Deoxycytidylate phosphorylation

Sugino el al. 17) have partly purified a deoxycytidylate kinase from calf thymus this preparation also phosphorylates cytidylate and uridylate, but not deoxyuridylate. The deoxyuridylate kinase activity was recovered in another protein fraction and so represents a distinct entity. As phosphate donors, only ATP and dATP were active in the cytidylate reaction. 

Thus, while existence of a cytidylate-deoxycytidylate kinase is established, there remains some uncertainty about the association of uridylate kinase activity with this enzyme. Phosphorylation of deoxyuridylate appears to be catalyzed by a separate enzyme activity, possibly by thymidylate kinase. 

Metabolism of the pyrimidine deoxyribonucleotides is more complex because, in addition to transfer of phosphoryl groups, deamination and methylation reactions occur at this level. Specifically, the thymidine phosphates are derived by methylation of deoxyuridylate, and the latter may be derived from the deoxycytidine phosphates by way of deoxycytidylate deaminase. The deoxycytidine phosphates are not formed by amination of deoxyuridine phosphates, but are derived entirely from the cytidine phosphates by enzymatic reduction (Chapter 16). 

Fractionation of extracts from calf thymus for kinase activity toward deoxyribonucleoside monophosphates has revealed the presence of at least four separate enzymes the kinase activities for dAMP, dGMP, dCMP, and dTMP are separate entities in this tissue 31). Each has been partly purified and examination of substrate specificities showed that the kinases for deoxyadenylate and deoxyguanylate also phosphorylate the ribosyl homologues, adenylate and guanylate, respectively. The deoxycytidylate kinase accepts as substrates both cytidylate and uridylate, but will not phosphor> late deoxyuridylate. The calf thymus thymidine monophosphate... 

Pyrimidines - The leukocytes of patients responding to l- -D-arabino-furanosylcytosine (ara-C) exhibited reduced DNA and elevated RNA and protein contents, whereas the normal bone marrow cells of these patients exhibited increased DNA, RNA, and protein contents. Despite extensive intracellular conversion of ara-C to nucleotides, only limited incorporation of the compounds into nucleic acids occurred. 2-Deoxy-cytidine reverses the effects of ara-C by inhibiting its phosphorylation, an anabolic step that must occur before it can inhibit DNA polymerase and, consequently, cell proliferation. Uridine exerts a sparing action on ara-C, and the effective level of this drug can be prolonged by coadministration of tetrahydrouridine, a potent inhibitor of deoxycytidyl-ate aminohydrolase. ... 

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