Alterations in mouse liver monooxygenases by benzothiadiazoles

Abstract

Administration of 1,2,3-benzothiadiazoles to mice had a biphasic effect on liver microsomal monooxygenases. During the first 15 hr of treatment, an inhibition of the in vivo metabolism of hexobarbital, as well as of the in vitro hydroxylation of naphthalene and N-demethylation of aminopyrine, was observed. An apparent decrease in cytochrome P-450 and in the activity of the NADPH-cytochrome c reductase also occurred. The levels of cytochrome b 5 and NADH-cytochrome c reductase activity were only slightly affected. A shift to 452 nm in the carbon monoxide difference spectrum was obtained with dithionite-reduced microsomes and this was not modified by ferricyanide. After the initial inhibitory phase, an enhancement of drug-metabolizing activities in vivo and in vitro and in the levels of some components of the mixed function oxidase system was observed. The carbon monoxide difference spectra of dithionite-reduced microsomes returned to a maximal absorption at 450 nm. The stimulatory effect on monooxygenase activity, elicited by benzothiadiazoles, was prevented completely by actinomycin D and was accompanied by increases in liver weight, microsomal protein, and incorporation of labeled amino acids into microsomal protein, as well as by proliferation of smooth and rough endoplasmic reticulum. Acrylamide gel analysis of liver microsomes from mice, given a single dose of 6-chloro-1, 2,3-benzothiadiazole 48 hr prior to being killed, showed preferential induction of cytochrome P-450 apoproteins of 50,000, 52,000 and 53,000 molecular weight.