Identification of a fertility-associated protein in bull seminal plasma as lipocalin-type prostaglandin D synthase.

The objective of this study was to characterize a 26-kDa seminal plasma protein previously shown to be prevalent in bulls of high fertility. Spots of this protein, excised and electroeluted from two-dimensional SDS-PAGE gels, were used for N-terminal amino acid sequencing and for preparation of antiserum in rabbits. The N-terminal amino acid sequence (ALQPNFEEDKFLGRWFTSGL) was 75% identical and 100% homologous to lipocalin-type prostaglandin (PG) D synthase isolated from human cerebrospinal fluid (CSF). Western blots of purified 26-kDa protein cross-reacted with polyclonal antibodies against lipocalin-type PGD synthase isolated from rat brain and human CSF. Immunoreactive bands at 26 kDa appeared in Western blots of seminal plasma and cauda epididymal fluid (CEF). A 29-kDa band appeared in blots of rete testis fluid (RTF). PGD synthase activity was detected in seminal plasma, CEF, and RTF. The cDNA for bovine lipocalin-type PGD synthase, isolated by reverse transcription-polymerase chain reaction, contained a coding region of 573 base pairs corresponding to 191 amino acids. The amino acid sequence was 63-80% identical to that of the enzyme of other mammals. These results establish that the 26-kDa fertility-associated protein in bull seminal plasma is lipocalin-type PGD synthase. Although we do not yet know the role of lipocalin-type PGD synthase in the male genital tract, we speculate that this protein may play an important role in both the development and the maturation of sperm.