Dephosphorylation of nitrobenzylthioinosine 5'-monophosphate by ecto 5'-nucleotidase of HeLa cells

HeLa cells as well as human and mouse erythrocytes possess membrane sites which bind the inhibitor of nucleoside transport, nitrobenzylthioinosine (NBMPR), reversibly but tightly ( K D , 10 −9–10 −10M). Site-specific binding of the ligand correlates with inhibition of nucleoside transport. The present study showed that the 5'-phosphate of NBMPR, NBMPR-P, was not transport inhibitory. Upon exposure to [ 35S]NBMPR-P or [G- 3H]NBMPR-P, HeLa cells retained the isotopic labels virtually exclusively in the form of NBMPR. The dephosphorylation of [G- 3H]NBMPR-P by HeLa cells, assayed by the production of extracellular [G- 3H]NBMPR, was competitively inhibited by AMP, but was not affected by the presence of 5 μM NBMPR, a concentration sufficient to completely occupy the transport inhibitory sites. Thus, the sites at which dephosphorylation of NBMPR occurs in HeLa cells are separate from and function independently of the high affinity sites which bind NBMPR.