Modulation of Phospholipase A2Activity Associated with Human Sperm Membranes by Divalent Cations and Calcium Antagonists

Phospholipase A2activity in sonicates and acid extracts of ejaculated, washed human sperm was measured using [1-14C] oleate-labeled autoclaved E. coliand 1-[1-14C] stearoyl-2-acyl-3-sn- glycerophosphorylethanolamine as substrates. Phospholipase A was optimally active at pH 7.5, was calcium-dependent, and exclusively catalyzed the release of fatty acid from the 2-position of phospholipids. The activity was membrane-associated, and was solubilized by extraction with 0.18 N H2SO4. Acid extracts of human sperm had the highest specific activity (1709 nmols /h per mg), followed by mouse, rabbit and bull, which were 105, 36 and 1.7 nmols /h per mg, respectively. para-bromophenacyl bromide inhibited human sperm phospholipase A2activity, but mepacrine was without effect. In the presence of 1.0 mM added CaCl2, phospholipase A2activity was inhibited by Zn2+and Mn2+; whereas Cu2+, Cd2+, Mg2+, or Sr2+had no effect. Zn2+stimulated activity at low concentrations (10−6to 10−8M), and inhibited activity in a dose-dependent manner at concentrations of 10−5M. The extent of stimulation by low concentrations of Zn2+was dependent on CA2+concentration; at 10−7M, Zn2+activity was stimulated 160% with 0.5 mM CaCl2, and only 120% with 1.0 mM CaCl2. At low concentrations (10−5to 10−7M), methoxyverapamil (D600) and trifluoperazine stimulated human sperm phospholipase A2activity, and trifluoperazine but not D600 produced almost complete inhibition between 10−5and 10−4M of the drug. The significance of human sperm phospholipase A2activity and its modulation by CA2+, Zn2+and Mn2+in the sperm acrosome reaction is discussed.