Regulation of Guinea Pig Sperm Adenylate Cyclase by Calcium1

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Abstract
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.