Deoxycytidine kinase substrate specificity

The 2 -chloro and 2 -bromo congeners of either 748 (FIAC) or 758 (FMAU) are more cytotoxic than FIAC and FMAU, suggesting that these chloro and bromo nucleosides, in contrast to the 2 -fluoro compounds, are comparatively better substrates for deoxycytidine kinase of human lymphocytes than the substrates for viral-specific thymidine kinase. The disposition of the 2 -fluoro group may also be important from the biological viewpoint. It should be noted that the structural difference between RNA and DNA is at the 2 -position. The ribo type of analog (738) of FIAC is 10 times less effective in suppression of HSV replication than is FIAC. Thus Fox, and Watanabe and coworkers concluded that the 2 - up fluorine disposition and the species of the substituent at C-5 are the two important factors influencing antiviral activity. Nevertheless, the mechanism of action of 2 -deoxy-2 -fluorocytidine (737) on certain herpes viruses, including HSV-1... 

Deoxyadenosine is also a substrate, though a relatively poor one. In some mammalian cells deoxycytidine kinase also phosphorylates deoxyadenosine and deoxyguanosine (see next section). In addition, indirect evidence suggests the existence of a specific deoxyadenosine kinase in human cells. Deoxyadenosine has received special attention as a substrate for kinases because congenital inability to catabo-... 

Habteyesus, A. Nordenskjoeld, A. Bohman, C. Eriksson, S. Deoxynucleo-side phosphorylating enzymes in monkey and human tissues show great similarities, while mouse deoxycytidine kinase has a different substrate specificity. Biochem. Pharmacol., 42, 1829-1836 (1991)... 

Usova, E.V. Eriksson, S. The effects of high salt concentrations on the regulation of the substrate specificity of human recombinant deoxycytidine kinase. Eur. J. Biochem., 248, 762-766 (1997)... 

The kinase which converts deoxycytidine to its 5 -monophosphate has been studied most extensively in preparations from calf thymus 35, 36). The preferred substrate is deoxycytidine, for which the Michaelis constant (5 X 10 M) is much lower than that of two other substrates, deoxyadenosine and deoxyguanosine. Cytidine, uridine, and thymidine are not phosphorylated by this enzyme. Deoxycytidine kinase is subject to a complex pattern of allosteric regulation by nucleotides. The end product of deoxycytidine phosphorylation, dCTP, is a potent inhibitor this inhibition is reversed by dTTP. The enzyme has a rather broad specificity for the phosphate donor, with the triphosphates of the natural ribo- and deoxyribonucleosides being substrates the inactivity of dCTP is a notable exception. 

In addition to thymidine, the enzyme will accept as substrates deoxyiui-dine and various 5-position derivatives including those with fluoro, bromo, and iodo substituents. The kinase is specific for deoxyribonucleosides and will not phosphorylate deoxycytidine. 

On the other hand, the most potent among these in the cytosine series are DMDC (3a) and FDMDC (3b), the cytotoxicities of which are comparable to that of SMDC (li). It is important to note that FDMDC would act as a 5-fluorocytosine derivative but not as a 5-fluorouracil derivative via the deamination by cytidine deaminase since DMDC was reported not to be a substrate of cytidine deaminase from mouse kidney.33 The 5-methylcytosine derivative 3f was about 170 times less active than DMDC and FDMDC. The other 5-halogeno- and 5-ethyl derivatives, 3c-e and h, were found to be devoid of the activity. Therefore, it is conceivable that the order of cytotoxicity might be related to the substrate specificity of the first activation enzymes, such as thymidine kinase or deoxycytidine kinase. 

Comparison of the substrate specificities of human thymidine kinase 1 and 2 and deoxycytidine kinase toward antiviral and cytostatic nucleoside analogues,... 

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