Multiple activation of chloroform in hepatic microsomes from uninduced B6C3F1 mice

The covalent binding of 14C-label to proteins and lipids was measured after incubation of hepatic microsomes from B6C3F1 mice with different concentrations of [14C]chloroform and oxygen. The effect of reduced glutathione on the covalent binding curves was also investigated. The results indicated that chloroform is activated through three processes: the first, oxidative, shows high affinity for chloroform and low affinity for oxygen; the second, also requiring oxygen, shows low affinity for chloroform and high affinity for oxygen; and the third, showing low affinity for chloroform, is inhibited by oxygen. The covalent binding associated with the oxidative processes is very effectively prevented by GSH. The reactive metabolites formed by the O2-inhibited mechanism are not efficiently scavenged by GSH and presumably are radicals that are produced reductively. The major conclusions which can be drawn from these results are: (i) The anoxic bioactivation of chloroform can cause high levels of covalent binding. This is at variance with the current opinion that the chloroform anoxic bioactivation occurs to a negligible extent. (ii) The damages produced under the usual in vitro experimental conditions by the oxidative biotransformation of chloroform, may be strongly limited by the physiological conditions of the liver. The features of the three processes described may help in understanding the mechanism of toxicity of chloroform.