Characterization of a calmodulin-stimulated adenylate cyclase from abalone spermatozoa

Abalone sperm adenylate cyclase activity is particulate in nature and displays a high Mg2+-supported activity (Mg2+/Mn2+ = 0.8) as compared to other sperm adenylate cyclases. Approximately 90% of the enzyme activity in crude homogenates is inhibited by EGTA in a concentration-dependent manner which is overcome by added micromolar free Ca2+. The EGTA-inhibited Ca2+-stimulated enzyme activity is also inhibited by phenothiazines. Added calmodulin, however, has no effect on enzyme activity prepared from crude homogenates. Preparation of a twice EGTA-extracted 48,000 X g pellet fraction yields a particulate enzyme activity that can be stimulated 10-65% by added calmodulin in the presence of micromolar free Ca2+. Detergent extraction (1% Lubrol PX) of the EGTA-washed 48,000 X g pellet solubilizes 2-5% of the total particulate adenylate cyclase activity, and this solubilized enzyme is activated up to 125% by calmodulin. The ability of the different enzyme preparations to be stimulated by calmodulin is inversely proportional to the endogenous calmodulin concentration. Calmodulin stimulation of the Lubrol PX-solubilized enzyme is specific to this Ca2+-binding protein and is mediated as an effect on the velocity of the enzyme. This stimulation is completely Ca2+ dependent and is fully reversible. These data suggest that the control of sperm cAMP synthesis by changes in Ca2+ conductance may be mediated via this Ca2+-binding protein.