L- and D-enantiomers of 2′,3′-dideoxycytidine 5′-triphosphate analogs as substrates for human DNA polymerases: Implications for the mechanism of toxicity

5′-Triphosphates of β-D and β-L-enantiomers of 2′,3′-dideoxycytidine (ddC), 2′,3′-dideoxy-5-fluorocytidine (FddC), 1,3-dioxolane-cytidine (OddC), and 1,3-dioxolane-5-fluorocytidine (FOddC) were evaluated as inhibitors and substrates for human DNA polymerases α, β, γ, δ, and ∈. L-ddCTP was not a substrate or inhibitor for any DNA polymerase studied; L-FddCTP was not an inhibitor or substrate for replicative DNA polymerases and was a less potent inhibitor of DNA polymerases γ and β than its D-enantiomer by 2 orders of magnitude. In contrast, all L-dioxolane analogs were potent inhibitors and chain terminators for all cellular DNA polymerases studied. The K_i values of their 5′-triphosphates for DNA polymerase γ were found to be in the following order: D-ddC < D-FddC L-OddC D-FOddC < L-FOddC ≪ L-FddC. The K_i values of L-OddCTP for the reactions catalyzed by DNA polymerases α, δ, ∈, β, and γ were 6.0, 1.9, 0.4, 3.0, and 0.014 μM, respectively, and those of L-FOddCTP were 6.5, 1.9, 0.7, 19, and 0.06 μM, respectively. The K_m values for incorporation of L-OddCTP into the standing points of primer extension were also evaluated and determined to be 1.3, 3.5, 1.5, 2.8, and 0.7 μM for DNA polymerases α, δ, ∈, β, and γ, respectively. The incorporation of dioxolane analogs into DNA by replicative DNA polymerases could explain their potent cellular toxicity.