Thymine oxidation, bacterial

The enzyme from this source has since been crystalized and extensively studied [for example, see 12)]. The dehydrogenase is a ilavoprotein containing FMN, FAD, and non-heme iron the reduced flavin in the purified enzyme can be oxidized directly by O and also by NAD+. The enzyme is specific for orotate and will not react with uracil, cytosine, thymine, or 5-methylcytosine. A related dehydrogenase has also been demonstrated in other bacterial species and in animal tissues. 

Bacterial cells that grow on pyrimidines contain adaptive enzymes that carry out the following reactions. Cytosine is deaminated to give uracil and 5-methylcytosine is similarly converted to thymine. The two dihydroxypyrimides are oxidized by an enzyme that reacts with methylene blue, but not with oxygen. The products are barbituric acid and 5-methylbarbituric acid from uracil and thymine, respectively (XII). Although intact bacteria oxidize thymine completely to CO and NH , cell-free extracts have not been found to carry the oxidation beyond 5-methylbarbituric acid. Barbituric add, on the other hand, is hydrolyzed to urea and malonic acid. The urea is further hydrolyzed and the malonic acid is oxidized as its CoA derivative to form acetyl CoA and CO. ... 

In the above studies, cytosine and 5-methylcytosine were probably deaminated to uracil and thymine, respectively, by a single enzyme. That deamination was the first catabolic step is in accord with earlier studies on the degradation of aminopyrimidines in bacteria. " The cell-free extracts of Wang and Lampen, however, were unable to oxidize cytosine, since the extracts did not contain cytosine deaminase. Evidence was presented nonetheless that the intact bacterial cell did metabolize cytosine cytosine was not utilized, but was first deaminated to uracil, which was in turn oxidized. Chargaff and Kream have reported the... 

The essential difference between the findings of Fink and collaborators and the bacterial studies reported above with thymine is that, in the former instance, an initial reduction at Ce to yield )8-aminoisobutyric acid was postulated, while to produce 5-methylbarbituric acid in bacteria, an oxidation must be postulated as the initial step. The possibility should not be overlooked that several degradative pathways may exist. The two pathways of thymine catabolism discussed above are shown in Fig. 15. 

Strains of Corynebacterium and Mycobacterium oxidized thymine and uracil to 5-methylbarbituric acid and barbituric acid, respectively, and it appeared that a single enzyme, uracil-thymine oxidase, was responsible for the oxidation of both bases (896) (Fig. 23). Uracil oxidase (398) and uracil-thymine oxidase (896) may be identical, since on purification of the former, the rates of oxidation of uracil and thymine remmned constant. A phosphorylated uracil-5-carbinol (2,4-dihydroxy-5-hydroxymethyl-pyrimidine) was isolated as an oxidation product of thymine by resting bacterial cells (398). However, the spectroscopic characteristics of the compound suggested that this compound may have been 5-methylbarbituric acid (896). 

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