Heparin and High-Density Lipoprotein Mediate Bovine Sperm Capacitation by Different Mechanisms1

Capacitation is an important process in bovine sperm maturation and is an obligatory step prior to fertilization. Two capacitating agents, namely heparin and high-density lipoprotein (HDL), have been shown to induce sperm capacitation. A family of major proteins of bovine seminal plasma designated BSP-A1/A2, BSP-A3, and BSP-30 kDa (collectively called BSP proteins) bind to the sperm surface upon ejaculation via their membrane choline phospholipids. Our previous studies with bovine epididymal sperm showed that BSP proteins potentiate sperm capacitation induced by heparin and HDL. This study was undertaken to clarify the mechanism of capacitation induced by heparin and HDL in the presence of BSP proteins. Washed bovine ejaculated sperm were incubated with heparin (12 microg/ml) or HDL (10-160 microg/ml) in the presence of polyclonal antibodies against purified BSP proteins (anti-BSP proteins). The percentage of capacitated sperm was evaluated after the induction of the acrosome reaction (AR) with lysophosphatidylcholine. When sperm were incubated for 5 h with heparin and anti-BSP proteins (40 microg/ml), the AR level was not significantly different from control levels (16. 8 +/- 0.9% vs. 12.9 +/- 0.9%). In contrast, incubation of sperm for 8 h with HDL and anti-BSP proteins did not inhibit the AR (42.4 +/- 1.1% vs. 17.1 +/- 1.6 for the control samples). We also investigated the effect of heparin and HDL on protein tyrosine phosphorylation associated with capacitation. The tyrosine phosphorylation of a group of proteins was increased in the presence of heparin. However, HDL did not significantly stimulate protein phosphorylation. The increase in phosphorylation was correlated with an increase in the AR after the incubation with heparin but not with HDL. These results indicate that heparin and HDL mediate capacitation via different mechanisms.