PHOSPHOLIPID-GLYCERIDE INTERACTIONS AS REGULATORS OF CARBOXYLESTER LIPASE ADSORPTION AND CATALYSIS

Abstract

Lipolysis occurs in the presence of non-substrate, surface-active molecules like phosphatidylcholines (PC’s). The role of PC in the regulation of carboxylester lipase (CEL) was investigated using mixed lipid monolayers. For each substrate an abrupt increase in substrate hydrolysis from <10% to >90%, i.e. switching, occurred at a substratedependent lipid composition. Similar results were obtained for free fatty acid 180 exchange catalyzed by CEL. The lack of hydrolysis at low substrate was not due to an absence of CEL at the interface. Enzyme adsorption to substrate.PC films is consistent with all surface being available, except for 43.5 AZ /molecule of PC. Adsorption of CEL obtained by extrapolation to infinite dilution of PC was 0.92 pmol/cm?, 1/4 of the expected value. CEL adsorption to PCefatty acid films fell short of the values predicted from substrate-containing films. These results suggest that adsorption of CEL to PC-rich films is regulated by clusters or domains of substrate or fatty acid molecules in the PC interface. Such clusters are predicted from stochastic distribution of components but appear to be modulated by lipid-lipid interactions. The stochastic argument further predicts that substrate domains should interconnect if >45% of total surface area is non-excluded by PC. Using data obtained from CEL binding to calculate non-excluded area at each lipid composition, hydrolysis and 180-exchange data are consistent with catalysis being regulated by domain connectivity, i.e. percolation. Being lipid-based, this regulation should occur with other lipases, but the consequences of the organization may be lipase specific.