T. Suzuki et al., DEGLYCOSYLATION SUSCEPTIBILITY AND BASE-PAIRING STABILITY OF 2'-DEOXYOXANOSINE IN OLIGODEOXYNUCLEOTIDE, Biochemistry, 36(26), 1997, pp. 8013-8019
We have demonstrated recently that nitrous acid or nitric oxide conver
ts 2'-deoxyguanosine (dGuo) into 2'-deoxyoxanosine (dOxo) [Suzuki, T.,
Yamaoka, R., Nishi, M., Ide, H., & Makino, K. (1996) J. Am. Chem. Sec
. 118, 2515-2516]. In the present study, we have measured susceptibili
ty of the N-glycosidic bond of dOxo to spontaneous hydrolysis and its
base-pairing stability to evaluate the biological significance of dOxo
as a new lesion in DNA. When oligodeoxynucleotide d(T5OT6) (O = dOxo)
, isolated from nitrous acid-treated d(T(5)GT(6)), was incubated at pH
4.0 and 70 degrees C, hydrolysis of the N-glycosidic bond of dOxo occ
urred with a first-order rate constant. Comparison of the rate constan
ts with those of dGuo and dXao indicates that the N-glycosidic bond of
dOxo was as stable as that of dGuo in d(T(5)GT(6)) and hydrolyzed 44-
fold more slowly than that of 2'-deoxyxanthosine (dXao), a simultaneou
sly generated damage by nitrous acid and nitric oxide. For the estimat
ion of the base-pairing stability, UV melting curves were measured for
the duplexes of d(T5OT6).d(A(6)NA(5)) (N = A, G, C, and T) at neutral
pH. The T-m values obtained were 15.3, 14.1, 19.3, and 16.3 degrees C
for N = A, G, C, and T, respectively, which are much lower than that
of the intact duplex containing a G . C pair at the same position [d(T
(5)GT(6)).d(A(6)CA(5)), T-m = 32.8 degrees C] but comparable with thos
e of d(T5XT6).d(A(6)NA(5)) (X = dXao, T-m = 14.8-22.3 degrees C). CD s
pectra of the four duplexes containing dOxo showed preservation of the
structure of the intact duplex at low temperature. UV and NMR pH-titr
ation studies indicated the pK(a) for the ring-opening and -closing eq
uilibrium to be 9.4, implying that dOxo is in the ring-closed form at
physiological pH. This structure appears to be not suitable geometrica
lly for the hydrogen bond formation with a specific counter base, thus
causing equally low T-m values for all the counter bases. Consequentl
y, these results imply that dOxo, a novel DNA lesion, may have an impo
rtant and unique role in mutagenic events in cells.