The dependence of lipid monolayer lipolysis on surface pressure

Brönsted-Bjerrum theory [Brönsted (1922) Z. Phys. Chem. 102, 169-207; (1925) Z. Phys. Chem. 115, 337-364; Bjerrum (1924) Z. Phys. Chem. 108, 82-100] as applied to reactions at interfaces is used to interpret published data on the lipolysis of dinonanoyl phosphatidylcholine monolayers by pancreatic phospholipase A2. Reasonable quantitative agreement between theoretical and experimental results occurs when the reported effects of surface pressure on the amount of adsorbed enzyme are used together with the assumption that the Langmuir-Syskowski [Lange (1967) Nonionic Surfactants (Schick, M., ed.), chap. 4, Marcel Dekker, London and New York] isotherm describes the adsorption of lipid. The equations of Mass Action kinetics are less successful. Equivalent data on the lipolysis of didodecanoyl phosphatidylglycerol by pancreatic lipase can also be interpreted by arguing that the adsorbed enzyme forms significant amounts of enzyme substrate complex which reacts to give products in accordance with the well-known Briggs-Haldane [Lehninger (1975) Biochemistry, 2nd edn., chap. 8, Worth, New York] mechanism. The positions of the observed maxima in rate versus surface pressure plots suggests that the adsorption of an enzyme molecule displaces less lipid than one expects from the pronounced differences in molecular sizes of the two adsorbed species.