The control of fatty acid composition in glycerolipids of the endoplasmic reticulum

Abstract

A functional relationship between constitutive enzymes of the endoplasmic reticulum that are involved in lipid metabolism and the proteins and phosphoglycerides constituting these membranes was examined by altering the protein and fatty acid composition. The specific activity of alkenyl glycerylphosphoryl choline hydrolyase did not change upon fasting, fasting, and refeeding or administration of phenobarbital, whereas the specific activities of both acyl-CoA hydrolyase and acyl-CoA:1-acyl glycerylphosphoryl choline acyltransferase(s) increased upon alteration of the nutritional state. Other enzymic activities also showed changes following the treatments. The constant specific activity of 1-alkenyl glycerylphosphoryl choline hydrolyase suggests that it is a true constitutive enzyme of the endoplasmic reticulum and a useful indicator of membrane biogenesis. Alteration in the fatty acid composition of microsomal phosphoglycerides by maintenance of animals under different dietary conditions, by in situ incorporation of fatty acids into microsomal phosphoglycerides, or by the addition of exogenous, micellar lecithins did not produce adaptive changes in the specificity for esterification of fatty acids to Position 2 of lecithins. Partial removal of microsomal phosphoglycerides by treatment with phospholipases failed to demonstrate a functional requirement for diacyl phosphoglycerides in acyltransferase activity. These considerations indicate that a functional role of phosphoglycerides in acyl-CoA:1-acyl glycerylphosphoryl choline acyltransferase activity is limited to the requirement of phosphoglycerides for the integrity of the membrane.