Binding of the Nucleoside Transport Inhibitor Nitrobenzylthioinosine to HeLa Cells

Nitrobenzylthioinosine (NBMPR), a potent inhibitor of nucleoside transport, was bound tightly but reversibly to HeLa cell membrane sites associated with the nucleoside transport mechanism. Site-specific binding was assayed with [ 33 S]NBMPR and a competing, nonisotopic congener. Mass law analysis of the binding data indicated that each HeLa cell possessed about 1O 3 binding sites of a single class which bound NBMPR tightly; the bound inhibitor had a dissociation constant of about 0.1 nM. Occupancy of these binding sites by NBMPR correlated with inhibition of uridine and thymidine uptake; however, the relationship between these parameters was not simple because, as binding saturation was approached (at about 5 nM NBMPR), a substantial fraction (25-30%) of the transport capability remained active but inhibitable by 5 µM NBMPR.