Mixed monolayers of natural and polymeric phospholipids: structural characterization by physical and enzymatic methods.

This study has focused on physical characterization and enzymatic hydrolysis of mixed monolayers of a natural phospholipid substrate and a polymerizable phospholipid analogue. Such a mixed system presents the possibility to stabilize model biomembranes, vary the molecular environment within the layer through polymerization and simultaneously examine these influences on monolayer structure. Phospholipase A2 was used here as a sensitive probe of the molecular environment within these mixed, polymerizable monolayers to complement information obtained from isotherm and isobar data. The results clearly show a strong influence of molecular environment on phospholipase A2 activity, even if differences in the physical state of mixed monolayers are not detectable with isotherm and isobar measurements. Physical characterization indicated that both monomeric and polymeric mixed monolayers were phase-mixed. Enzyme hydrolysis, however, showed large differences in the ability of the enzyme to selectively hydrolyze the natural phosphatidylcholine component from the monomeric as opposed to the polymeric mixtures. This demonstrates a high sensitivity of phospholipase A2 to distinguish subtle differences in molecular arrangement within mixed monolayers on a molecular level.