Protamine in the rat, its fate in vivo, and its degradation in vitro by egg homogenate

Spermatogenesis in the male rat involves chemical and physical transformation of chromatin. There is a shift in the chemical composition of rat testicular sperm chromatin containing histones to nucleoprotamines in the mature sperm. Rat protamine contains a high percentage of arginine. The biosynthesis and kinetics of H³‐arginine incorporation into rat sperm protamine was at its peak in the testis 2 days after intratesticular injection. The specific activity of protamine in the testis was reduced to 50% 5 days after injection. Specific activity of protamine in the epididymis was maximal at 9 days after injection and remained at a similar level for 12 days. The kinetics of H³‐arginine and H³‐thymidine in ejaculated sperm were compared. It was found that maximal specific activity of H³‐arginine (H³‐Arg/No. of sperm) in the spermatozoa was obtained 20–22 days following its intratesticular injection. The peak of H³‐thymidine specific activity (H³‐thymidine/No. of sperm) was obtained 53–59 days following the injection. H³‐Arg labeled sperm were capacitated in vitro and were allowed to penetrate zona free rat eggs in vitro. The disappearance of the label from the sperm head was observed 1–2 hr after penetration. Protamine labeled in vitro with I¹²⁵ incubated with extracts of freshly ovulated rat eggs shows that protamine could be degraded by the cytoplasmic extract (10–30%). The proteolytic activity is maximal at pH 7.0–8.0. Cytoplasmic extracts from cumulus cells incubated with I¹²⁵ rat protamine under identical conditions did not show any significant proteolytic activity. Trypsin degradation of rat protamine shows a different pattern of degradation. It is assumed that the H³‐labeled protamine in the sperm chromatin is removed and probably degraded early after penetration, making possible the remodeling of the sperm chromatin by embryonic histones.