Modulation of phospholipase A2 activity associated with human sperm membranes by divalent cations and calcium antagonists

Abstract

Phospholipase A2 activity in sonicates and acid extracts of ejaculated, washed human sperm was measured using [1-14C]oleate-labeled autoclaved Escherichia coli and 1-[14C]stearoyl-2-acyl-3-sn-glycerophosphorylethanolamine as substrates. Phospholipase A was optimally active at pH 7.5, was Ca-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 nmol/h per mg), followed by mouse, rabbit and bull, which were 105, 36 and 1.7 nmol/h per mg, respectively. p-Bromophenacyl bromide inhibited human sperm phospholipase A2 activity, but mepacrine was without effect. In the presence of 1.0 mM added CaCl2, phospholipase A2 activity was inhibited by Zn2+ and Mn2+; whereas Cu2+, Cd2+, Mg2+ or Sr2+ had no effect. Zn2+ stimulated activity at low concentrations (10-6 to 10-8 M), and inhibited activity in a dose-dependent manner at concentrations of 10-5 M. The extent of stimulation by low concentrations of Zn2+ was dependent on Ca2+ concentration; at 10-7 M, Zn2+ activity was stimulated 160% with 0.5 mM CaCl2 and only 120% with 1.0 mM CaCl2. At low concentrations (10-5 to 10-7 M), methoxyverapamil (D600) and trifluoperazine stimulated human sperm phospholipase A2 activity, and trifluoperazine but not D600 produced almost complete inhibition between 10-5 and 10-4 M of the drug. The significance of human sperm phospholipase A2 activity and its modulation by Ca2+, Zn2+ and Mn2+ in the sperm acrosome reaction is discussed.