Hydrolysis of neutral lipid substrates by rat hepatic lipase

Rat hepatic lipase, an enzyme whose involvement in the catabolism of lipoproteins remains poorly defined, has both neutral lipid and phospholipid hydrolyzing activity. We determined the substrate specificity of hepatic lipase for 1-oleoyl-sn-glycerol, 1,2-dioleoyl-sn-glycerol, and 1,3-dioleoyl-sn-glycerol in the Triton X-100 mixed micellar state, and compared these results to those obtained previously in our laboratory for the phospholipid substrates phosphatidic acid (PA), phosphatidylethanolamine (PE), and phosphatidylcholine (PC). Vmax values were determined by diluting the substrate concentration in the surface of the micelle by Triton X-100. The Vmax values obtained were 144 mumol/min/mg for 1-oleoyl-sn-glycerol, 163 mumol/min/mg for 1,2-dioleoyl-sn-glycerol, and 145 mumol/min/mg for 1,3-dioleoyl-sn-glycerol. These values were higher than those obtained earlier for phospholipids which were 67 mumol/min/mg for PA, 50 mumol/min/mg for PE and 4 mumol/min/mg for PC. In addition, the mole fraction of lipid substrate at half maximal velocity (K) in the surface dilution plot was lower for the neutral lipid substrates as compared to those obtained for the phospholipid substrates. When the hydrolysis of 1,3-dioleoyl-sn-glycerol mixed micelles was studied as a function of time, cleavage at the sn-1 and sn-3 positions occurred at the same rate, suggesting that hepatic lipase is not stereoselective with respect to 1,3-diacyl-sn-glycerol substrates.(ABSTRACT TRUNCATED AT 250 WORDS)