The presence in human sperm nuclear proteins of a limited amount of unoxidized thiol groups, stabilized by reversible binding to zinc ions, has been presumed to play a role in the decondensation of sperm within the oocyte. In the present study, the number and molecular localization of free sullhydryls in the major proteins of human sperm chromatin, protamines P1 and P2, were determined by alkylation of reactive thiols with 14 C-iodoacetamide, isolation of protamines, and peptide mapping.

Less than 1.5% of the cysteines of protamines were found as reactive thiols, a proportion strikingly lower than that reported previously for whole human sperm proteins. The amount of sulthydryls was unaffected by the zinc chelating agent EDTA. Labeling was evenly distributed on every cysteine of protamines P1 and P2. The results confirm the extensive stabilization of sperm chromatin by disulfide bridges and show that the unoxidized cysteines remaining at the end of epididymal transit in some protamine molecules may be one of the six (protamine P1) or five (protamine P2) cysteines present in the sequence of each class of protamines. This even distribution of the reactive cysteines could facilitate decondensation of sperm nuclei initiated by a thioldisulfide exchange.