A critical role for uridine nucleotides in the regulation of deoxycytidine kinase and the concentration dependence of 1-beta-D-arabinofuranosylcytosine phosphorylation in human leukemia cells.

The intracellular concentration of 1-beta-D-arabinofuranosylcytosine (ara-C) for half-maximal phosphorylation by leukemic blasts obtained directly from patients was 2.1 +/- 2.5 microM (median, 1.3 microM, N = 25), and the rate of ara-C accumulation actually declined at concentrations above 20 microM in 35% of these cell populations. These apparent Km values for cellular phosphorylation were an order of magnitude lower than the Km of deoxycytidine (dCyd) kinase for ara-C with ATP as phosphate donor. dCyd kinase was purified from human leukemia cells and assayed for [3H]ara-C kinase activity with a mixture of 7 nucleotides at their approximate cellular concentrations or with a single nucleotide deleted. At low or high ara-C concentrations, ATP, GTP, CTP, or dTTP could be eliminated without significantly altering the rate. The only potential phosphate donor that was clearly important was UTP, since its deletion reduced the rate to only 25% of that with the complete mix. As anticipated, eliminating dCTP, the end product of this salvage pathway, moderately increased the rate by 50% at 0.4 microM ara-C or by 26% at 40 microM ara-C. At 40 microM ara-C, deleting UDP from the mix increased the rate more than deleting dCTP. dCTP was less inhibitory against 1 mM UTP (50% inhibitory concentration, 26 microM) than against 4 mM ATP (50% inhibitory concentration, 2.2 microM). In kinetic assays with 4 mM ATP and variable ara-C, UDP was a potent uncompetitive inhibitor with a Ki of 4 microM; the Ki for ADP was 1000-fold higher. Direct fit of kinetic data to the Michaelis equation yielded a Km for ara-C of 49 microM with 4 mM ATP as the phosphate donor; however, there was evidence of negative cooperativity with a Hill coefficient of 0.7. High ara-C Km values were also obtained with GTP and CTP, but with no evidence of cooperativity. With 1 mM UTP, the Km was 1.5 microM with moderate substrate inhibition; thus the kinetic data with UTP were similar to those for ara-C phosphorylation by intact cells. UDP was less potent versus UTP than versus ATP. It lowered the Vmax and enhanced the ara-C substrate inhibition without altering the Km. When 1 mM UTP and 4 mM ATP were mixed, the kinetic pattern was similar to that for UTP alone. The Km for UTP with [3H]dCyd as the phosphate acceptor of 0.8 microM was 25-fold lower than the Km for ATP of 20 microM.(ABSTRACT TRUNCATED AT 400 WORDS)