A mechanism of inhibition of short-wavelength ultraviolet light-induced deamination of pyrimidine bases by ascorbic acid

Abstract

The relation of ascorbic acid to the ultraviolet light-induced deamination of cytosine (to uracil) and 5-methylcytosine (to thymine) was examined using gas chromatography-mass spectrometry. There was an inverse relation between the uracil content measured in solutions of cytosine after exposure to short wavelength (254 nm) ultraviolet light and the ascorbic acid concentration of the solution indicating inhibition of deamination of cytosine by ascorbic acid. Ascorbic acid also inhibited ultraviolet light-induced deamination of 5-methylcytosine. The ascorbic acid was only partially consumed during the exposure suggesting that protection from deamination was not due entirely to the antioxidant properties of ascorbic acid. Mercaptoethanol did not prevent ultraviolet light-induced deamination of either cytosine or 5-methylcytosine. However, near identical protection from ultraviolet light-induced deamination was provided by 4-aminobenzoic acid and ascorbic acid at equivalent ultraviolet light absorbing concentrations. This observation suggests that ascorbic acid prevents short wavelength ultraviolet light-induced deamination through absorbance of ultraviolet light rather than through antioxidant mechanisms.