Lung surface-active fraction as a model system for macromolecular ultrastructural studies with Crotalus atrox venom

The dog lung surface-active fraction and phosphatidylcholine constituents were subjected to hydrolysis by Crotalus atrox phospholipase A(2). Relative rates of hydrolysis were: dipalmitoyl glycerophosphorylcholine > phosphatidylcholine isolated from the surface-active fraction > phosphatidylcholine as an integral component of the intact surface-active macromolecular structure. Cholesterol markedly inhibited, whereas tripalmitin increased, the rate of hydrolysis with both pure phosphatidylcholine substrates. The effect of temperature on the velocity indicated the enzyme was most active when the substrates were in the gel state. These kinetic results, in conjunction with surface chemistry studies, can be interpreted to indicate that the phosphatidylcholine in the intact surface-active macromolecular particle is liquid crystalline due to molecular interactions with other constituents. Gas-liquid chromatographic analysis of the 2-lysophosphatidylcholines and fatty acids produced from the enzymatic hydrolysis of the intact surface-active fraction indicated that palmitoyl residues were more accessible to the enzyme, perhaps because they occupied positions near the surface of the particle.