Hydrolysis of monoacylglycerol in lipoprotein remnants catalyzed by the liver plasma membrane monoacylglycerol acyltransferase

Experiments were carried out to study the role played by the extrahepatic and hepatic lipolytic enzymes in lipoprotein catabolism. Chylomicra and very low density lipoproteins containing [2-³H]glyceryl triacylglycerols radiolabeled in vivo were incubated with purified milk lipoprotein lipase to produce lipoprotein remnants rich in monoacylglycerol. The primary products of the milk lipoprotein lipase-catalyzed reaction were free fatty acid and monoacylglycerol; hydrolysis of monoacylglycerol by this enzyme was dependent on the migration of the acyl group in position 2 to position 1. The monoacylglycerol product either was retained within the lipoprotein remnant or became bound to albumin, depending on the availability of lipid-binding sites on the albumin in the reaction mixture. The lipoprotein remnant and albumin were then separated by gel filtration and characterized by their chemical composition. After incubation with milk lipoprotein lipase, further degradation of triacylglycerol and monoacylglycerol in the lipoprotein remnants was observed; this did not occur for albumin-bound monoacylglycerol. Interestingly, plasma blocked complete degradation of triacylglycerol of the lipoprotein remnant by lipoprotein lipase, as shown by the accumulation of monoacylglycerol. Both bound forms of monoacylglycerol were excellent substrates for liver plasma membrane monoacylglycerol acyltransferase; conversely liver plasma membrane monoacylglycerol acyltransferase did not degrade triacylglycerol in either chylomicra or remnant lipoproteins. These results further support the proposed role for liver plasma membrane monoacylglycerol acyltransferase in the catabolism of lipoprotein remnants by liver. In addition, the utilization of monoacylglycerol bound to albumin by liver plasma membrane monoacylglycerol acyltransferase suggests a new and alternate pathway for monoacylglycerol transport and uptake by the liver.