Products Derived from the Thymine Moiety

The products derived from T that have been detected in free-radical damaged DNA are depicted in Fig.12.7. 

5HmU and 50H5MeHyd are readily detected by GC/ MS after excision from the damaged DNA and trimethylsilylation (Chap. 13.2). For the Tg lesion, an ultrasensitive assay is also available that even allows its detection in cells at a dose as low as 0.25 Gy and to follow its repair (Le et al. 1998). 

The attack of OH at the methyl group of T was first followed by the release of 3H20 from DNA 3H-labeled at this position (Swinehart and Cerutti 1975). With bX174 DNA y-irradiated in aqueous solution and also within the phage an in situ protection factor of 9.4 was noticed, and for E. coli the corresponding value was 790. Interestingly, for the formation of the Tg lesion, the protection factors were noticeably lower, 1.3 and 140, respectively. In the phage, 200 of the latter lesions are formed at a dose equivalent to 37% survival as indicated by 

5-Hydroxy-5,6-dihydro- 5,6-Dihydrothymine 5-Hydroxymethyl-thymine (50H5,6H2T) (5,6H2T) uracil (5HmU) 

As expected (Chaps 10.3 and 10.4), 5,6H2Thy and 50H5,6H2Thy are only formed, when 02 is absent (Table 12.5). 

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