In vivo quantitation of N-3 and N-7 alkylpurines induced by alkylnitrosoureas

General method for determining ethylene oxide and related N7-guanine DNA adducts by gas chromatography-electron capture mass spectrometry

A 112-micrograms sample of DNA was spiked with 103 pg of N7-(2'-hydroxyethyl)guanine and 100 pg of N7-(2'-hydroxyethyl-d4)guanine, the internal standard. The sample was subjected to the following sequence of steps: heating at 100 degrees C, precipitation of the DNA with HCl, reaction with nitrous acid to form the corresponding xanthines, reaction twice with pentafluorobenzyl bromide (first to derivatize NH, then OH), solid-phase extraction on silica and detection by gas chromatography-electron capture mass spectrometry. The absolute, overall yield of final product for both the analyte and internal standard was 9.7%. Conveniently, the three chemical reactions are conducted sequentially in the same vial and, aside from a washing step, are separated only by evaporations. Corresponding N7-guanine methyl, phenyl and styrene oxide adducts were detected at about the 50-ng level by the procedure, to indicate the generality of the method.

Specific DNA alkylation damage and its repair in carcinogen-treated rat liver and brain

The in vivo formation and repair of specific DNA lesions produced by alkylating agents of contrasting carcinogenic potencies were investigated. Male Sprague-Dawley rats were treated with direct-acting alkylating agents methylmethane sulfonate (MMS) or methylnitrosourea (MNU). The amounts of N-3-methyladenine (3-meA), N-7-methylguanine (7-meG), and methylphosphotriesters (mePTE) in the DNA of liver and brain were determined following selective removal of the methylated bases by enzyme 3-meA N-glycosylase from Micrococcus luteus and thermal depurination at neutral pH. Both enzyme- and heat-induced alkali-labile apurinic sites were converted to single-strand breaks on incubation with 0.1 M NaOH. The number of such sites was quantitated following centrifugation of the DNA in alkaline sucrose gradients, fluorescent detection of unlabeled DNA, and estimation of number-average molecular weight. The results show a carcinogen dose-dependent initial linear increase in the number of enzyme- and heat-induced DNA strand breakage in both liver and brain DNA. With a half-life of approximately 3 h, 3-meA was removed from the tissues, whereas 45 to 55% of 7-meG remained unrepaired at 48 h. The study of the alkylation damage induced by MNU treatment of rats showed that the kinetics of repair for 3-meA and 7-meG was similar to the MMS-treated tissues and that mePTE persisted over a 7-day period. The technique developed does not require the use of radiolabeled reagents of DNA and allows for the selective quantitation of DNA alkylation lesions like 3-meA and 7-meG in the presence of nitrosourea-induced phosphotriesters.