Correlation studies between the binding of aflatoxin B1 to chromatin components and the inhibition of RNA synthesis

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Abstract
Aflatoxin B 1 (AFB 1 is a potent inhibitor of rat liver nucler and nucleolar RNA synthesis. However, since after activation AFB 1 , binds to both DNA and chromosomal proteins, the question is which form of binding is responsible for the inhibition of RNA synthesis. Male Sprague—Dawley rats (200 g) were given i.p. injections of 10, 50, 100, 300 and 500 μg AFB 1 per 100 g body wt containing 50μCi [ 3 H] AFB 1 (sp. act. 25 Ci/mmol), and the animals sacrificed 2 h later. Liver nuclei, nucleoli and P-3 (a transcriptionally active subnucleolar fraction that is 3.4-fold enriched in active rDNA) were isolated and the binding of AFB 1 to DNA and protein of each fraction was determined by DNase I digestion and 5% trichloroacetic acid TCA) hydrolysis. We found that the binding of AFB 1 to both nuclear and nucleolar DNA plateaus at 300 μg AFB 1 per 100g body wt, with values around 100 and 400 pmol AFB 1 , per mg nuclear and nucleolar DNA, respectively. On the other hand, the binding to protein is linear, although with different slopes, for both nuclear and nucleolar fractions even at 500 μg AFB 1 per 100g body wt, the highest dose used. Since AFB 1 , inhibition of nuclear and nucleolar RNA synthesis plateaus respectively at 60% and 90% inhibition levels at the dose of 300 μg AFB 1 per 100g body wt, these results suggest the binding of AFB 1 to DNA, but not to protein, is responsible for the inhibition of RNA synthesis. Further support for this contention is obtained by comparing the binding and the inhibition data between P-3 and nucleoli. p-3 is three times more transcriptionally active than nucleoli and, as a result, is more sensitive to AFB 1 inhibition. This greater sensitivity is reflected by the specific binding activity of AFB 1 to P-3 DNA, which is >50% higher than to nucleolar DNA. In contrast, this effect is not reflected by the specific binding activity of AFB> to protein which is exactly the same for both fractions.