Abstract
Biochemical aspects of post‐testicular sperm maturation have been studied. Partial purification of either human or ram seminal plasma yields a heat‐stable, acidic protein which is a Ca2+‐dependent regulator of 3′:5′‐cyclic nucleotide phosphodiesterase. This protein fraction has been provisionally named calsemin. Human calsemin activates the flagellar plasma membrane Ca2+‐ATPase of ram epididymal and ejaculated spermatozoa by 290% and 20%, respectively. Activation is abolished by trifluoperazine. The addition of calsemin plus Ca2+ to isolated ram caudal spermatozoa results in a three‐fold stimulation of flagellar beat activity. These results lead us to propose that only after interaction between calsemin and the sperm flagellar plasma membrane can the Ca2+‐ATPase obtain full expression to maximise Ca2+ efflux from the flagellum and thereby cause a stimulation and coordination of sperm flagellar beat activity. This finding may provide a partial biochemical explanation for the development of mammalian sperm motility.
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