LACK OF ENANTIOSPECIFICITY OF HUMAN 2'-DEOXYCYTIDINE KINASE - RELEVANCE FOR THE ACTIVATION OF BETA-L-DEOXYCYTIDINE ANALOGS AS ANTINEOPLASTIC AND ANTIVIRAL AGENTS

We demonstrate that human 2'-deoxycytidine kinase (dCK) is a nonenanti oselective enzyme because it phosphorylates beta-D-2'-deoxycytidine (D -dCyd), the natural substrate, and beta-t-2'-deoxycytidine (L-dCyd), i ts enantiomer, with the same efficiency. Kinetic studies showed that L -dCyd is a competitive inhibitor of the phosphorylation of D-dCyd with a K-j value of 0.12 mu M, which is lower than the K-m value for D-dCy d (1.2 mu M). Chemical modifications of either the base or the pentose ring strongly decrease the inhibitory potency of L-dCyd. L-dCyd is re sistant to cytidine deaminase and competes in cell cultures with the n atural D-dCyd as substrate for dCK, thus reducing the incorporation of exogenous [H-3]dCyd into DNA. L-dCyd had no effect on the pool of dTT P deriving from the salvage or from the de novo synthesis, does not in hibit short term RNA and protein syntheses, and shows little or no cyt otoxicity. Our results indicate a catalytic similarity between human d CK and herpetic thymidine kinases, enzymes that also lack stereospecif icity. This functional analogy underlines the potential role of dCK as activator of L-deoxycytidine analogs as antiviral and antineoplastic agents and lends support to the hypothesis that herpesvirus thymidine kinase might have evolved from a captured cellular dCK gene, developin g the ability to phosphorylate thymidine and retaining that to phospho rylate deoxycytidine.