Summary. Both the low-density lipoprotein fraction of egg yolk (LDF) and sonicated lecithin liposomes provided an equal measure of protection for ram spermatozoa during cold shock, but LDF was superior to lecithin during cold storage. The protective activity of LDF during storage at 5°C was not altered by a subfractionation procedure which did not alter the molecular organization. Removal of the protein from the surface of LDF particles gave preparations with altered lipid:protein ratios. Neither the low-lipid fraction nor bovine plasma albumin protected spermatozoa but the high-lipid fraction was as protective as LDF. Survival of spermatozoa decreased as the lipid:protein ratio fell below 1·67 compared with a ratio of 4·76 for LDF. The absolute lipid content was more important than the ratio except at low ratios. Lipid-depleted preparations bound more effectively to the plasma membrane than did LDF whereas the lipid-enhanced preparation showed little binding capability.
It is concluded that the phospholipid of LDF provides the protection to the sperm cell membrane. The protein of LDF serves to solubilize the lipid and bind it to the cell membrane. The importance of the role of the protein during cold storage is discussed.