Gamete Biology

Structural Features of Sterols Required to Inhibit Human Sperm Capacitation

Abstract

Ejaculated mammalian sperm must undergo a final maturation (capacitation) before they can acrosome-react and fertilize eggs. Loss of cholesterol is an essential step in the capacitation of human sperm. Experimentally maintaining a high level of cholesterol inhibits capacitation, but the mechanism is unknown. This study investigated the structural features that are required for cholesterol's inhibitory activity. Human sperm also contain much desmosterol, and it is lost from sperm during capacitation. Preventing the loss of desmosterol inhibited capacitation (assessed by acrosomal responsiveness), with effectiveness about equal to cholesterol. Other structural analogs were added to the incubation medium to replace sperm cholesterol and desmosterol. Most inhibited capacitation, including those that lacked cholesterol's 3-OH group (cholesterol methyl ether and epicholesterol) and those with modified C17 groups (ergosterol and diosgenin). Two steroids did not inhibit capacitation well. Coprostanol, which has a nonplanar steroid nucleus, had low inhibitory activity that could be explained by an elevated endogenous cholesterol concentration. Epicoprostanol, which has a nonplanar ring structure and a 3-OH group, promoted capacitation rather than inhibiting it. The inhibitory activity of the analogs was correlated with their ability to promote order of egg phosphatidylcholine as measured by fluorescence anisotropy. In summary, a planar ring structure is required for sterol inhibitory activity but a 3-OH group and a saturated cholesterol-like aliphatic tail on C17 are not required. The results support the hypothesis that sperm sterols block capacitation by increasing order of phospholipids.