Role of lipid packing in the activity of phospholipase C-delta(1) as determined by hydrostatic pressure measurements

Previous studies with phospholipid monolayers revealed a large decrease in the activity of phosphoinositide-specific phospholipase C-delta(1) (PLC-del ta(1)) which catalyses the hydrolysis of PtdIns(4,5)P-2 as lateral pressure is applied to the membrane. If stress on the membrane is the sole inhibito r of PLC-delta(1) activity, the enzyme must penetrate the membrane surface to engage its substrate. To test the effect on PLC-delta(1) activity of lip id packing in the absence of a directional stress, we examined the effects of increasing hydrostatic pressure on enzymic activity. We find that, in co ntrast with monolayer studies, increasing lipid packing by hydrostatic pres sure does not affect membrane binding and increases enzymic activity by 90 % in going from atmospheric pressure to 10(8) Pa (approx, 1000 atm). The in crease in activity could be accounted for mainly by electrostriction of wat er around the multiply-charged product. Our results show that when there is no net stress on the monolayer, lipid packing does not alter PLC-delta(1) activity, possibly because penetration of the enzyme into the membrane surf ace is shallow. We suggest that, in biological membranes, the activity of t his and possibly other interfacial proteins is independent of headgroup pac king.