Identification of a Species Specific Regulatory Site in Human Pancreatic Cholesterol Esterase

All mammalian pancreatic cholesterol esterases (CEase) bind to membrane-associated heparin at a single site on the intestinal brush border membrane with a dissociation constant of 100 nM. While the enzyme is bound to the membrane, the activity of the human and bovine enzymes is enhanced 2-fold when compared to the activity of the enzyme in solution. On the other hand, soluble heparin potently inhibits the human CEase-catalyzed hydrolysis of cholesterol oleate with an IC50 of 2 x 10(-4) mg/mL, a value that is about 10(4) times more potent than that found with the bovine enzyme. The C-terminal portion of the human enzyme contains 16 proline-rich repeats of 11 amino acids each, while that from other species contains only a few of these repeat units. To determine if the unique human C-terminus is responsible for this inhibition, two chimeras containing either the human N-terminus (residues 1-445) and the bovine C-terminus (residues 446-557), HB, or the bovine N-terminus (residues 1-445) and the human C-terminus (residues 446-722), BH, were prepared. The cholesterol oleate hydrolytic activity of these chimeras was similar to that for the recombinant human and bovine enzymes. Importantly, each chimera was inhibited by heparin with IC50 values of 0.03 and 0.1 mg/mL for HB and BH, respectively. These intermediate IC50 values indicate that human CEase has two structural regions that contribute to is unique inhibition by this sulfated glycosaminoglycan, and these could regulate cholesterol uptake in humans.