Regulation of fatty acid oxygen-18 exchange catalyzed by pancreatic carboxylester lipase. 2. Effects of lateral lipid distribution in mixtures with phosphatidylcholine

Abstract

The lipase-catalyzed exchange of the carboxyl oxygens of 13,16-cis,cis-docosadienoic acid (DA) was studied in the presence of a nonsubstrate matrix lipid, 1-palmitoyl-2-oleoylphosphatidylcholine. For mixed lipid films at the argon-water interface exposed to pancreatic carboxylester lipase (EC 1.1.1.13), the extent of oxygen exchange showed an abrupt increase as the abundance of DA in the interface was increased from 0.5 to 0.6 mole fraction. This compositional range was independent of the level of enzyme used and of the surface pressure, i.e., lipid packing density, of the film. Concomitant with the transition was a change in the apparent mechanism of exchange from coupled to random sequential. Like the extent of oxygen exchanged, the shift in mechanism was independent of all variables except the lipid composition of the interface. The absence of any chemical or physical change accompanying the exchange reaction precludes mechanistic explanations based on the generation of reaction products by the enzyme. Instead, the results suggest that the lateral distribution of DA in phosphatidylcholine-DA interfaces regulates the expression of carboxylester lipase activity and its apparent mechanism. Preliminary measurements give an average cluster size of 1825 molecules of DA when its mole fraction is 0.35. As the DA content of the interface reaches 0.5-0.6, there appears to be a lipid head-group based percolative transition in which DA becomes the continuum. Because this transition involves the lateral organization of the lipids themselves, other interfacially active enzymes may be regulated similarly.