EVIDENCE FOR A NOVEL ADENYLYL-CYCLASE IN HUMAN EPIDIDYMAL SPERM

Since cAMP is considered to play a major role in the acquisition of ma turation and fertilizing capacity of mammalian sperm, we investigated the expression of cAMP-synthesizing adenylyl cyclase (AC) in sperm ret rieved directly from the human epididymis. Particulate fractions were prepared from purified epididymal sperm samples and AC was monitored b y the direct conversion of ATP into cAMP. We report that in great cont rast to human ejaculated sperm and other mammalian sperm cells, the hu man epididymal sperm do not express a Mn2+-sensitive AC. However, a fu nctional AC was readily detectable in these sperm cells in the presenc e of saturating concentrations of Ca2+ (50mM) and bicarbonate (HCO3-, 50mM), a combination that causes maximal activation in human ejaculate d sperm. Using these conditions, human epididymal sperm AC showed simi lar capacity to generate cAMP compared to human ejaculated sperm AC. W hen assays were performed in the presence of Mg2+ and a saturating con centration of GMP-P(NH)P (50muM), the hydrolysis-resistant GTP analog, and forskolin (100muM), no activity was detected indicating that the epididymal sperm AC differs from that in somatic cells. These data dem onstrate that human epididymal sperm contain an AC that is unique and different from the enzyme system described in somatic cells and other mammalian sperm cells, including human ejaculated sperm.