Unusual Two-Step Claisen-type Rearrangement Reaction under Physiological Conditions

An unprecedented free radical mechanism for the formation of DNA adducts by the carcinogenic N-sulfonated metabolite of aristolochic acids

The carcinogenicity of aristolochic acids (AAs) has been attributed mainly to the formation of stable DNA-aristolactam (DNA-AL) adducts by its reactive N-sulfonated metabolite N-sulfonatooxyaristolactam (N-OSO3--AL). The most accepted mechanism for such DNA-AL adduct formation is via the postulated but never unequivocally-confirmed aristolactam nitrenium ion. Here we found that both sulfate radical and two ALI-derived radicals (N-centered and C-centered spin isomers) were produced by N-OSO3--ALI, which were detected and unequivocally identified by complementary applications of ESR spin-trapping, HPLC-MS coupled with deuterium-exchange methods. Both the formation of the three radical species and DNA-ALI adducts can be significantly inhibited (up to 90%) by several well-known antioxidants, typical radical scavengers, and spin-trapping agents. Taken together, we propose that N-OSO3--ALI decomposes mainly via a new N-O bond homolysis rather than the previously proposed heterolysis pathway, yielding reactive sulfate and ALI-derived radicals, which are together and in concert responsible for forming DNA-ALI adducts. This study presents strong and direct evidence for the production of free radical intermediates during N-OSO3--ALI decomposition, providing an unprecedented free radical perspective and conceptual breakthrough, which can better explain and understand the molecular mechanism for the formation of DNA-AA adducts, the carcinogenicity of AAs and their potential prevention.

Structure-Activity Relationship Investigation on Reaction Mechanism between Chlorinated Quinoid Carcinogens and Clinically-Used Aldoxime Nerve-Agent Antidote under Physiological Condition

Pyridinium aldoximes are best-known therapeutic antidotes used for clinical treatment of poisonings by organophosphorus nerve-agents and pesticides. Recently, we found that pralidoxime (2-PAM, a currently clinically used nerve-agent antidote) could also detoxify tetrachloro-1,4-benzoquinone (TCBQ), which is a carcinogenic quinoid metabolite of the widely used wood preservative pentachlorophenol under normal physiological conditions, via an unusually mild and facile Beckmann fragmentation mechanism accompanied by radical homolysis. However, it is not clear whether the less-chlorinated benzoquinones (C_nBQs, n ≤ 3) act similarly; if so, what is the structure-activity relationship? In this study, we found that (1) The stability of reaction intermediates produced by different C_nBQs and 2-PAM was dependent not only on the position but also the degree of Cl-substitution on C_nBQs, which can be divided into TCBQ- and DCBQ (dichloro-1,4-benzoquinone)-subgroup; (2) The pK_a value of hydroxlated quinones (C_n-1BQ-OHs, the hydrolysis products of C_nBQs), determined the stability of corresponding intermediates, that is, the decomposition rate of the intermediates depended on the acidity of C_n-1BQ-OHs; (3) The pK_a value of the corresponding C_n-1BQ-OHs could also determine the reaction ratio of Beckmann fragmentation to radical homolysis in C_nBQs/2-PAM. These new findings on the structure-activity relationship of the halogenated quinoid carcinogens detoxified by pyridinium aldoxime therapeutic agents via Beckmann fragmentation and radical homolysis reaction may have broad implications on future biomedical and environmental research.