Positional Specificity of Lipoprotein Lipase

The stereochemical course of hydrolysis of radioactive triolein was determined using lipoprotein lipase of cow's milk and rat postheparin plasma. [3H]Glycerol trioleate or glycerol [1-14C]trioleate was emulsified with total egg yolk lipids to provide 0.2 to 2.5 µCi of 3H or 0.03 µCi of 14C per µm substrate. The hydrolysis products were isolated at 1 to 45 min and free fatty acids and the various positional isomers of mono- and diglycerides were resolved by thin layer chromatography. In a total yield of 0.03 to 0.16 µmole of diglyceride, 9 to 30% was 1,3- and 70 to 91%, 1,2-(2,3-)diglyceride. The 2,3-isomer was 73 to 96% of the latter. The yield of free fatty acids and monoglycerides varied with the length of incubation. In 15 of 17 determinations, the 1-(3-)isomer accounted for 51 to 87% of the monoglyceride. It is suggested that lipoprotein lipase attacks preferentially position 1 in sn-glycerides and follows it by hydrolysis of the positions 2 and 3. An intermediate formation of 2,3-diglycerides during lipoprotein lipase hydrolysis may be important in avoiding stimulation of triglyceride and phospholipid biosynthesis which proceed via 1,2-diglycerides.