In vitro polymerization of histones by carcinogenic nickel compounds

This study was undertaken to explore whether nuclear chromatin constituents can participate in and/or be affected by redox reactions catalyzed by nickel, like those of nickel complexes with small peptides, e.g. tetraglycine (G4) and oxygen. Calf thymus DNA, nucleohistone (NH) or free histones were incubated at 37 degrees C, pH 7.6, for up to 96 h with nickel(II)acetate (NiAcet) or nickel subsulfide (Ni3S2) and/or G4. The effects on DNA and NH were studied by means of melting profiles. Free individual histones and histones extracted from NH prior to and after exposure to nickel compounds and/or G4 were examined by electrophoresis on polyacrylamide gels. Two-day exposure of DNA to NiAcet, G4, or NiAcet + G4 had no significant effect on its melting temperature. Incubation of NH with NiAcet, however, markedly increased its melting temperature by 2.2 +/- 0.3 degrees C after 24 h and 3.0 +/- 0.5 degrees C after 96 h (P less than 0.01 versus NH alone at either time). Incubation of NH with NiAcet + G4 also resulted in a significant rise of the melting temperature by 1.4 +/- 0.3 degrees C after 24 h (P less than 0.05) and 5.5 +/- 0.3 degrees C after 96 h (P less than 0.0001). G4 alone had no effect. Exposure of NH to NiAcet + G4, but not to the individual chemicals, slowly decreased solubility of the histone components in 0.2 M H2SO4. Only trace amounts of histones could be extracted from NH with acid after 72-h exposure to NiAcet + G4. Treatment of free histones with NiAcet, Ni3S2 and/or G4 resulted in a slow random polymerization of the proteins by NiAcet + G4, Ni3S2 + G4 and Ni3S2 alone, but not NiAcet or G4 alone. The action of Ni3S2 alone was slower than that of either nickel compound combined with G4. The present findings indicate that nickel carcinogens NiAcet and Ni3S2, in the presence of G4 or even alone (Ni3S2), are capable of causing protein-protein and perhaps also protein-DNA crosslinking. Reactions of this type may be involved in the mechanism(s) of nickel carcinogenesis.