The soluble calmodulin-sensitive isoform of adenylyl cyclase isolated from equine sperm is unique because it requires Mn2+ rather than Mg2+ for activity. To gain insight into the molecular action of metals on sperm adenylyl cyclase, the kinetics of Mn2+ and ATP effect was examined. A biphasic response to increases in ATP concentration was observed when metal was held constant. When [Mn2+] exceeded [ATP], however, greatly enhanced enzyme activity was observed. The kinetic profiles were consistent with allosteric activation of adenylyl cyclase by Mn2+. Linear transformation of the data yielded an apparent K-m for Mn-ATP of 5.8 mM and calculated V-max of 12 nmol cyclic AMP formed/min/mg. Data analysis using calculated equilibrium concentrations of free and complexed reactants provided similar estimates of these kinetic parameters. (C) 2003 Elsevier Inc. All rights reserved.