Effect of albumin on products formed from chylomicron triacylglycerol by lipoprotein lipase in vitro

Abstract

The effect of albumin and Ca 2+ on the action of purified bovine milk lipoprotein lipase on chylomicron triacylglycerol in vitro was studied with rat lymph chylomicrons containing triacylglycerol labeled with [ 14C]oleic acid and [ 14H]glycerol. Lipoprotein lipase hydrolyzed chylomicron triacylglycerol to mostly glycerol and fatty acids when incubated in medium containing sufficient albumin to bind all of the fatty acids formed. There was, however, transient accumulation of monoacylglycerol during the first 5 min of incubation. It is suggested that the latter represented monoacylglycerol undergoing isomerization within chylomicrons before being hydrolyzed to glycerol and fatty acid. Hydrolysis of monoacylglycerol was markedly inhibited (67%) when the incubation mixture contained a 9-fold excess of albumin. This effect on hydrolysis probably resulted from binding of monoacylglycerol to albumin in the medium, thus making the monoacylglycerol inaccessible to the enzyme. When the incubation mixture contained limited albumin, hydrolysis of all acylglycerols was reduced, and diacylglycerol and monoacylglycerol accumulated. Although some of the monoacylglycerol was released to the medium, the molar ratio of monoacylglycerol to albumin in the medium never exceeded 0.35. In contrast, the molar ratio of fatty acids to albumin in the medium was sometimes as high as 7. Ca 2+ was a poor substitute for albumin as an acceptor for fatty acids produced by the action of lipoprotein lipase on chylomicrons. The rate of hydrolysis of chylomicron triacylglycerol by lipoprotein lipase in vitro was related directly to the amount of enzyme added up to 1.7 μg (34 pmol) of enzyme per μmol of triacylglycerol. It was calculated that a nearly maximal rate was obtained when 43 molecules of enzyme were added per chylomicron.