A comparison of the activity of phosphatidylinositol phosphodiesterase against substrate in dispersions and as monolayers at the air water interface

The activity of phosphatidylinositol phosphodiesterase, purified from rat brain, against substrate in 3 forms: multibilayer liposomes, single bilayer vesicles of phosphatidylinositol and phosphatidylinositol oriented as monolayers at the air water interface, was examined. The reaction rate was similar against the 2 substrate dispersions prepared with the same phospholipid concentration, although there was a large difference in substrate surface area available to the enzyme, and this similarity could not be accounted for by any differences in the microviscosity of the hydrocarbon region of the phospholipid bilayers. The reaction showed apparent zero order reaction kinetics until about 10% of the substrate had been degraded, whereupon the rate decreased. The reaction against monolayers of phosphatidylinositol was linear throughout the entire digestion of the film, provided that more than 0.25 mg of enzyme was present in the subphase. The pH optimum was 6.6. Bivalent ions (Ca²⁺, Mg²⁺, Co²⁺, Ni²⁺ and Mn²⁺) facilitated enzyme penetration into substrate monolayers, but the enzyme was only activated by Ca²⁺ (optimal concentration, 1 mM) and to a lesser extent by Mg²⁺. The reaction rate was independent of initial surface pressures of less than about 22 mN x m^-1 but at higher pressures the rate decreased. This decrease could be prevented by the addition of 10 mol of octadecylamine/90 mol of phosphatidylinositol to the substrate monolayer; the amine did not increase the rate of reaction in films of less than 22 mN x m^-1.