Some studies of the effects of aflatoxin B1 in vivo and in vitro on nucleic acid synthesis in rat and mouse

Abstract

The mouse compared with the rat, is more resistent to the acute toxic action of aflatoxin B1 and is refractory to its hepatocarcinogenic properties. Aflatoxin B1 inhibits DNA synthesis more strongly than RNA synthesis in the rat, and both nucleic acid syntheses more strongly in rat than in the mouse. Mouse hepatic microsomes, like those of the rat, are capable of metabolizing aflatoxin B1 in vitro in the presence of NADPH, to an active form which binds to DNA both in solution and in intact nuclei and also inhibits nuclear RNA synthesis. Non NADPH-dependent binding of aflatoxin B1 to nuclei is not effective in inhibiting RNA polymerase and is largely removed by washing with lipid solvents. Mouse nuclear RNA polymerases particularly Mn 2+ (NH4)2SO4 primed acitivity are more resistant to inhibition in vitro by activated aflatoxin B1 than are the corresponding enzyme activities in rat liver nuclei. This would appear to be due to the bound aflatoxin B1 being less efficient in the case of the mouse nucleus, in inhibiting RNA synthesis. Mouse liver slices exhibit a much lesser degree of inhibition of RNA synthesis by aflatoxin B1 than do rat liver slices. Accompanying this is a lower level of binding of aflatoxin B1 to subcellular particulate fractions in the mouse liver slice compared to the rat, this disparity being most marked in the case of the nuclear fraction. The suggestion is made that the resistance of RNA synthesis in the mouse liver, to aflatoxin B1, and perhaps also resistance to its toxicity, is dependent, not on a lower capacity to activate the toxin, but (a) on a less efficient inhibition of RNA synthesis by nuclear bound toxin, and (b) a detoxifying mechanism at least partially situated in the cytosol fraction.