Aflatoxin B1, a potent hepatocarcinogen, causes rapid but reversible in vivo inhibition of DNA-dependent RNA polymerase in isolated nuclei. This study was designed to determine whether the consequent decrease of RNA formation occurs as a result of direct alteration of RNA polymerase. Aflatoxin B1, 1 mg/kg body weight, was administered i.p. to male rats; nuclei were isolated from liver homogenates at various times up to 24 hr thereafter. RNA polymerase was solubilized by sonic disruption, concentrated, and dialyzed. Some preparations were further purified by fractionation with diethylaminoethyl Sephadex chromatography. Methods were selected to measure nucleoplasmic or nucleolar RNA polymerase in intact nuclei and in crude solubilized or fractionated preparations with exogenous template (denatured calf thymus DNA). Nucleoplasmic enzyme activity of all three preparations was inhibited to 40% of control values at 2 through 9 hr after aflatoxin administration, but it returned to control values at 24 hr. Fractionated nucleolar polymerase activity 2 and 24 hr after toxin administration was not significantly different from control values. Addition of aflatoxin B1in vitro to nuclei and crude solubilized polymerase did not produce altered nucleotide incorporation. Aflatoxin B1in vivo appears directly to alter nucleoplasmic rat liver DNA-dependent RNA polymerase function. In the dose selected for this study, aflatoxin impairs nucleolar RNA polymerase much less, if at all. The failure of direct addition of aflatoxin B1 to alter 3H-labeled uridine 5′-triphosphate incorporation in vitro provides further support to the theory that it is altered metabolically to a toxic form.

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This work was supported in part by USPHS Research Grant AM08686 and American Cancer Society Grant E-480. A preliminary report of these data has been presented at the 1971 fall meeting of the American Association for the Study of Liver Diseases (27).

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