Effects of phospholipids on hydrolysis of trioleoylglycerol by human serum carboxylesterase.

Human serum carboxylesterase (EC 3.1.1.1), purified by affinity chromatography on trimethylammonium anilinium-Sepharose, hydrolyzed the short-chain fatty acid ester tributyrin (40 mumol/mg protein per h), but scarcely hydrolyzed the long-chain fatty acid ester triolein (less than 0.2 mumol/mg protein per h). Phospholipids enhanced triolein hydrolysis by carboxylesterase to various extents, cardiolipin causing the most enhancement (2.5 mumol/mg protein per h). Phosphatidylserine and phosphatidylinositol also enhanced carboxylesterase-catalyzed hydrolysis of triolein (450-980 nmol/mg protein per h). The optimal pH for tributyrin hydrolysis was pH 8.0, but the pH range for triolein hydrolysis was broad, being pH 4.5-7.5. The rates of hydrolyses of monoolein, diolein and triolein by carboxylesterase in the absence and presence of 100 micrograms/ml cardiolipin were 3.9, 0.5 and 0.2 mumol/mg esterase per h and 2.0, 0.6 and 4.0 mumol/mg protein per h, respectively. Thus, on addition of cardiolipin, triolein hydrolysis was enhanced, but tributyrin hydrolysis was reciprocally decreased. Triton X-100 (0.1%) and NaCl (1.0 M) decreased triolein hydrolysis, but did not decrease tributyrin hydrolysis. Mercaptoethanol decreased triolein hydrolysis, but not tributyrin hydrolysis. These results suggest that cardiolipin modifies the interaction of carboxylesterase with substrates in such a way as to facilitate its interaction with a hydrophobic substrate, and that disulfide bonding might be involved in the substrate recognition site.