The Purification and Properties of Deoxycytidylate Deaminase from Chick Embryo Extracts.

The importance of this pathway to E. coli for the synthesis de nouo of dTMP becomes apparent from studies indicating only a dubious contribution toward the formation of dTMP from Reaction Sequence 1, because of the low levels of deoxycytidylate deaminase in this organism (5). It remains to be seen whether an analogous pathway to Reaction Sequence 2 can be observed in animal tissues. Some support has been obtained from the finding that uridine-2-14C can be incorporated into the thymine of chick embryo DNA in the presence of sufficient 6-diazo-5oxo-L-norleucine to prevent the conversion of uridine to cytidine U-3. Since as discussed previously (7, 8), dTMP synthesis may be the rate-limiting step in DNA synthesis, it becomes essent.ial to determine the extent to which each pathway contributes to the formation of dTMP, providing both are present within an organism. It would also be of interest to determine whether these pathways are subject to similar regulatory mechanisms. It is possible that the role of deoxycytidylate deaminase in the synthesis of dTMP may be only a minor one; however, studies showing the variation of this enzyme with the degree of mitosis in regenerating liver (8), the association of the deaminase with rapidly growing normal as well as with neoplastic tissues (2, 9ll), and the incorporation of dCM32P into the dTMP of DNA