Mediated transport of nucleosides in human erythrocytes. Specific binding of the inhibitor nitrobenzylthioinosine to nucleoside transport sites in the erythrocyte membrane

Abstract

Transport of purine and pyrimidine nucleosides in human erthrocytes, which occurs by facilitated diffusion, is inhibited by 6-((4-nitrobenzyl)thio)-9-β- d-ribofuranosylpurine (nitrobenzylthioinosine) and related compounds. The present studies with nitrobenzylthioinosine indicated that inhibition of nucleoside transport resulted from binding of this compound to nucleoside transport sites in the erythrocyte membrane. Two modes of retention of nitrobenzylthioinosine by intact erythrocytes were apparent: saturable binding with an apparent dissociation constant of 10 −9 M and non-saturable intracellular accumulation of inhibitor. High affinity binding of nitrobenzylthioinosine was significantly reduced by the related transport inhibitors, 6-(methylthio)-9-β- d-ribofuranosylpurine and 2-amino-6-((2-hydroxy-5-nitrobenzyl)thio)-9-β- d-ribofuranosylpurine. The high affinity binding sites were identified as transport sites by demonstrating that inhibition of uridine transport was proportional to the number of high affinity sites occupied by nitrobenzylthioinosine. The results presented indicate that nitrobenzylthioinosine interacts specifically with nucleoside transport elements of the human erythrocyte.