The reaction of 3-methyl-1,2,3-oxadiazolinium tosylate
10, a close model for a putative reactive
intermediate
in the carcinogenic activation of ethanol nitrosamines such as
(2-hydroxyethyl)methylnitrosamine 1, with
various
guanine derivatives, including acycloguanosine 12,
deoxyguanosine, deoxyguanosine monophosphate, and cyclic
guanosine monophosphate, various DNA oligomers, and calf-thymus DNA has
been examined to determine whether
this compound methylates and hydroxyethylates guanine residues as
proposed. In all of the transformations, 7-(2-(methylnitrosamino)ethyl)guanine (14) is the major
product, following acidic hydrolysis, and exceeds the
formation
of 7-methylguanine by ratios ranging from 4:1 to 48:1, depending upon
the guanine bearing substrate.
O
6-(2-(Methylnitrosamino)ethyl)deoxyguanosine (20) was
prepared from the Mitsunobu coupling of 1 and a
protected
deoxyguanosine derivative. 20 is not produced in the
reaction of 10 and deoxyguanosine and decomposes to
1 and
guanine upon mild acid treatment, suggesting possible neighboring group
participation in its facile hydrolytic cleavage.
All of the major products from the reaction of 10 and
12 have been characterized, including the direct
alkylation
product, 7-(2-(methylnitrosamino)ethyl)acycloguanosine
(13), and
N
2-(2-(methylnitrosamino)ethyl)guanine,
which
was independently synthesized. Elucidation of the reactions of DNA
with 10 and other electrophiles was facilitated
by the development of both partial and total enzymatic hydrolysis
assays utilizing 32P-5‘-labeled DNA
oligotetramers
containing one of each base type and HPLC with radiometric detection.
The partial hydrolysis assay gives information
as to the type of base being modified, and the total hydrolysis assay
permits a determination of the number of
adducts produced for a given base. The assays permit a comparison
between reactions where the same type of base
adduct could be expected. Comparisons of the reactions of ethylene
oxide and 10 using this methodology showed
that 10 does not hydroxyethylate guanine in
DNA.