Surface Pressure-Dependent Interactions of Secretory Phospholipase A2with Zwitterionic Phospholipid Membranes.

The hydrolytic activity of secretory phospholipase A2(PLA2) is regulated by many factors, including the physical state of substrate aggregates and the chemical nature of phospholipid molecules. In order to achieve strong binding of PLA2on its substrates, many previous works have used anionic lipid dispersion to characterize the orientation and penetration depth of PLA2molecules on membrane surfaces. In this study, we applied monolayer technique with controllable surface area to investigate the PLA2s of Taiwan cobra venom and bee venom on zwitterionic phophatidylcholine monolayers and demonstrated an optimum hydrolytic activity at a surface pressure of 18 and 24 mN/m, respectively. By combining polarized attenuated total reflection Fourier-transform infrared spectroscopy and monolayer-binding experiments, we found that the amount of membrane-bound PLA2decreased markedly as the surface pressure of the monolayer was increased. Interestingly, the insertion area of the PLA2s decreased to near zero as the surface pressure increased to the optimum pressure for hydrolytic activity. On the basis of the measured infrared dichroic ratio, the orientation of the PLA2s bound to zwitterionic membranes was similar to that observed on a negatively charged membrane and was independent of the surface pressure. Our findings suggest that both PLA2s were located on the membrane surface rather than penetrating the membrane bilayer and that the deeply inserted mode is not a favorable condition for the hydrolysis of phospholipids in zwitterionic phospholipid membranes. The results are discussed in terms of the easy access of catalytic water for the PLA2activity and the mobilization of its substrate and product to facilitate the catalytic process. [ABSTRACT FROM AUTHOR]