Plasma membrane bound Ca2+-ATPase activity in bull sperm.

The acrosome reaction in vertebrate sperm has been shown to be essential for fertilisation [ 11. The process involves fusion and vesiculation of sperm head plasma membranes and the underlying outer acrosomal membrane, resulting in the exocytotic release of acrosomal enzymes. Yanagamachi and Usui have demonstrated that sperm have an absolute requirement for Ca*’ during the acrosome reaction [2]. A net uptake of Ca*’ has been identified [3] and experiments with the Ca*+ ionophore A23 187 suggest that Ca*+ transmembrane fluxes govern the induction of the acrosome reaction [4]. On the basis of electron microscopic observations Gordon has suggested that an ATPase, which is not stimulated by Cap’, transports Ca*’ into the periacrosomal space and a Ca*+ dependent ATPase located on the outer acrosomal membrane transport this Ca*’ into the acrosome, thus playing a crucial role in the induction of the acrosome reaction [ 5,6]. However, this hypothesis has been challenged by reports of the absence of Ca**-ATPase activity in guinea pig sperm [2], though earlier reports identified Ca*‘-ATPase activity on sperm membranes by cytochemical methods [5]. In the course of biochemical characterisation of the various subcellular fractions of bull sperm, we have detected the presence of a Ca*‘-ATPase in the plasma membrane fraction. In this communication we report the observation of Mg*+-independent Ca*+ATPase activity, confined primarily to a fraction enriched in sperm plasma membranes. This enzyme is inhibited by the anionic, hydrophobic (fluorescent