Is lateral phase separation required for fatty acid to stimulate lipases in a phosphatidylcholine interface?

Lipase-catalyzed oxygen exchange between 13,16-cis,cis-docosadienoic acid and water in liquid-expanded monolayers with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine exhibits abrupt, lipid composition-dependent changes in extent and mechanism [e.g., Muderhwa, J. M. and Brockman, H. L. (1992) J. Biol. Chem. 267, 24184-24192]. The critical nature of this transition suggests possible lateral phase separation of the lipids. This has been addressed by substituting for either lipid species one which can exist in more condensed monolayer states. Analysis of phase transition surface pressures as a function of lipid composition shows that each set of fatty acid-phosphatidylcholine mixtures exhibits a finite range of miscibility in liquid-expanded monolayers. These results strongly suggest that 13,16-cis,cis-docosadienoic acid and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine are miscible under the conditions of the oxygen-exchange experiments. Furthermore, to address more directly the relation of lateral lipid phase separation to lipase regulation, oxygen exchange catalyzed by pancreatic carboxylester and triglyceride lipases was studied using mixed monolayers of [18O]2-docosadienoic acid and 1-myristoyl-2-palmitoyl-sn-glycero-3-phosphocholine. These lipids are miscible in the liquid-expanded state at all compositions. The lipid composition dependencies of both the extent and mechanism of lipase-catalyzed oxygen exchange were essentially identical to those obtained earlier. Thus, lateral lipid phase separation is not required for the critical transition in substrate accessibility to lipases. This finding supports a percolation-based model of lipase regulation within a single surface phase and suggests the "topo-temporal" regulation of lipid-mediated signaling in cells.