PHOSPHOLIPASE A(2) DOMAIN FORMATION IN HYDROLYZED ASYMMETRIC PHOSPHOLIPID MONOLAYERS AT THE AIR WATER INTERFACE/

Phospholipase A(2) (PLA(2)) catalyzed hydrolysis of asymmetric 1-capro yl-2-palmitoyl-phosphatidylcholine (6,16-PC) and 1-palmitoyl-2-caproyl -phosphatidylcholine (16,6-PC) lipid monolayers at the air/water inter face was investigated. Surface pressure isotherms, surface potential a nd fluorescence microscopy at the air/water interface were used to cha racterize the asymmetric monolayer systems. Cobra (N. naja naja) and b ee venom PLA, exhibit hydrolytic activity towards 6,16-PC and 16,6-PC monolayers at all surface pressures up to monolayer collapse (37 mN m( -1)). Pancreatic PLA(2) hydrolytic activity, however, was observed to be blocked at a lateral surface pressure of approx. 18 mN m(-1) for bo th 6,16-PC and 16,6-PC monolayers. For 6,16-PC monolayers, fluorescenc e microscopy revealed that monolayer hydrolysis by PLA(2) from cobra, bee, and bovine pancreatic sources all produced monolayer microstructu ring. Fluorescence microscopy also showed that PLA(2) is bound to thes e monolayer microstructures. Very little PLA(2)-induced microstructuri ng was observed to occur in 16,6-PC monolayer systems where caproic ac id (C6) hydrolysis products were readily solubilized in the aqueous mo nolayer subphase. Surface potential measurements for 16,6-PC monolayer hydrolysis indicate dissolution of caproic acid reaction products int o the monolayer subphase. Monolayer molecular area as a function of 6, 16-PC monolayer hydrolysis time indicates the presence of monolayer-re sident palmitic acid reaction products. With bovine serum albumin pres ent in the monolayer subphase, PLA, domain formation was observed only in hydrolyzed 6,16-PC monolayers. These results are consistent with l aterally phase separated monolayer regions containing phospholipid and insoluble fatty acid reaction products from PLA(2) monolayer hydrolys is electrostatically driving PLA(2) adsorption to and enzyme domain fo rmation at the heterogeneous, hydrolyzed lipid monolayer interface.