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 from livers of B6C3F1 hybrid mice and incubated with 2.5, 5.0 and 10.0 mumol [14C]CCl4 in the presence of microsomes isolated from the same tissue, at 4 mg protein, and an NADPH-regenerating system at 37 degrees C for varying incubation times. Binding of [14C]CCl4 to histones and NHCP was also determined in the presence of 5 mM L-cysteine. The results show that the activated intermediate of CCl4 bound more to histones than to NHCP in a dose- and time-dependent manner, and that 5 mM L-cysteine inhibited the binding of the activated intermediate of CCl4 to histones by 59%, without affecting the binding to NHCP. These data suggest different extents of alkylation or acylation between histones and NHCP by metabolically activated CCl4 under aerobic in vitro conditions, and differential inhibition of CCl4-alkylation-acylation by cysteine. This suggestion does not exclude other possible mechanisms of action.