Biochemical Distinctions between the Nuclear and Microsomal Membranes from Rat Hepatocytes: THE EFFECT OF PHENOBARBITAL ADMINISTRATION

Abstract

The nuclear membrane from rat hepatocytes has been examined for components of the DPNH and TPNH electron transport chains as well as two drug-metabolizing enzymes, benzo[a]pyrene hydroxylase and an aminoazo dye N-demethylase, in control and phenobarbital-treated animals. The enzyme induction phenomenon characteristic of the microsomes was not observed with the nuclear membrane for any of the enzymes or pigments studied. The nuclear electron transport enzyme DPNH-cytochrome c reductase and cytochrome b5 were found to follow the same pattern as their microsomal counterparts. Thus, in each case, the reductase level was depressed in the treated animals while the cytochrome b5 level was not significantly altered. In control animals, the specific activity of TPNH-cytochrome c reductase in the nuclear membrane was approximately one-third that found for the same enzyme associated with the microsomes. The specific activity of this enzyme remained unchanged after phenobarbital was administered. The occurrence of cytochrome P-450 in the nuclear membrane was variable. No cytochrome P-450 was demonstrable in membranes from phenobarbital-treated animals, but low levels were detected in three out of 10 individual control preparations. Measurable amounts of aryl hydroxylase and N-demethylase were found in the nuclear membrane which corresponded to 7 and 11% of the specific activity obtained for the same enzymes in the microsomal membrane, respectively. Failure of these enzymes to be induced suggests that they are not of microsomal origin. Clearly, the differential response of the nuclear and microsomal membranes to phenobarbital indicates that quite distinct regulatory mechanisms must be operative for the two membranes with respect to the metabolic control of TPNH-cytochrome c reductase and the drug-metabolizing enzymes.