SURFACE BEHAVIOR OF HUMAN PANCREATIC AND GASTRIC LIPASES

The kinetics of the adsorption of human gastric lipase (HGL) and human pancreatic lipase (HPL) were studied by recording the changes in the surface pressure with time in the absence and presence of an egg phosphatidylcholine (PC) monomolecular film spread at the air/water interface. In the presence of PC film, the tensioactivity of HGL increased considerably compared with its behaviour at the air/water interface, whereas HPL exhibited a comparable degree of tensioactivity whether or not a phospholipid monolayer was present at the interface. This difference in surface behaviour is consistent with the higher penetration capacity attributed to HGL. Procolipase considerably increased both the initial adsorption rate and the final surface pressure reached by HPL compared with its adsorption without colipase. The kinetics of the hydrolysis of 1,2-didecanoyl-sn-glycerol (dicaprin) monolayers by HGL and HPL were measured using a ''zero-order'' trough. The large differences between the calculated characteristic adsorption times and the measured lag times indicate that the partition of the lipase molecules between the subsurface and the interface was probably limited by an energy barrier. The amplitude of this energy barrier can be partly attributed to the drastic conformational change in the enzyme, associated with the interfacial activation. The area per dicaprin molecule (56 angstrom2) corresponding to the maximal activity of HPL was compared with the dimension of the hydrophobic cleft surrounding the serine (Ser 152) of the catalytic triad of HPL, as recently demonstrated by H. Van Tilbeurgh and co-workers (Nature, 359 (1992) 159; 362 (1993) 814) in their studies on the ''open'' and ''closed'' forms of the respective three-dimensional crystalline structures. The catalytic triad was not accessible to a sphere 8.4 angstrom in diameter, mimicking the van der Waals envelope of the dicaprin molecule, due to the steric hindrance of the side chains of aromatic and cyclic residues F 215, F 77, Y 114 and H 263. It can be concluded that the substrate molecule must also undergo some conformational changes at the contact of the enzyme to be accommodated in the active site.