Milk caseins decrease the binding of the major bovine seminal plasma proteins to sperm and prevent lipid loss from the sperm membrane during sperm storage.

Milk is used as a medium for sperm preservation. Caseins, the major proteins of milk, appear to be responsible for the protective effect of milk on sperm. Recently, we have shown that egg yolk, which is also widely used to preserve semen, protects sperm functions by preventing the binding to sperm of the major proteins of bull seminal plasma (BSP proteins), thereby preventing BSP protein-mediated stimulation of lipid loss from the sperm membrane. In the present study, we investigated whether milk caseins protect sperm in the same manner as egg yolk. Bovine ejaculates were diluted with skimmed milk permeate (skimmed milk devoid of caseins) or permeate that was supplemented with caseins and stored at 4 degrees C for 4 h. In the semen diluted with permeate, sperm viability and motility decreased in a time-dependent manner. However, in semen diluted with milk or permeate supplemented with caseins, sperm functions were maintained. In addition, lower amounts of the BSP proteins were associated with sperm in semen diluted with milk or permeate supplemented with caseins, as compared to semen diluted with permeate. No milk proteins were detected in the sperm protein extracts. Furthermore, sperm diluted with milk or permeate supplemented with caseins showed 3-fold lower losses of cholesterol and choline phospholipids than sperm diluted with permeate during storage. Thus, milk caseins decreased the binding of BSP proteins to sperm and reduced sperm lipid loss, while maintaining sperm motility and viability during storage. These results support our view that milk caseins prevent the detrimental effects of BSP proteins on the sperm membrane during sperm preservation.

Low-density lipoprotein fraction from hen's egg yolk decreases the binding of the major proteins of bovine seminal plasma to sperm and prevents lipid efflux from the sperm membrane.

For sperm preservation, semen is generally diluted with extender containing egg yolk (EY), but the mechanisms of sperm protection by EY are unclear. The major proteins of bull seminal plasma (BSP proteins: BSP-A1/A2, BSP-A3, and BSP-30-kDa) bind to sperm surface at ejaculation and stimulate cholesterol and phospholipid efflux from the sperm membrane. Since EY low-density lipoprotein fraction (LDF) interacts specifically with BSP proteins, it is proposed that the sequestration of BSP proteins in seminal plasma by EY-LDF represents the major mechanism of sperm protection by EY. In order to gain further insight into this mechanism, we investigated the effect of seminal plasma, EY, and EY-LDF on the binding of BSP proteins to sperm and the lipid efflux from the sperm membrane. As shown by immunodetection, radioimmunoassays, and lipid analysis, when semen was incubated undiluted or diluted with control extender (without EY or EY-LDF), BSP proteins bound to sperm in a time-dependent manner, and there is a continuous cholesterol and phospholipid efflux from the sperm membrane. In contrast, when semen was diluted with extender containing EY or EY-LDF, there was 50%-80% fewer BSP proteins associated with sperm and a significant amount of lipid added to sperm membrane during incubation. In addition, sperm function analysis showed that the presence of EY or EY-LDF in the extender preserved sperm motility. These results show that LDF is the constituent of EY that prevents binding of the BSP proteins to sperm and lipid efflux from the sperm membrane and is beneficial to sperm functions during sperm preservation.