Inhibition of Rat‐Liver RNA Polymerase in vitro by Aflatoxin B₁ in the Presence of a Microsomal Fraction

Abstract

Aflatoxin B 1 inhibits rat liver nucleoplasmic RNA polymerase B (40–50%) and cytoplasmic RNA polymerase C activities (25–35%) if applied in vivo . Nucleolar RNA polymerase A activity is not inhibited under the same conditions. Aflatoxin B 1 has no effect on the activities of purified RNA polymerase enzymes A, B and C or on [ 3 H]UTP incorporation of isolated rat liver nuclei in vitro . Aflatoxin B 1 , upon preincubation with a rat liver microsomal fraction, however, is apparently converted to a compound which then inhibits the activities of purified nucleoplasmic RNA polymerase (20–40%), cytoplasmic RNA polymerase (10–20%) and the incorporation of [ 3 H]‐UTP into isolated nuclei (38%). Nucleolar RNA polymerase activity is not affected under these conditions. The microsomal fraction, which most effectively converts aflatoxin B 1 to an inhibitor of RNA polymerase is also the most effective cellular fraction catalysing the metabolism of aflatoxin B 1 .