Transport of deoxycoformycin in human erythrocytes: Measurement by adenosine deaminase titration and radioisotope assays

The assay of residual adenosine deaminase (ADA) activity was used as a sensitive measure of the transport of deoxycoformycin (dCF) into human erythrocytes. Contrary to prior reports from this laboratory, the inactivation of intraerythrocytic ADA by dCF was linear rather than log-linear, with time. Linear inactivation rates were also seen when erythrocytes were preloaded with a 5-fold excess of calf intestinal ADA. The uptake of tritium-labeled dCF molecules and the rate of inactivation of ADA molecules showed an approximate 1:1 stoichiometry. The nucleoside transport inhibitors, 6-[(4-nitrobenzyl)thio]-9-β-D-ribofuranosylpurine (NBMPR) and dipyridamole, and the permeant, uridine, inhibited dCF transport with K i values of 35 nM, 45 nM, and 340 μM respectively. The affinity of dCF for the nucleoside transporter was low with a K i of approximately 10 mM for the inhibition of adenosine influx.