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5,6-dihydrouracil can be formed from cytosine after exposure of DNA to ionizing radiation under anoxic conditions [Ref]. What are other ways by which 5,6-dihydrouracil is formed in DNA? What about 5,6-dihydroxyuracil?

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Dihydrouracil oxidase can catalyse the reduction of uracil to dihydrouracil in the presence of hydrogen peroxide. The reduction of uracil to dihydrouracil can also be catalysed by dihydrouracil dehydrogenase using NADH.

According to this paper, 5,6-dihydroxycytosine can be formed by treating cytosine or DNA with osmium tetroxide, an extremely strong oxidising agent. Since they are both pyrimidines (and in fact cytosine often spontaneously hydrolyses to uracil), uracil will also be affected by the same oxidation. 5,6-dihydroxyuracil was also detected within the oxidised DNA sample as a result of oxidation and demethylation of thymine.

The presence of 5,6-dihydroxyuracil (peak 11 in Fig. 1) can be explained analogously to the formation of 5-hydroxy-5-methylbarbituric acid from thymine on permanganate oxidation (lida & Hayatsu, 1970, 1971) (Scheme 2). The actual oxidation products ofcytosine, i.e. structures (I) and (II), were not observed in DNA. As the calf thymus DNA used in this work did not contain uracil, the uracil derivatives discussed above could not have been formed from uracil in DNA.

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