Hypertriacylglycerolemia in the chick: Effect of estrogen on hepatic microsomal enzymes of triacylglycerol and phospholipid synthesis

Abstract

Hepatic microsomal enzymes of triacylglycerol and phospholipid synthesis were investigated in chicks made hyperlipidemic by estrogen treatment. The total activities of two liver microsomal enzymes common to the triacylglycerol and phospholipid biosynthetic pathways, the fatty acid CoA ligase (AMP) (EC 6.2.1.3) and the sn-glycerol 3-phosphate acyl-CoA acyltransferase (EC 2.3.1.15), and an enzyme unique to triacylglycerol synthesis, the diacylglycerol acyltransferase (EC 2.3.1.20), increased 2.5–3.6-fold, as did total liver protein, relative to the activities and protein from controls. Upon subcellular fractionation, little change in the specific activities of these biosynthetic enzymes was observed. The microsomal marker activity NADPH cytochrome C reductase (EC 1.6.2.a) also increased proportionately with liver protein. However, the total activity of a phospholipid biosynthetic enzyme, diacylglycerol cholinephosphotransferase (EC 2.7.8.2) increased only 32% after a 5-day diethylstilbestrol course, while the specific activity of this enzyme decreased 40%. The total activity of succinic dehydrogenase (EC 1.3.99.1), a mitochondrial marker activity, increased only 22%, further demonstrating the differential effect of estrogen on hepatic enzyme activities. The augmentation of triacylglycerol synthesis may be mediated, in part, by increases in total activities of two enzymes common to the triacylglycerol and phospholipid synthetic pathways and/or by regulation at the diacylglycerol branch point of triacylglycerol and phospholipid synthesis.