Physicochemical studies on the interaction of pancreatic phospholipase A2 with a micellar substrate analogue.

Porcine pancreatic phospholipase A2 interacts with micelles of the substrate analogue n-octadecylphosphocholine to form a specific complex over considerably wide concentration ranges of both lipid and protein. UV absorption difference spectroscopy measurements indicate that the ratio of lipid to protein molecules in the complex is approximately 50. This number is confirmed by using other techniques to study the composition of the complex, namely, ultracentrifugation experiments and light scattering. The latter techniques furthermore demonstrate that the lipid--protein complex consists of 100 lipid and 2 enzyme molecules. Thus, the number of lipid molecules in the free micelle (200) is halved when the complex with phospholipase is being formed. The consequences of the results are discussed in relation to a theoretical model of the lipid--protein interaction.