Effect of Sublytic Concentrations of Sodium Cholate on Phospholipase C Hydrolysis of Phospholipid Bilayers

Abstract

Phospholipase C activity has been assayed with phosphatidylcholine as substrate in the presence of sodium cholate at concentrations well below those producing lipid solubilization. With short-chain phosphatidylcholine, which exists in monomeric form in aqueous solution, cholate has little or no effect. However, when the substrate is egg phosphatidylcholine in the form of bilayers, small cholate concentrations (below 1 mM, corresponding to an effective surfactant:lipid ratio below 0.05) increase the maximum enzyme rates by about threefold, while decreasing drastically the latency periods of enzyme activity. Previous studies from this laboratory have associated the phospholipase 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 phospholipase C through a hitherto undescribed mechanism. Sodium cholate concentrations above 1 mM decrease further the enzyme lag time, but they are less effective in enhancing enzyme rates. These observations may be pertinent in the analysis of biochemical data with purified lipases, as well as in physiological studies of biliary function.