RAPID DECREASES IN INDIGENOUS COVALENT DNA MODIFICATIONS (I-COMPOUNDS) OF MALE FISCHER-344 RAT-LIVER DNA BY DIQUAT TREATMENT

I-compounds are indigenously appearing covalent DNA modifications that can be detected by P-32-postlabeling assay in tissues of normal anima ls without known exposure to any carcinogens or toxins. Although these compounds have not been structurally identified, indirect evidence fr om earlier work suggested the possibility of involvement of molecular fragments derived from lipid peroxides. Diquat is a herbicide that sti mulates lipid peroxidation and massive intrahepatic oxidant stress thr ough redox cycling-mediated generation of reactive oxygen species. In the present study, we examined the effects of diquat on hepatic I-comp ounds of male Fischer-344 rats. Two groups of rats, approximately 14 w eeks and 8 weeks old, were given a hepatotoxic dose (0.1 mmol/kg) of d iquat or equal volumes of saline, i.p. Two and 6 h later plasma alanin e aminotransferase(ALT) activities were measured and hepatic DNA I-com pound levels were examined by nuclease PI-enhanced P-32-postlabeling. Elevated ALT activities were observed in some animals in both groups, at both time points, but considerable inter-animal variation was seen. A total of 15-16 I-compound fractions were measured in control and in diquat-treated animals, but no extra spots indicative of treatment-in duced adducts were detected. Despite the qualitative similarities, the quantities of individual I-compounds were markedly decreased at 2 h i n diquat-treated animals of both age groups. In 14 week old rats the h epatic I-compound contents were decreased at 2 h by 22-59%, which was statstically significant (ANOVA, P < 0.05) for all of the 9 polar I-co mpound fractions and none of the non-polar fractions. Eleven I-spots f rom this group showed significant negative linear correlations (P < 0. 05) with ALT values. In 8 week old rats treated with diquat a 22-43% d epletion in I-compound contents was statistically significant for 4 of the 7 nonpolar and 2 of the 8 polar adduct fractions, but there was n o significant correlation of I-compound contents with ALT values at th e 2 h time point. By 6 h most of the I-spot levels had returned to nor mal or above normal values in both groups of animals. While most I-spo ts from 14 week old rats did not correlate with ALT levels at 6 h, two I-spots displayed positive correlations in the 8 week group. Overall, the susceptibility to diquat-associated DNA alterations appeared to d iffer with age. Decreases in hepatic I-compound contents of diquat-tre ated Fischer-344 rats and the absence of additional I-compounds in the se animals were not consistent with the hypothesis that I-compounds ar e derived from lipid peroxide fragments. However, the rapid but transi ent decrease in I-compound levels is notable because of similar trends previously observed during a number of procarcinogenic experimental m anipulations.