Effector studies of 3'-azidothymidine nucleotides with human ribonucleotide reductase

The 5'-mono-, di- and triphosphate derivatives (N3dTMP, N3dTDP and N3dTTP respectively) of 3'-azidothymidine (N3dThd), a new drug for the treatment of the acquired immune deficiency syndrome (AIDS), were synthesized. The abilities of these analog nucleotides to mimic the effector properties of the corresponding thymidine nucleotides with human ribonucleotide reductase were studied. Surprisingly, the mode of inhibition of CDP reduction by dTTP and dTDP was found to be competitive versus CDP. The Ki values were 22 and 78 microM respectively. Inhibition by N3dTTP and N3dTDP was considerably weaker, with Ki values of 1200 and 550 microM. Neither dTMP nor N3dTMP produced significant inhibition at concentrations up to 500 microM. dTTP was an essential activator for GDP reduction. In the presence of the accessory activator, ATP, the activation constant for dTTP was 7.8 microM. N3dTTP was neither an activator of GDP reduction nor an inhibitor of the activation by dTTP. In view of the intracellular concentrations of these analog nucleotides reached after incubations with N3dThd [Furman et al., Proc. natn. Acad. Sci. U.S.A. 83, 8333 (1986)] and the weakness of their interactions with ribonucleotide reductase, it is unlikely that the antiviral or toxic effects of N3dThd can be attributed to direct effects on this enzyme. The possible indirect effects caused by alterations in the pools of the natural effectors are discussed.