Deterioration of goat sperm viability in milk extenders is due to a bulbourethral 60-kilodalton glycoprotein with triglyceride lipase activity.

Abstract

The aim of this work was to purify, identify, and characterize the component of goat bulbourethral gland secretion (BUS) responsible for goat sperm deterioration in skim milk extender. BUS extracts promote a decrease in the percentage of motile spermatozoa, deterioration in the quality of movement, breakage of acrosomes, and cellular death of goat spermatozoa diluted in skim milk. A 55- to 60-kDa monomeric glycoprotein (BUSgp60) was purified by cation-exchange, concanavalin A, and heparin-affinity chromatography and was identified as the only BUS component responsible for the deterioration of spermatozoa in milk. The BUSgp60 was shown to display triacylglycerol hydrolase activity, and partial sequences (37 amino acids in all) exhibited 50-70% homology with sequences of various types of pancreatic lipases (PLs), especially PL-related protein 2 (PL-RP2). In addition, porcine PL produced deterioration of goat sperm properties in milk as effectively as BUS and BUSgp60. These results support the preliminary identification of the goat BUSgp60 as a bulbourethral lipase belonging to the PL-RP2 family. Since seminal plasma also contains factors favorable for sperm survival, it is envisaged that, for the best preservation of goat semen, specific inhibitors of this family of lipase could be added into milk-based extenders without eliminating seminal plasma.