tissue components was then investigated. In rats fed the low casein diet, the specific activity (cpm/µg P) of mitochondrial and microsomal phosphatidylcholine and phosphatidylethanolamine was increased at 1.5 and 4 hours after 32P injection, with the largest increase in phosphatidylcholine at the earlier time interval. Phosphatidylcholine had a lower specific activity than phosphatidylethanolamine, a difference which was most evident in the microsomal fraction. The specific activity values of these major phospholipids were similarly reduced in mitochondrial and microsomal fractions when rats were fed the 25% casein diet. The ratio of the relative specific activity of phosphatidylethanolamine in mitochondria to that in microsomes was uniformly lower than that of phophatidylcholine and this difference was unaffected by diet. The proportions of major fatty acids of the mitochondrial and microsomal fractions resembled one another and were similarly altered in rats fed the low casein diet. These changes included a decrease in the proportions of arachidonic and stearic acids, and an increase in palmitic, oleic, and docosapentaenoic acids. A low intake of dietary protein also affected the proportions of mitochondrial and microsomal phospholipids. The principal changes in each organelle were an increase in the concentration of phosphatidylethanolamine and a decrease in that of phosphatidylcholine.