Formycin B, a C-
nucleoside analog of
inosine, is not catabolized by human erythrocytes and mouse
P388 leukemia cells and is only very inefficiently phosphorylated in these cells. This relative inertness allows the measurement of its transport into and out of the cells uncomplicated by metabolic conversions. We have measured the zero-trans and equilibrium exchange flux of
formycin B in these cells by rapid kinetic techniques. The Michaelis-Menten constants and maximum velocities for
formycin B transport in both types of cell were similar to those previously reported for
uridine and
thymidine. Nevertheless, the differential mobility of the substrate-loaded and empty carrier of human erythrocytes was less for
formycin B than
uridine as substrate.
Formycin B influx was inhibited by other
nucleosides in accordance with their affinities for the carrier, but unaffected by
purines. The inhibition of
formycin B influx by nitrobenzylthioinosine and
dipyridamole was also identical to that observed with
uridine as substrate (IC50 = 10 and 30 nM, respectively).
Formycin B accumulated in both types of cell to 30-40% higher concentrations than were present in the medium. This concentrative accumulation was not due to active transport, metabolism or partitioning into
membrane lipids. It seems to reflect binding of
formycin B to intracellular components, but does not interfere significantly with measurements of its transport.