Dihexanoylphosphatidylethanolamine: effect of head group charge on rates of alkaline and phospholipase A2 catalyzed hydrolyses.

Dihexanoylphosphatidylethanolamine (DiC6-PE) was prepared by phospholipase D catalyzed transphosphatidylation of dihexanoylphosphatidylcholine (DiC6-PC). Below the critical micellar concentration the pKa of the amino group is 9.4 +/- 0.05. The critical micellar concentration of the zwitterionic species is 5.3 +/- 0.2 mM, while that of the anionic species is 11.0 +/- 0.05 mM. Based on the pH dependence of the rate of hydroxide ion catalyzed hydrolysis, the second order rate constant for hydrolysis of the zwitterionic species is 0.70 +/- 0.021 s-1 M-1, while that for the anionic species is 0.040 +/- 0.011 s-1 M-1. The pH-dependence of phospholipase A2 catalyzed hydrolysis at substrate concentrations below the critical micellar concentration shows that the zwitterionic species is the preferred substrate, and the anionic species is either a competitive inhibitor of the hydrolysis of the zwitterionic species or poor substrate. DiC6-PE is hydrolyzed by C. adamanteus at about 1% the rate of DiC6-PC.