REGULATION OF GUINEA-PIG SPERM ADENYLATE-CYCLASE BY CALCIUM

The effect of various metal ions on guinea pig sperm adenylate cyclase activity was determined. In the presence of 4.5 mM free metal, relative enzyme activity with Mn2+, Ca2+, Mg2+, Co2+, Zn2+, and Ba2+ was 1.00, 0.16, 0.10, 0.10, 0.05 and 0.02, respectively. Added Ca2+, specifically, appeared to activate the enzyme in the presence of Mn2+ or Mg2+. The guinea pig sperm adenylate cyclase was stimulated ~4-fold by low concentrations (μM) of free Ca2+ in the presence of Mg2+ (5 mM). This Ca2+-dependent increase in adenylate cyclase activity was inhibited by trifluoperazine (0.3-0.5 mM), a known inhibitor of calmodulin. Basal adenylate cyclase activity measured in the presence of Mg2+ (5 mM) and in the absence of Ca2+ was not affected by the addition of trifluoperazine (0.5 mM). Treatment of the sperm homogenate with ethylene-glycol-bis (β-aminoethyl ether) N,N'-tetra-acetic acid (EGTA) under a variety of conditions failed to completely remove the Ca2+-sensitivity of the particulate adenylate cyclase; such treatment also failed to remove the membrane associated calmodulin. After detergent solubilization, the sperm Mg2+-dependent adenylate cyclase activity was less than 0.5% of the Mn2+-dependent activity and was not stimulated by added Ca2+. These results suggest that a component of the guinea pig sperm adenylate cyclase complex is regulated by Ca2+. Whether the Ca2+-sensitive component is calmodulin remains unclear.