Separate monoacylglycerol and diacylglycerol acyltransferases function in intestinal triacylglycerol synthesis

Abstract

The monoacylglycerol and diacylglycerol acyltransferases in microsomes from the rat small intestinal epithelium have been examined in order to: (1) differentiate between the monoacylglycerol acyltransferase and diacylglycerol acyltransferase and (2) establish whether or not there is more than one diacylglycerol acyltransferase involved in the two pathways of triacylglycerol synthesis which operate in these cells. Assays were established for the monoacylglycerol acyltransferase using sn-2-monoolein as substrate and for the diacylglycerol acyltransferase using three different substrates: exogenous sn-2-diolein; diacylglycerol synthesized endogenously from monoolein; or diacylglycerol synthesized endogenously from glycerol phosphate as substrate. Compared to the diacylglycerol acyltransferase, the monoacylglycerol acyltransferase was distributed more widely throughout the intestine and was also relatively insensitive to inhibition by mercuric chloride. In addition, exogenous diolein did not inhibit the monoacylglycerol acyltransferase activity. These observations are consistent with separate monoacylglycerol and diacylglycerol acyltransferase enzymes. The distribution of the diacylglycerol acyltransferase activity assayed either with exogenous diolein or diacylglycerol produced from monoolein was similar in different sections of the intestine and in samples from different rats. Further, the diacylglycerol acyltransferase activity was strongly inhibited by mercuric chloride with all three assays. However, the activity assayed with exogenous substrate was selectively inhibited by dithiothreitol and the non-ionic detergents, Triton X-100 and Tergitol NP-40. This is unlikely to represent the presence of separate enzymes since the same reagents inhibit the activity of the microsomal CDPcholine: diacylglycerol cholinephosphotransferase when assayed with exogenous diolein but not with endogenous diacylglycerol from glycerol phosphate. It is concluded that these reagents interfere with the association of exogenous diolein with the microsomal membrane and that there is only one diacylglycerol acyltransferase active in the small intestinal epithelium.