Time-related binding of the hepatocarcinogen carbon tetrachloride to hepatic chromatin proteins in vitro

The in vitro covalent binding of ¹⁴C-labelled carbon tetrachloride[¹⁴C]CCl₄ to histones and non-histone chromosomal proteins (NHCP)under microsome-mediated aerobic conditions was determined.Whole chromatin was prepared from purified nuclei isolated fromlivers of B6C3F1 hybrid mice and incubated with 2.5, 5.0 and10.0 µmol [¹⁴C]CCl₄, in the presence of microsomes isolatedfrom the same tissue, at 4 mg protein, and an NADPH-regeneratingsystem at 37°C for varying incubation times. Binding of[¹⁴C]CCl₄ to histones and NHCP was also determined in the presenceof 5 mM L-cysteine. The results show that the activated intermediateof CCl₄ bound more to histones than to NHCP in a dose- and time-dependentmanner, and that 5 mM L-cysteine inhibited the binding of theactivated intermediate of CCl₄ to histones by 59%, without affectingthe binding to NHCP. These data suggest different extents ofalkylation or acylation between histones and NHCP by metabolicallyactivated CCl₄ under aerobic in vitro conditions, and differentialinhibition of CCl₄-alkylation-acylation by cysteine. This suggestiondoes not exclude other possible mechanisms of action.