Incorporation of selenium-75 into seminal plasma and spermatozoa of the bull

75Se was given intravenously into 5 bulls. Multiple blood and semen samples were taken and during slaughter 5, 10, 15, 20 and 80 days later samples of various reproductive and other organs were collected. After injection, 75Se in blood reached a peak at 6 h followed by a rapid decline. The label was mainly found in serum with very low levels in erythrocytes. Initially the serum 75Se was bound to a macromolecule with a mw of 80 kDa, but later a larger molecule (100 kDa) was observed. In semen 75Se was first mainly found in seminal plasma, where a plateau level was reached at 5 d followed by a gradual decline after 12 d. The total semen level, however, increased after 14 d and this increase was due to a rapid appearance of the label in spermatozoa. The sperm 75Se level reached a plateau at 20 d and remained high until 40 days, after which a gradual decline ensued. The seminal plasma 75Se eluted in gel filtration coincident with glutathione peroxidase. The highest levels of 75Se were found in the kidney followed by seminal vesicles and testicles. The seminal vesicle secretion was particularly rich in 75Se and its fractionation resembled that of the seminal plasma. 75Se appeared in the epididymal caput within 5 days and passed through the epididymis in 20 days. It is concluded that 75Se is actively incorporated in the bull seminal vesicles into GSH-Px, while in the testis it is incorporated into a structural sperm protein during spermatogenesis.