A 34 kD selenoprotein purified from rat testis was identified as a specific sperm nuclei glutathione peroxidase (snGPx) with similar properties to phospholipid hydroperoxide glutathione peroxidase (PHGPx). The determination of its primary structure by analysis of its first N‐terminal amino acids, database search, polymerase chain reaction, and sequencing of the cDNA showed that it differs from PHGPx in its N‐terminal sequence. This sequence, which is encoded for by an alternative exon in the first intron of the PHGPx gene, shows more than 50% homology to the protamine sequences and contains a nuclear localization signal. In rats, snGPx is highly expressed in the nuclei of the late spermatids where it is the only selenoprotein present. Its appearance coincides with the reorganization of DNA, which leads to highly condensed chromatin stabilized by cross‐linked protamine thiols. In selenium‐depleted rats where the concentration of snGPx had decreased to one‐third of the normal level, chromatin condensation was severely disturbed. We provided evidence that snGPx acts as a protamine thiol peroxidase responsible for disulfide cross‐linking by reduction of reactive oxygen species. Its dual function in chromatin condensation and the protection of sperm DNA against oxidation is necessary to ensure male fertility and sperm quality.

By a combination of trace techniques and various biochemical methods, information about the characteristics of a 15-kDa selenoprotein was obtained. After labeling of rats in vivo with [(75)Se]selenite, subcellular fractionation of the homogenates of the prostate, lung, brain, thyroid gland, and large intestine, and gel electrophoretic separation of the proteins and subcellular fractionation, 15-kDa (75)Se was found in the cytosols of the tissues prostate > brain > lung > thyroid gland > large intestine after autoradiography. After coelectrophoresis of the separated 15-kDa labeled band obtained from each cytosolic fraction, the 15-kDa (75)Se band migrated in the same way as the combined bands isolated from the five tissue cytosols. After proteolytic cleavage in the gel of the 15-kDa labeled band obtained from the cytosol of each tissue and re-electrophoresis, the same labeled peptide pattern was found in each gel slice after autoradiography. By means of reversed-phase HPLC, we characterized a selenocysteine-containing protein that has enzymatic activity like that of glutathione peroxidase.