Mass spectral characterisation of the major DNA--carcinogen adduct formed from the metabolically activated carcinogen 15,16-dihydro-11-methylcyclopenta[a]phenanthren-17-one.

E. coli DNA, labelled with [14C]adenine and [14C]-guanine, was allowed to react with the [3H]-labelled carcinogen 15,16-dihydro-11-methylcyclopenta[a]phenanthren-17-one in the presence of a microsomal metabolising system. Enzymatic hydrolysis of the DNA followed by Sephadex LH20 chromatography of its constituent nucleosides established that the major DNA - carcinogen adduct involved guanine, and not adenine. This was confirmed by submitting calf thymus DNA, which had been allowed to react with the unlabelled carcinogen, to pyrolysis electron impact mass spectrometry without further derivatisation. Analysis of a selected ion product (m/z 368) by means of mass-analysed kinetic energy spectrometry, a technique which allows study of the further fragmentation of the single, selected ion, revealed that the guanine moiety was attached via the nitrogen atom of its exocyclic amino group to C-1 of a 1,2,3,4-tetrahydro-2,3,4-trihydroxy derivative of the original carcinogen.