Macromolecular binding of [3H]6-nitrobenzo[a]pyrene in male rats

Generally labelled [3H]6-nitrobenzo[a]pyrene (6NBP) when administered to male rats by i.p. injection was widely distributed throughout the body. After a single injection of 4.2 mg/kg the highest concentration was found in the liver (5% at 2 h) followed by the spleen greater than kidney greater than lung. Maximal covalent binding to macromolecules occurred between 8 and 24 h depending upon the organ; the order of binding/mg macromolecule was RNA greater than DNA greater than protein for all organs studied. In the liver maximal binding was found 8 h after 6NBP administration at which time there were 45 +/- 7 pmol 6NBP/mg bound to DNA and 114 +/- 40 pmol/mg to RNA following a dose of 4.2 mg/kg. These levels fall to 14 +/- 4 and 12 +/- 6 pmol/mg, respectively, after 168 h, the longest time point so far studied. At a lower dose of 1.4 mg/kg the values for DNA at 8 h were 14 +/- 2 pmol/mg and at 168 h 3 +/- 2 pmol/mg while for RNA the corresponding values were 29 +/- 8 and 15 +/- 3 pmol/mg. After gastric intubation at a dose of 4.2 mg/kg there was no difference in liver macromolecular binding at 24 h between conventional and germ-free animals. Enzymic digestion of liver DNA from [3H]6NBP injected animals and reversed-phase h.p.l.c. of the digests showed two peaks of radioactivity. The percentage of total radioactivity in each peak varied from preparation to preparation suggesting that one peak may be a degradation product of the other although experimental variability cannot be ruled out. The implications of these findings in relation to the potential carcinogenicity of 6NBP are discussed.