DOI 10.1095/biolreprod.102.013052 Decrease in Order of Human Sperm Lipids During Capacitation1

Ejaculated mammalian sperm must undergo a final matura-tion (capacitation) before they can acrosome-react and fertilize eggs. Loss of the sperm sterols, cholesterol and desmosterol, is an obligatory step in the capacitation of human sperm. Because sterols can increase the order of membrane phospholipids, it has been suggested that the importance of sterol loss is that it de-creases membrane lipid order. The present study tested the hy-potheses that sterol loss decreases sperm membrane lipid order during capacitation and that lipid disorder is a sufficient stim-ulus for capacitation. Steady-state fluorescence anisotropy of the membrane probe, 1,6-diphenyl-1,3,5-hexatriene, decreased during capacitation, indicating a decrease in lipid order. The de-crease was dependent on the loss of sperm sterols, suggesting that it reflected diminished sterol-mediated phospholipid order-ing. However, the lipid-fluidizing agents, benzyl alcohol and 2-(2-methoxyethoxy)ethyl 8-(cis-2-n-octylcyclopropyl) octanoate, did not cause sperm capacitation or overcome inhibition by cho-lesterol. In summary, loss of sperm sterols caused a significant decline in lipid order during capacitation; however, decreased bulk lipid order was not sufficient to trigger the subsequent events that complete capacitation. acrosome reaction, gamete biology, sperm, sperm capacitatio