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 μmol/min/mg for 1‐oleoyl‐sn‐glycerol, 163 μmol/min/mg for 1,2‐dioleoyl‐sn‐glycerol, and 145 μmol/min/mg for 1,3‐dioleoyl‐sn‐glycerol. These values were higher than those obtained earlier for phospholipids which were 67 μmol/min/mg for PA, 50 μmol/min/mg for PE and 4 μmol/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 thesn‐1 andsn‐3 positions occurred at the same rate, suggesting that hepatic lipase is not stereo‐selective with respect to 1,3‐diacyl‐sn‐glycerol substrates. To determine if the presence of one lipid could affect the hydrolysis of the other, all possible dual combinations of 1‐oleoyl‐sn‐glycerol, 1,2‐dioleoyl‐sn‐glycerol, and 1,3‐dioleoyl‐sn‐glycerol, in the same micelle were made and the hydrolysis rate of each substrate was determined. Interaction occurred only for the 1,2‐dioleoyl‐sn‐glycerol/1,3‐dioleoyl‐sn‐glycerol mixture where the hydrolysis of 1,2‐dioleoyl‐sn‐glycerol was slightly inhibited and that of 1,3‐dioleoyl‐sn‐glycerol slightly activated compared to the predicted theoretical rate. These findings demonstrate that when presented in similar physical states, the neutral lipid substrates tested were hydrolyzed at a higher rate by hepatic lipase than the phospholipid substrates.