1. Aflatoxin B1, a known hepatocarcinogen, is found to induce polysomal disaggregation in rat liver. 2. The entry of [14C]orotic acid into liver RNA is also inhibited by this toxin. A significant inhibition is obtained at a dose as low as 6.5 μg per 100 g body weight and the inhibition seems to be almost complete at 150 μg per 100 g body weight. At lower doses the ribosomal RNA's showed greater susceptibility to inhibition than the smaller RNA species. 3. Aflatoxin B1 did not inhibit an in vitro RNA synthesizing system containing rat-testicular RNA polymerase (nucleoside triphosphate: RNA nucleotidyltransferase, EC 188.8.131.52). 4. These results suggest that a metabolite of aflatoxin B1 rather than the toxin itself is a binding inhibitor of DNA-dependent RNA synthesis.
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