Abstract
The work described in this study separates naturally into two parts. Firstly an examination of the persistence of binding in vivo of a carcinogenic polycyclic aromatic compound to the DNA in mouse skin, a tissue in which the chemical is a carcinogen, was carried out. The results of this experiment prompted the work of the second part of this study which entailed an examination and comparison of the binding of a number of polycyclic aromatic hydrocarbons to nuclear and mitochondrial DNA. The binding of the carcinogen 15,16-dihydro-l l-methyl-cyclopenta(a)-phenanthren-17-one to mouse skin was examined over a period of time. The major hydrocarbon-deoxyribonucleoside adducts separated by enzymic hydrolysis of the DNA and chromatography on Sephadex LH 20 were found to be removed quickly over the 72 hour period following the maximum binding of the hydrocarbon, but the removal of adduct then slowed so that after five weeks approximately 10% of the original adduct still remained. The covalent binding of six polycyclic aromatic hydrocarbons to the nuclear and mitochondrial DNA of whole mouse embryo tertiary cells in culture was examined. The binding of the hydrocarbons to mitochondrial DNA was generally 50 to 100 times greater than to nuclear DNA. No relationship between the known carcinogenicity of the compounds and the binding to nuclear DNA was observed, but a relationship was found for binding to mitochondrial DNA. In general the same species of adducts were present in mitochondrial and nuclear DNA although the proportions varied. Often, late eluting adducts, possibly formed from an epoxide, rather than a dihydrodiol epoxide, represented a far greater proportion of the total adducts in mitochondrial DNA than in nuclear DNA. The repair of adducts of two of the hydrocarbons, benzo(a) pyrene and 3-methylcholanthrene, was examined. In both cases the adducts were removed efficiently from the nuclear DNA, but in the case of benzo(a) pyrene the removal of adducts from mitochondrial DNA was inefficient, while the removal of 3-methyI-cholanthrene adducts from mitochondrial DNA paralleled in efficiency that of the nuclear DNA. Benzo(a) pyrene was shown to reduce the proportion of mitochondrial DNA extractable as closed circles, and increase the proportion of open circles and possibly catenated dimers. These results are discussed with reference to the carcinogenic process.