Human plasma carboxyl esterase-catalyzed triolein hydrolysis. Existence of promoting factor in serum

The possibility that some factor in serum changes the substrate specificity of purified human plasma carboxyl esterase, which hydrolyzes the short chain fatty acid ester, tributyrin, was investigated. The purified carboxyl esterase from human plasma hydrolyzed 48 mmol of tributyrin/mg of protein/h, monoolein at 1560 mumol of released fatty acids/mg of protein/h, diolein at 133 mumol of released fatty acids/mg of protein/h, and triolein at less than 10 mumol of released fatty acids/mg of protein/h. When human serum was applied to phenyl-Sepharose, a triolein hydrolysis-promoting factor (THPF) for purified carboxyl esterase was bound to the gel and was eluted with water. This partially purified human serum THPF enhanced carboxyl esterase-catalyzed triolein hydrolysis about 30-fold, diolein hydrolysis 2-fold, and monoolein hydrolysis 1.5-fold. Hydrolysis of triolein in very low density lipoproteins (d less than 1.006) and intermediate lipoproteins (1.006 less than d less than 1.019) by carboxyl esterase was also enhanced by addition of THPF. THPF activity was reduced by treatment of delipidation, but resistant to trypsin treatment or heating at 50 degrees C. These results indicated that serum carboxyl esterase can hydrolyze the long chain fatty acid ester, triolein, in the presence of triolein hydrolysis-promoting factor in serum.