Shedding of a rat epididymal sperm protein associated with infertility induced by ornidazole and α-chlorohydrin

The protein composition of epididymal fluid and sperm extracts of rats treated with the nitroimidazole compound ornidazole was investigated by two-dimensional gel electrophoresis. Epididymal luminal fluid from the corpus and cauda regions of male animals rendered infertile by ornidazole treatment contained a prominent protein (contraception-associated protein 1, CAP1) with a molecular mass of similar to 25 kDa and an isoelectric point (pI) of 5.8; it was not found in fluids, but was present in sperm, from fertile vehicle-fed rats. Infrared matrix-assisted laser desorption/ionization mass spectrometry indicated that the molecular mass of CAP1 was 20 420 +/- 120 daltons. Analysis of 17 amino acids demonstrated 49% homology to a diuretic hormone from an insect (Acheta domesticus). Densitometric quantitation of CAP1 on silver-stained gels indicated its presence in greater amounts in cauda than in corpus fluid from treated animals, whereas fluid from the rete testis lacked CAP1. In vitro incubations of tissue from the caput, corpus, and cauda epididymidal regions with [S-35]methionine gave no hint that CAP1 was a secretion product of the epididymal epithelium. The absence of CAP1 from luminal fluid obtained from the sperm-depleted corpus epididymidis of efferent duct-ligated ornidazole-fed rats suggested a spermatozoal origin. CAP1 was present in spermatozoa from the caput epididymidis but not from the rete testis in control animals. Less CAP1 was present in detergent extracts of cauda sperm from ornidazole-treated rats than in sperm from control animals, suggesting a contraceptive-related displacement of protein from sperm to fluid. The association of ornidazole-and alpha-chlorohydrin-induced infertility with the presence of CAP1 in epididymal fluid, probably originating from spermatozoa, suggests a critical role for this protein in fertilization.