Effect of high fat/high erucic acid diet on phosphatidate synthesis and phosphatidate phosphatase in the subcellular fractions of rat heart and liver

Rats of weaning age were fed for a period of 1,3 or 6 weeks either a control diet (laboratory stock diet) or a semisynthetic diet containing 20% by weight of either mustard seed oil (1/3 of the total fatty acids were comprised of erucic acid) or corn oil (2/3 of the total fatty acids consisted of linoleic acid). Mitochondrial and microsomal fractions were isolated from the hearts and livers of these rats, and the rate of acylation ofsn‐[U‐14C] glycerol 3‐phosphate (P) was examined using palmitoyl‐CoA or erucoyl‐CoA as the acyl donor. In addition, activities of phosphatidate phosphatase of the mitochondrial, microsomal and soluble fractions were assayed. Studies on the acylation of glycerol 3‐P with palmitoyl‐CoA demonstrated that feeding of the high fat/high erucic acid diet for 1,3 or 6 weeks significantly increased the rate of formation of monoacylglycerol 3‐P by the cardiac subcellular fractions as compared to the control. The rate of formation of diacylglycerol 3‐P also increased but to a lesser degree. Feeding the high fat/high linoleic acid diet tended to increase acylation of glycerol 3‐P by cardiac subcellular fractions. However, neither high fat diet influenced acyltransferase activities of the hepatic subcellular fractions or phosphatase activities of the cardiac and hepatic fractions. Studies on the acylation of glycerol 3‐P with erucoyl‐CoA demonstrated that the rate of acylation was ca. 1/10 that measured using palmitoyl‐CoA in all experiments; in particular, the formation of diacylglycerol 3‐P was extremely slow, suggesting that erucoyl‐CoA is an unsuitable substrate for the position‐2 of the monoacylglycerol 3‐P. The rate of acylation by the cardiac and hepatic subcellular fractions was not influenced by the feeding of the high‐fat diets. The rate of glycerol 3‐P acylation by both cardiac and hepatic mitochondrial fraction was ca. 2/3 of the rate of acylation by the respective microsomal fraction. In addition, the ratio of monoacyl‐to diacylglycerol 3‐P synthesized by the mitochondrial fraction was smaller than that by the microsomal fraction. These results suggest that acylation of glycerol 3‐P by the mitochondrial cannot be attributed to the action of the contaminating microsomal enzymes.