Hydrolysis of extracellular adenine nucleotides by equine epidydimal spermatozoa

Ectoenzymic activities capable of hydrolyzing ATP sequentially to adenosine are present on equine epidydimal spermatozoa membranes. Kinetic parameters for ATPase, ADPase and 5'-nucleotidase were obtained by analysis of progress reactions curve when ATP, ADP and AMP were supplied as initial substrates. These values are not different from those found when the substrates were supplied from the preceding reactions. Feed-forward inhibition on 5'-nucleotidase by ATP/ADP was taken into account to fit simulated data to the experimental results. None of the substrates supplied by the preceding reactions showed a preferential delivery to ADPase and/or 5'-nucleotidase. We therefore conclude that the model that fits the equine spermatozoa is that already proposed for pig aortic endothelial cells.