Mammalian sperm preservation in extenders containing egg yolk (EY) and/or milk has been used for over half a century. However, the mechanism by which EY or milk protects sperm during storage remains elusive. Studies conducted over the past two decades in our laboratory have revealed that a family of lipid-binding proteins (BSP proteins) present in bull seminal plasma is detrimental to sperm preservation since these proteins induce cholesterol and phospholipid removal from the sperm membrane. Interestingly, these detrimental factors of seminal plasma interact with the low-density lipoproteins (LDL) present in EY. This interaction minimizes lipid removal from the sperm membrane, which positively influences sperm storage in liquid or frozen states. Based on several lines of evidence, we suggest that the sequestration of BSP proteins by LDL (BSP proteins: lipoprotein interaction) is the major mechanism of sperm protection by EY. Skimmed milk, which is devoid of lipoproteins, also protects sperm during storage. Several studies indicate that the active components involved in sperm protection by milk are casein micelles. Thus, it appears that the mechanism by which milk protects sperm involves a BSP protein: casein micelle interaction. In view of these new insights, novel strategies have been suggested to improve the efficiency of semen preservation.

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.

Over the past 60 years, egg yolk (EY) has been routinely used in both liquid semen extenders and those used to cryopreserve sperm. However, the mechanism by which EY protects sperm during liquid storage or from freezing damage is unknown. Bovine seminal plasma contains a family of proteins designated BSP-A1/-A2, BSP-A3, and BSP-30-kDa (collectively called BSP proteins). These proteins are secretory products of seminal vesicles that are acquired by sperm at ejaculation, modifying the sperm membrane by inducing cholesterol efflux. Because cholesterol efflux is time and concentration dependent, continuous exposure to seminal plasma (SP) that contains BSP proteins may be detrimental to the sperm membrane, which may adversely affect the ability of sperm to be preserved. In this article, we show that the BSP proteins bind to the low-density fraction (LDF), a lipoprotein component of the EY extender. The binding is rapid, specific, saturable, and stable even after freeze-thawing of semen. Furthermore, LDF has a very high capacity for BSP protein binding. The binding of BSP proteins to LDF may prevent their detrimental effect on sperm membrane, and this may be crucial for sperm storage. Thus, we propose that the sequestration of BSP proteins of SP by LDF may represent the major mechanism of sperm protection by EY.