Pancreatic carboxyl ester lipase (CEL) hydrolyzes cholesteryl esters (CE), triglycerides (TG), and lysophospholipids, with CE and TG hydrolysis stimulated by cholate. Originally thought to be confined to the gastrointestinal system, CEL has been reported in the plasma of humans and other mammals, implying its potential in vivo to modify lipids associated with LDL, HDL (CE, TG), and oxidized LDL (lysophosphatidylcholine, lysoPC). We measured the concentration of CEL in human plasma as 1.2 Ϯ 0.5 ng/ml (in the range reported for lipoprotein lipase). Human LDL and HDL 3 reconstituted with radiolabeled lipids were incubated with purified porcine CEL without or with cholate (10 or 100 M, concentrations achievable in systemic or portal plasma, respectively). Using a saturating concentration of lipoprotein-associated CE (4 M), with increasing cholate concentration there was an increase in the hydrolysis of LDL-and HDL 3 -CE; at 100 M cholate, the percent hydrolysis per hour was 32 Ϯ 2 and 1.6 Ϯ 0.1, respectively, indicating that CEL interaction varied with lipoprotein class. HDL 3 -TG hydrolysis was also observed, but was only ‫ف‬ 5-10% of that for HDL 3 -CE at either 10 or 100 M cholate. Oxidized LDL (OxLDL) is enriched with lysoPC, a proatherogenic compound. After a 4-h incubation with CEL, the lysoPC content of OxLDL was depleted 57%. Colocalization of CEL in the vicinity of OxLDL formation was supported by demonstrating in human aortic homogenate a cholate-stimulated cholesteryl ester hydrolytic activity inhibited by anti-human CEL IgG. We conclude that CEL has the capability to modify normal human LDL and HDL composition and structure and to reduce the atherogenicity of OxLDL by decreasing its lysoPC content.

Pancreatic carboxyl ester lipase is a glycoprotein that requires millimolar concentrations of trihydroxy bile salts, such as cholate, for maximal catalytic activity against cholesteryl esters and triglycerides. Binding of cholate, with subsequent activation, has been proposed to occur in the carboxy-terminal region of carboxyl ester lipase, which contains multiple sites for O-linked glycosylation (1). To investigate the role of O- and N-linked glycosylation in the secretion of carboxyl ester lipase by cells and its activation by cholate, rat carboxyl ester lipase cDNA was transfected into the mutant chinese hamster ovary cell line, IdID, and the ability of the cells to modify the expressed carboxyl ester lipase by N- and O-linked glycosylation was modulated by using various incubation conditions and metabolic inhibitors. The results showed that, similar to other lipases, maximal secretion of carboxyl ester lipase activity required N-linked glycosylation. In contrast, O-linked glycosylation did not affect the secretion of carboxyl ester lipase activity. In addition, the cholate stimulation of hydrolysis was also independent of O-linked glycosylation.