A comparison of the enantioselectivities of human deoxycytidine kinase and human cytidine deaminase

The stereoselectivities of recombinant human deoxycytidine kinase (EC 2.7.1.74) (dCK) and of recombinant human cytidine deaminase (EC 3.5.4.5) (CDA) were investigated with respect to a series of cytidine analogs, most of them having the unnatural L-stereochemistry. The enantioselectivity of dCK was always low and generally favored the L-enantiomers in the case of beta-2',3'-dideoxycytidine (beta-ddC), 5-fluoro-beta-2',3'-dideoxycytidine (beta-FddC) and beta-cytidine (beta-riboC). Concerning beta-2'-deoxycytidine, dCK showed a preference for the D-enantiomer. All other examined beta-L-cytidine analogs, [1-beta-L-lyxofuranosyl cytosine (beta-L-lyxoC), l-beta-L-xylofuranosyl cytosine (beta-L-xyloC), and 5-fluoro-1-beta-L-xylofuranosyl cytosine (beta-L-Fxylo C)], were substrates of dCK regardless of the nature of the pentose. None of the studied alpha-L-anomers (alpha-L-riboC, alpha-L-araC, alpha-L-lyxoC, or alpha-L-xyloC) was a substrate of dCK. Contrasting with the relaxed enantioselectivity of dCK, CDA had a strict requirement for D-cytidine analogs since none of the already listed beta-L- or alpha-L analogs was a substrate or an inhibitor of the enzyme. The conjunction of the preceding stereochemical properties of dCK and CDA confers to L-cytidine analogs important potentialities in antiviral and anticancer therapies.