Mass spectrometric analysis of 7-sulfoxymethyl-12-methylbenz[a]anthracene and related electrophilic polycyclic aromatic hydrocarbon metabolites

The Meso‐region theory of polycyclic aromatic hydrocarbon (PAH) carcinogenesis predicts that the development of pronounced carcinogenicity depends on the introduction of a good leaving group on alkyl side‐chains attached to the exceptionally reactive meso‐anthracenic or L‐region positions of PAHs. Thus, the first step in carcinogenesis by methylated PAHs such as 7,12‐dimethylbenz[a]anthracene (DMBA) would be the hydroxylation of the L‐region methyl groups, particularly the 7‐methyl group. The second would be the formation of a metabolite, e.g. a sulfate ester, which is expected to be a good leaving group capable of generating a highly reactive benzylic carbocation. 7‐Hydroxymethyl‐12‐methylbenz[a]anthracene (7‐HMBA) is a metabolite of DMBA, and sulfation of 7‐HMBA to a 7‐sulfoxymethyl metabolite (7‐SMBA) is a known Phase II metabolic process designed to facilitate excretion, but actually enabling more destructive side‐reactions. These side‐reactions occur with generation of an electrophilic 7‐methylene carbonium ion, and/or by in vivo halide exchange to provide neutral side‐products more capable of entering cells, especially those of DMBA target tissues. Electrospray ionization mass spectrometry (MS) enabled us to visualize 7‐SMBA as an intact m/z 351 conjugate anion by negative mode, and as a released m/z 255 carbonium ion by positive mode. Upon prolonged refrigeration, 7‐SMBA accumulated an m/z 383 photooxide, which appeared capable of re‐evolving the starting material as visualized by tandem quadrupole MS, or MS/MS. The 7‐SMBA carbonium ion provided interpretable fragments when studied by fragment ion MS/MS, including those representing the loss of up to several protons. Subtle differences in this property were encountered upon perturbing 7‐SMBA, either by warming it at 37 °C for 2 h or by substituting the initial sulfoxy group with an iodo group. Side‐reactions accounting for such proton losses are proposed, and are of interest whether they occur in the mass spectrometer, in solution or both; these proposals include acidity at the 12‐methyl position and cyclization between the 12‐methyl group and the adjacent C‐1 position. It is also suggested that such side‐reactions may comprise one route to relieving steric strain arising between the 12‐methyl group and the angular benzo ring of 7‐SMBA. Copyright © 2004 John Wiley & Sons, Ltd.