Adenosine 5'-triphosphate modulation of nitrobenzylthioinosine binding sites in plasma membranes of bovine chromaffin cells.

Nitrobenzylthioinosine (NBTI) is a high affinity probe for facilitated diffusion nucleoside transporters. Kinetic analysis of the binding of [3H]NBTI to plasma membranes of chromaffin cells was conducted in the presence or absence of adenosine 5'-triphosphate (ATP). Similar curvilinear plots with a Hill number of 1.32 were obtained in both conditions. ATP significantly increased the number of NBTI binding sites in these preparations showing Bmax values of 1.62 +/- 0.20 pmol/mg protein for controls and 3.22 +/- 0.31 pmol/mg protein in the presence of ATP. However, the affinity constant (KD) was not significantly modified. The non-metabolizable ATP analogue, 5'-adenylyl imidodiphosphate (AMP-PNP) and diadenosine tetraphosphate (Ap4A) can mimic the stimulatory ATP effect, but adenosine monophosphate (AMP) has no effect on the NBTI binding to plasma membranes. These results indicate a modulatory role for ATP, non-hydrolysis dependent, on nucleoside transport in chromaffin cells. Therefore, a nucleotide binding site on the nucleoside transporter similar to that described for glucose transporter could be suggested.