The binding of aflatoxin B1 to rat liver nuclear proteins and its effect on DNA-dependent RNA synthesis.

This paper reports studies on the binding of aflatoxin B1 (AFB1) to rat liver nuclear proteins in vivo and in vitro, and its effect on RNA synthesis. Two hours after rats (200 g) were given a single i.p. injection of 300 micrograms AFB1 containing 50 microCi [3H]AFB1/100 g body wt, AFB1 was found bound to the free nuclear proteins (29.7 pmol/mg protein), histones (20.3 pmol/mg protein) and chromatin-bound non-histone proteins (13.8 pmol/mg protein). The binding of AFB1 to histones was further studied in vitro. We found that for a given type of histone, the binding level varied greatly depending on the conditions used. Under both in vivo and in vitro conditions, however, H3 was always the most efficient substrate, and H4/H2B always the least efficient substrates for AFB1 binding. These results suggest that the binding preference was mainly related to the intrinsic properties of the histone type, and was little affected by the geometric arrangement of the histones in chromatin. Using nuclear proteins added to the RNA synthesizing system in vitro, we found that only the histone fraction had a strong inhibitory effect. Further studies, however, indicated that this inhibition was not due to histones per se, but rather to poly-ADP-ribosylated histones present in the histone preparations. No detectable difference in effect was found between control and AFB1-bound nuclear proteins on RNA synthesis. Moreover, higher levels of AFB1 binding to histones did not potentiate the inhibitory effect. We therefore conclude, and in direct support to our previous correlation studies (see the preceding paper), that the binding of AFB1 to nuclear proteins has no inhibitory effect on RNA synthesis.