Evidence for a novel adenylyl cyclase in human epididymal sperm

Since cAMP is considered to play a major role in the acquisition of maturation and fertilizing capacity of mammalian sperm, we investigated the expression of cAMP-synthesizing adenylyl cyclase (AC) in sperm retrieved directly from the human epididymis. Particulate fractions were prepared from purified epididymal sperm samples and AC was monitored by the direct conversion of ATP into cAMP. We report that in great contrast to human ejaculated sperm and other mammalian sperm cells, the human epididymal sperm do not express a Mn(2+)-sensitive AC. However, a functional AC was readily detectable in these sperm cells in the presence of saturating concentrations of Ca2+ (50mM) and bicarbonate (HCO3-, 50mM), a combination that causes maximal activation in human ejaculated sperm. Using these conditions, human epididymal sperm AC showed similar capacity to generate cAMP compared to human ejaculated sperm AC. When assays were performed in the presence of Mg2+ and a saturating concentration of GMP-P(NH)P (50 microM), the hydrolysis-resistant GTP analog, and forskolin (100 microM), no activity was detected indicating that the epididymal sperm AC differs from that in somatic cells. These data demonstrate 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.