EFFECT OF MONOLAYER SURFACE PRESSURE ON THE ACTIVITIES OF PHOSPHOINOSITIDE-SPECIFIC PHOSPHOLIPASE-C-BETA-1, PHOSPHOLIPASE-C-GAMMA-1, AND PHOSPHOLIPASE-C-DELTA-1

Three isoforms of phospholipase C, either PLC-beta(1), PLC-gamma(1), o r PLC-delta(1), were added to the aqueous subphase beneath phospholipi d monolayers formed at an air-solution interface, and the initial rate of hydrolysis of phosphatidylinositol 4,5-bisphosphate was measured a fter addition of 10 mu M free Ca2+. The monolayers were formed from mi xtures of phosphatidylcholine (65% PC), phosphatidylserine (33% PS), a nd phosphatidylinositol 4,5-bisphosphate (2% PIP2). Increasing the sur face pressure of the monolayer, pi, from 15 to 25 mN/m decreases the r ate of hydrolysis 16-, 13-, and 5-fold for PLC-beta(1), PLC-gamma(1), and PLC-delta(1), respectively. The simplest interpretation of these r esults is that a portion of each of the enzymes of area A(p) must inse rt into the monolayer, doing work pi A(p), prior to hydrolysis of PIP2 ; binding studies with simple model compounds of known cross-sectional area are consistent with this interpretation. Removing the monovalent acidic lipid PS from the monolayer decreases the initial rates of hyd rolysis of PIP2 about 3-fold for each PLC isoform, which suggests that negative electrostatic surface potentials increase the PLC activity.