Fig. 2.

The endogenous dCK substrate dC competes with [³H]CFA uptake at concentrations found in rodent plasma but not in human or NHP plasma. (A) Extracellular dC competes with CFA uptake. [³H]CFA (9.25 kBq) uptake for 1 h in CEM WT cells in the presence of varying concentrations of dC ± DI-82 (1 µM). n = 3 per group. (B) Plasma dC, (C) dT, (D) dA, and (E) dG concentrations for indicated species. The dT concentration in the NHP plasma was below the lowest limit of detection for the assay (10 nM). n = 5 subjects per group for human and C57BL/6 murine plasma; n = 3 subjects per group for NHP and rat plasma. (F) Mice were fasted for 24 h before plasma collection. Plasma dN concentrations were measured by LC-MS/MS-MRM and were normalized to those of the control unstarved group. n = 4 mice per group. N.S., nonsignificant; *P < 0.05; ***P < 0.001. (G) [U-¹³C₉,¹⁵N₃]dC (5 µM) is more rapidly deaminated to [¹³C₉,¹⁵N₂]dU upon 60-min incubation with human plasma (Top) than mouse plasma (Bottom). The deamination of dC was blocked by treating human plasma samples with 100 µM THU, a specific CDA inhibitor (Middle).