A comparison between different types of covalent DNA modifications (I-compounds, persistent carcinogen adducts and 5-methylcytosine) in regenerating rat liver

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Abstract
I-Compounds have been recently identified as adduct-like nonpolar covalent DNA modifications that are detectable by 32P-postlabeling assay in tissues of untreated experimental animals and increase with age. Additional I-compounds have now been observed in liver DNA of male Sprague-Dawley rats when the chromatographic conditions were modified to allow for the detection of more polar adducts exhibiting low affinity to polyethyleneimine (PEI).sbd.cellulose anion-exchange thin-layer material. The total I-compound level in 10-month-old animals was as high as one modification in .apprx. 107 nucleotides. This represented a minimum estimate since 100% recovery of all rat liver I-compounds in 32P-labeled form presumably was not achieved by the procedures used. The I-compound pattern was reproducible and variation of I-compound levels among individual animals of the same age was small. We have used regenerating rat liver herein as a model to compare the properties of I-compounds with those of persistent 2-acetylaminofluorene (AAF)-induced DNA adducts and of 5-methylcytosine (m5C), a normal enzymatic DNA modification. Eight- to 10-month-old male Sprague-Dawley rats were given 2-AAF (50 mg/kg in DMSO) or vehicle (DMSO) by i.p. injection. Partial hepatectomy was performed 6 weeks later (i.e. after AAF adduct levels had stabilized) and regenerating liver samples were taken 1 week after the operation for DNA analysis. Consistent with the restoration of cell and tissue loss, the overall levels of I-compounds and 2-AAF adducts were reduced to .apprx. 47% and .apprx. 45% respectively of control in regenerating liver by dilution with newly synthesized DNA, while the m5C level was not affected. Thus, in regenerating liver, I-compounds resembled carcinogen-DNA adducts and not m5C. This supports our hypothesis that the formation of these DNA modifications may be due to the binding to DNA of small amounts of reactive electrophilic by-products of normal metabolic activities, leading to the slow accumulation of I-compounds in tissue DNA with ageing.

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