Relationships between carcinogen metabolism, adduct binding and DNA damage in 3-methylcholanthrene-exposed lung

The extent to which structural damage in DNA, isolated from rats receiving an intratracheal dose of 3-methylcholanthrene (3-MC), was affected by change in microsomal mixed-function oxidase activity, has been examined. Instillation of 3-MC causes a characteristic pattern of structural change in lung DNA as determined by stepwise elution from benzoylated DEAE-cellulose (BD-cellulose). The proportion of lung DNA containing single stranded regions was increased biphasically, relative maxima occurring 14 and 72 h after treatment, the first increase being proportional to the dose of 3-MC. Binding of 3-MC to lung DNA increased progressively for 24 h after treatment and decreased rapidly from 48 h onward. Cytochrome P-450 content of pulmonary, and also of hepatic microsomes from treated animals, was measured. On this basis, 3-MC metabolism was modified, carbon tetrachloride being an effective inhibitor when administered 4 h in advance of treatment whilst maximum self-induction of 3-MC metabolism required 24 h. BD-cellulose analysis of lung DNA of 3-MC-treated rats subjected to inhibiting or inducing treatment suggested that the extent of structural damage was primarily determined by the capacity of lung tissue to metabolise the carcinogen. In particular, inhibition of 3-MC metabolism by prior treatment with carbon tetrachloride prevented production of single stranded regions, whilst modifying 3-MC binding to DNA. The data indicate that structural analysis of DNA is a sensitive means of assessing levels of genomic injury by carcinogens. This procedure may be used to study the effects of complicated treatments, and specifically agents which modify carcinogen metabolism.