Localization of phosphatase activity on the membranes of the mammalian sperm head

Abstract

Abstract Epididymal and ejaculated rabbit sperm and ejaculated human sperm were incubated at pH 7.0 and pH 9.0 to test for phosphatase activity. With the substrates tested, ATP, ADP, AMP, ITP, p‐nitro‐phenyl phosphate, and glucose‐6‐phosphate, enzymes capable of dephosphorylating ATP at pH 7.0 were localized on the periacrosomal portion of the plasmalemma in epididymal but not in ejaculated sperm. The enzyme was active without divalent cations. In both epididymal and ejaculated sperm the outer membrane of the acrosome was reactive at pH 9.0 with ATP and Ca ++ . These strikingly different properties of the phosphatases on the plasmalemma and acrosome membrane indicate that the enzymes may be adapted to respond to particular environmental stimuli and function in tandem to bring about the capacitated state and acrosome reaction. The fact that the plasmalemmal enzyme could not be elicited in ejaculated sperm suggests that seminal plasma may contain an inhibitor for this enzyme which is removed in utero. Calcium ions are necessary for activation of acrosomal proteinases. During capacitation in the uterus, the plasmalemmal psphatase may move exogenous Ca ++ into the cytosol surrounding the acrosome, thus stimulating the acrosomal membrane enzyme which may then transport Ca ++ into the acrosomal vesicle. Calcium ions may also bind the plasma lemma to the outer acrosome membrane and so prepare the membranes for the acrosome reaction.