Phosphatidic acid synthesis in the heart. 1. Effect of age and species difference in the mitochondrial and microsomal synthesis.

Rates of syntheses of monoacyl- and diacyl-glycerol 3-P (phosphate) were determined in the mitochondrial and microsomal fractions prepared from hearts of rats and rabbits, to compare characteristics of the acylation reactions by the two subcellular fractions. The assays were carried out with the subcellular fractions prepared from (i) hearts of hyperthyroid animals, and (ii) hearts of newborn and weanling rats. In addition, the effect of an addition of bovine serum albumin in the assay system was examined. (1) Administration of thyroid hormones increased the acyltransferase activity in rabbit hearts but not that in rat hearts. (2) Mitochondrial and microsomal fractions of hearts of newborn rats acylated glycerol 3-P at a rate 1.3-4 times greater than those of adult rats. The rate of acylation by the mitochondrial fraction of weanling rats was also high, but the rate of microsomal acylation was slightly lower than that of adult rats. By contrast, in newborn rats, diacylglycerol 3-P formation by the liver microsomes was not greater than that of the adults, although its formation by the newborn liver mitochondria was greater. (3) The accumulation of monoacylglycerol 3-P during the assay was accelerated by the addition of increasing amounts of bovine serum albumin. Therefore, the monoacylglycerol 3-P formation was less than 10% of the diacylglycerol 3-P formation in the assay containing no albumin and with the rat subcellular fractions, whereas nearly four times more monoacyl- than diacyl-glycerol 3-P was synthesized in the presence of 20 mg albumin. (4) The ratio of monoacyl- to diacyl-glycerol 3-P formation by the mitochondrial fraction was greater than that of microsomal fraction at any concentration of albumin in both rats and rabbits. At an equal albumin concentration in the assay, relatively more diacyl- than monoacyl-glycerol 3-P was formed in the mitochondrial fraction of newborn rat hearts as compared with adult hearts; (5) In conclusion, our data concerning the age and species differences in acyltransferase activities support a view that the mitochondrial fraction of both rat and rabbit hearts, in addition to the microsomal enzymes, is capable of catalyzing the de novo synthesis of phosphatidic acid.