Histone H3 disulfide reactions in interphase, mitotic, and native chromatin

In order to obtain information possibly relevant to the mechanisms of initiation and maintenance of chromosome condensation at mitosis, the histones of interphase and mitotic cultured Chinese hamster ovary cells have been compared with special reference as to whether disulfide linked dimers of histone H3 exist in vivo. Double label experiments show that the relative mass proportions of histones H2A, H2B, H3, and H4 are identical between interphase and mitotic chromatin, although mitotic chromatin is enriched in phosphorylated forms of histones H1 and H3. Reconstruction experiments show that if histone H3 disulfide dimers exist in vivo, they would be reduced upon extraction from either interphase or mitotic cells under a wide variety of conditions. Therefore, whether histone H3 dimers exist is an unanswered question, and earlier studies addressing this issue are open to considerable doubt. A further complication associated with quantitating histone H3 dimers is that chromatin bound H3 monomer was found to have a propensity to form disulfide bridges under conditions of homogenization at intermediate ionic strength. Histone H3 of mitotic chromatin had about a 3 fold greater tendency to dimerize artifactually compared to the H3 of interphase chromatin, reaching levels of up to 40% of the total H3 as dimer, even in the presence of 5 mM iodoacetamide. The ease of histone H3 dimerization suggested that a close proximity of H3 sulfhydryl groups may exist in chromatin. However, chemical oxidation of H3 thiols with 40 mM hydrogen peroxide did not lead to detectable inter or intramolecular disulfide bridges in calf thymus chromatin samples isolated by gentle means, shown to have intact polynucleosomal structures. In contrast, exposure of calf thymus chromatin to shear forces resulting in nucleosome damage created H3 homopolymers cross linked through disulfide bridges up to pentamers.