The sequence specificity in the in vitro
DNA photobinding of
khellin and
visnagin, two naturally occurring furochromones proposed for
chemotherapy of
vitiligo, was investigated by using
DNA sequencing methodology. The
3'-5' exonuclease associated with the T4
DNA polymerase served as a tool for determining photoadducts distribution on
DNA fragments of the lac I gene of Escherichia coli. The photoadduct distribution of
psoralen is also studied for comparison. Upon UVA irradiation,
visnagin mainly forms monoadducts with
thymine and to a lower extent with
cytosine. Alternating (A-T)n sequences are hot spots for
visnagin photoaddition. This is a property shared with
furocoumarins. TTT sites are also quite reactive to
visnagin, as they are to methylated
angelicins. In contrast, with
psoralen derivatives, there is no preferential photobinding in 5'-TpA sites, and 5'-ApT sites react as well. Furthermore, many sites such as T in the GC context, and C in any context, react, although weakly. The
visnagin photoadduct distribution resembles very much the photoadduct distribution of methylated
angelicins as described by Miolo et al. The photoreaction of these two series of compounds is less sequence dependent than the photobinding of
psoralen derivatives as described by Sage and Moustacchi and by Boyer et al. The sequence specificity in
khellin-
DNA photobinding is the same as for
visnagin, even though it forms much fewer photoadducts. The absence of photo-oxidation of
DNA after treatment with
visnagin or
khellin plus UVA suggests that furochromones do not present any photodynamic effect on
DNA.