Hydrolysis of retinyl esters by pancreatic triglyceride lipase

Previously [van Bennekum, A. M., et al. (1999) Biochemistry 38, 4150-4156] we showed that carboxyl ester lipase (CEL)-deficient (CELKO) mice have normal levels of pancreatic, bile salt-dependent retinyl ester hydrolase (REH) activity. In the present study, we further investigated this non-CEL REH activity in pancreas homogenates of CELKO and wild-type (WT) mice, and rats. REH activity was detected in both the presence and absence of tri- and dihydroxy bile salts in rats, WT mice, and CELKO mice. In contrast, pancreatic cholesteryl ester hydrolase (CEH) activity was only detected in the presence of trihydroxy bile salts and only in rats and WT mice, consistent with CEL-mediated cholesteryl ester hydrolysis. Enzyme assays of pancreatic triglyceride lipase (PTL) showed that there was a colipase-stimulated REH activity in rat and mouse (WT and CELKO) pancreas, consistent with hydrolysis of retinyl ester (RE) by PTL. Pancreatic enzyme activities related to either CEL or PTL were separated using DEAE-chromatography. In both rats and mice (WT and CELKO), REH activity could be attributed mainly to PTL, and to a much smaller extent to CEL. Finally, purified human PTL exhibited similar enzymatic characteristics for triglyceride hydrolysis as well as for retinyl ester hydrolysis, indicating that RE is a substrate for PTL in vivo. Altogether, these studies clearly show that PTL is the major pancreatic REH activity in mice, as well as in rats.