Bioactivation and covalent binding of halothane in vitro: studies with [3H]- and [14C]halothane

To determine if the hydrogen atom of halothane (CF3CHBrCl) is retained on the reactive intermediates that covalently bind to microsomal lipids and protein, [3H]halothane and [14C]halothane were incubated with rat hepatic microsomes and a NADPH generating system. Both [3H]- and [14C]halothane were bioactivated and bound to a greater degree when incubations were performed in a N2 atmosphere rather than an O2 atmosphere. Binding of [3H]- and [14C]halothane equivalents was significanty enhanced when heaptic microsomes from phenobarbital- or Aroclor 1254-treated rats were used in the incubations. Omission of NADPH or incubation with CO was inhibitory to the binding of both [3H]- and [14C]halothane. The apparent kinetic constants for binding or halothane equivalents, Km and Vmax, indicate a significantly higher Km but lower Vmax for the formation and/or binding of 3H-binding equivalents. The results indicate tht halothane is primarily bioactivated under conditions that promote its reductive metabolism and that this reactive metabolism does not involve cleavage of the carbon-hydrogen bond of halothane. Differences in binding under N2 and O2 as well as between [3H]- and [14C]halothane suggest that multiple reactive intermediates may form during the biotransformation of halothane.