Absence of binding sites for the transport inhibitor nitrobenzylthioinosine on nucleoside transport-deficient mouse lymphoma cells.

Cells of an adenosine-resistant clone (AE1) of S49 mouse lymphoma cells were compared with cells of the parental line with respect to (a) characteristics of nucleoside transport, (b) high affinity binding of the inhibitor of nucleoside transport, nitrobenzylthioinosine (NBMPR), and (c) the antiproliferative effects of the nucleoside antibiotics, tubercidin, arabinosyladenine and showdomycin. Rates of inward transport of uridine, thymidine, adenosine, 2'-deoxyadenosine, tubercidin, showdomycin, and arabinosyladenine in AE1 cells were less than 1% of those in cells of the parental S49 line. The inhibitor of nucleoside transport, NBMPR, reduced rates of inward nucleoside transport in S49 cells to levels comparable to those seen in the transport-defective mutant. S49 cells possessed high affinity sites that bound NBMPR (6.6 X 10(4) sites/cell, Kd = 0.2 nM), whereas site-specific binding of NBMPR to AE1 cells was not demonstrable, indicating that loss of nucleoside transport activity in AE1 cells was accompanied by loss of the high affinity NBMPR binding sites. Relative to S49 cells, AE1 cells were resistant to the antiproliferative effects of tubercidin and showdomycin, but differences between the two cell lines in sensitivity toward arabinosyladenine were minor, suggesting that nucleoside transport activity was required for cytotoxicity of tubercidin and showdomycin, but not for that of arabinosyladenine.