Ni(II) specifically cleaves the C-terminal tail of the major variant of histone H2A and forms an oxidative damage mediating complex with the cleaved-off octapeptide

The acetyl-TESHHK-amide peptide, modeling a part of the C-terminal "tail" o f histone H2A, was found previously by us to undergo at pH 7.4 a Ni(II)-ass isted hydrolysis of the E-S peptide bond with formation of a stronger Ni(II ) complex with the SHHK-amide product [Bal, W., et al. (1998) Chem. Res. To xicol. 11, 1014-1023]. To further characterize the hydrolysis and test the resulting Ni(II) complex for redox activity, bovine histone H2A and three p eptides were investigated: acetyl-LLGKVTIAQGGVLPNIQAVLLPKKTESHHKAKGK (H2A(3 4)), modeling the entire "C-tail" of H2A; SHHKAKGK (H2A(8)), modeling the c utoff product of hydrolysis; and acetyl-KTESHKAKGK (H2A(10)), modeling a pu tative Ni(II) binding site in a minor variant H2A.4 of human histone H2A. T he Ni(II)-assisted hydrolysis of H2A and H2A(34) was found to proceed appro ximately 7-fold faster than that of the Ni(II)- acetyl-TESHHK-amide complex under comparable conditions. In both cases, the Ni(II) complex with H2A(8) was the smaller product of the hydrolysis, indicating a high site specific ity of the reaction. Of three other metals tested with H2A(34), only Cu(II) cleaved the E-S bond, although much less efficiently than Ni(II); Co(II) a nd Zn(II) had no effect whatsoever. The H2A(10) peptide appeared to be full y resistant to hydrolytic cleavage and did not exhibit any redox activity v ersus H2O2 in the presence of Ni(II) at pH 7.4. Likewise, redox-inactive wa s the Ni(II)-H2A(34) complex. In contrast, the Ni(II)-H2A(8) complex promot ed oxidative damage of pUC19 DNA by H2O2, evidenced by a significant increa se in the number of single strand breaks and nucleobase modifications typic al for a hydroxyl radical-like species attack on DNA. Interestingly, instea d of 8-oxopurines, the corresponding formamidopyrimidines were the major pr oducts of the damage. The difference in redox activity between the Ni(II)-H 2A(34) and Ni(II)-H2A8 complexes is most likely associated with their diffe rent geometries: octahedral and square planar, respectively. Incubation of the Ni(II)-H2A8 complex with H2O2 also resulted in degradation of the pepti de ligand, especially at its Ser and His residues. Thus, binding of Ni(II) to the ESHHK motif of the histone H2A C-tail is damaging to the histone C-t erminal tail and to histone-associated DNA. The results support a dual mech anism of Ni(II)-induced carcinogenesis, including both genotoxic and epigen etic effects.