Activation of Phosphatidylinositol-Specific Phospholipase C toward Inositol 1,2-(Cyclic)-Phosphate

Abstract

Phosphatidylinositol-specific phospholipase C (PI-PLC) from Bacillus thuringiensis catalyzes the hydrolysis of phosphatidylinositol (PI) in discrete steps: (i) an intramolecular phosphotransferase reaction to form inositol 1,2-(cyclic)-phosphate (cIP), followed by (ii) a cyclic phosphodiesterase activity that converts cIP to inositol 1-phosphate. Water-soluble cIP was used as the substrate to study the cyclic phosphodiesterase activity and interfacial behavior of PI-PLC. Different detergent micelles and phospholipid vesicles were used to examine if "interfacial activation" of the enzyme could occur toward a soluble substrate. Almost all detergents examined activated the enzyme at least 2-fold, with PC species yielding the largest increases in PI-PLC specific activity. Kinetic parameters were measured in the absence and presence of several representative detergents (e.g., Triton X-100 and diheptanoylphosphatidylcholine (diC7PC)). Gel filtration experiments showed that, under these conditions, the cIP did not partition to any measurable extent with these detergent micelles. The concentration at which half the maximum activation was observed occurred near the detergent CMC. Both Km and Vmax were altered by the presence of a surface: Km decreased to different degrees depending on the detergent, while Vmax increased substantially. The Km for cIP was 90 mM without detergent and decreased to 29 mM with diC7PC micelles added; Vmax increased almost 7-fold in the presence of diC7PC micelles. The enzyme efficiency (Vmax/Km) in the presence of diC7PC increased more than 21-fold, but it was still 20-fold lower than initial phosphotransferase activity for monomeric dihexanoylphosphatidylinositol. The poor efficiency of the cyclic phosphodiesterase activity is largely due to substrate binding affinity. The dependence of rate on substrate concentration exhibits cooperative behavior, especially without detergent. This cooperativity is discussed in terms of protein aggregation and ligand binding sites on the enzyme.