Water insoluble fraction of egg yolk maintains porcine sperm motility by activating adenylate cyclase.

The decrease in motility of porcine cauda epididymal sperm was less than that of caput epididymal sperm in the medium containing bicarbonate. This may be due to the difference of sensitivity of adenylate cyclase to bicarbonate between mature and immature sperm; activation of mature sperm enzyme by bicarbonate was higher than that of immature sperm. Nondialysable fraction of egg yolk prevented the decrease in motility of immature sperm in the presence of bicarbonate, but it was not effective for the motility of mature sperm under the same condition, because only bicarbonate is sufficient for the maintenance of its motility. In the absence of bicarbonate, both mature and immature sperm required egg yolk to maintain motility. The favorable effect of egg yolk on the motility is ascribed to the enhancement of intracellular cAMP level. Partial fractionation of egg yolk showed that water-insoluble lipoprotein fraction contains factor(s) which activates adenylate cyclase in sperm plasma membrane. This is the first report in which high molecular weight activator of the sperm enzyme was demonstrated.