Effect of sublytic concentrations of sodium cholate on phospholipase C hydrolysis of phospholipid bilayers

Phospholipase C activity has been assayed with phosphatidylcholine as subst rate in the presence of sodium cholate at concentrations well below those p roducing lipid solubilization. With short-chain phosphatidylcholine, which exists in monomeric form in aqueous solution, cholate has little or no effe ct. However, when the substrate is egg phosphatidylcholine in the form of b ilayers, small cholate concentrations (below 1 mM, corresponding to an effe ctive surfactant:lipid ratio below 0.05) increase the maximum enzyme rates by about threefold, while decreasing drastically the latency periods of enz yme activity. Previous studies from this laboratory have associated the pho spholipase enhancing activity of a variety of amphiphiles to their ability to facilitate the formation of inverted hexagonal phospholipid structures, yet sodium cholate has the opposite effect, stabilizing the lamellar versus the inverted hexagonal phase. This suggests that cholate is activating pho spholipase C through a hitherto undescribed mechanism. Sodium cholate conce ntrations above 1 mM decrease further the enzyme lag time, but they are les s effective in enhancing enzyme rates. These observations may be pertinent in the analysis of biochemical data with purified lipases, as well as in ph ysiological studies of biliary function. (C) 1999 Academic Press.