Monolayers of long chain lecithins at the air/water interface and their hydrolysis by phospholipase A2

The behavior of phosphatidylcholine monolayers at the air/water interface was studied by measuring their surface isotherm, surface potential, surface viscosity, and rate of hydrolysis by the dimeric phospholipase A2 from the venom of Crotalus atrox. The monolayers showed typical liquid-expanded behavior. In this phase, the surface potential was linearly dependent on surface concentration and extrapolated at zero concentration to a value characteristic of a liquid hydrocarbon/water interface. The rate of the reaction was measured by monitoring changes in area at constant surface pressure for 1,2-dioctanoyl- and 1,2-didecanoyl-3-sn-phosphatidylcholines, and by monitoring changes in surface potential for 1,2-dimyristoyl-, 1,2-dipalmitoyl-, 1-palmitoyl-2-oleoyl-, and 1-oleoyl-2-palmitoyl-sn-glycero-3-phosphocholines. The enzymatic hydrolysis is first order with respect to the enzyme-calcium complex which forms with a Kd = 1.5 mM. A mechanism is proposed to account for the dependency of the reaction rates on the surface concentration of the substrate. We postulate that the rate-limiting step is the decomposition of a quaternary complex formed from two phospholipid molecules, one calcium ion and one dimeric enzyme. The rate is independent of the surface pressure per se; addition of inert lipids to a monolayer at constant area, and hence constant surface concentration of the substrate, increases the surface pressure without changing the surface density of the substrate yielding maximal enzymatic rate. The enzyme is specific for loosely packed substrate molecules in the liquid-expanded state: transition into the liquid-condensed state or compression of the liquid-expanded layer beyond 80 A2/phospholipid strongly inhibits the enzymatic reaction. Our results show that surface recognition is a direct consequence of a bifunctional active site since it is only at a phospholipid surface that the distance between two substrate molecules is optimal for forming a catalytically competent enzyme-Ca2+-(substrate)2 complex.