The hypoxanthine transporter of Novikoff rat hepatoma cells exhibits directional symmetry and equal mobility when empty or substrate-loaded

Abstract

The kinetics of hypoxanthine transport were measured in hypoxanthine phosphoribosyltransferase-deficient Novikoff cells by rapid kinetic techniques applying both zero- trans and equilibrium exchange protocols. The data indicate operation of a simple carrier with directional symmetry and equal mobility when substrate loaded and empty. Zero- trans influx and efflux were about equivalent and so were zero- trans influx and equilibrium exchange flux. The apparent Michaelis-Menten constant and maximum velocity were about 500 μM and 100 pmol/s per μl cell H 2O, respectively. The time courses of accumulation of radioactively labeled hypoxanthine at a concentration above the Michaelis-Menten constant differed noticeably in zero- trans and equilibrium exchange mode, but computer simulations showed that the difference is predicted by the symmetrical carrier model and does not reflect trans-stimulation.