Freezing–thawing induces alterations of the nucleoprotein–DNA binding through the breaking of disulfide bonds in boar sperm

Previous reports proved that freezing–thawing alters the main nucleoprotein structure in boar sperm. The main aim of this work was to gain insight in the mechanisms, by which freezing–thawing induces such alteration. For this purpose, the alteration of freezing–thawing on the two main molecular mechanisms that modulate the protein–DNA linking, protein tyrosine phosphorylation and protein–DNA disulfide bounds was investigated. Protein tyrosine phosphorylation was analysed through Western blot analyses in samples previously subjected to immune-precipitation. Furthermore, the putative changes on disulfide bounds were studied through analysis of free cysteine radical levels. Freezing–thawing did not have any significant effect on the tyrosine phosphorylation levels of both protamine 1 and histone H1. However, thawed samples showed a significant (p < 0.05) increase in the free cysteine radical content (from 3.1 ± 0.5 nmol/lg protein in fresh samples to 6.7 ± 0.8 nmol/lg protein). In summary, our results suggest that freezing–thawing induces significant alterations in the nucleoprotein structure of boar sperm head by mechanism(s) linked with the rupture of disulfide bonds among the DNA. These mechanisms seem to be unspecific, affecting both the protamine–DNA unions and the histones– DNA bonds in a similar way.