Adenine and hypoxanthine transport in human erythrocytes: distinct substrate effects on carrier mobility

Transport of adenine and hypoxanthine in human erythrocytes proceeds via two mechanisms: (1) a common carrier for both nucleobases and (2) unsaturable permeation 4-5-fold faster for adenine for hypoxanthine. The latter process was resistant to inactivation by diazotized sulfanilic acid. Carrier mediated transport of both substrates was investigated using zero-trans and equilibrium exchange protocols. Adenine displayed a much higher affinity for the carrier (Km approximately 5-8 microM) than hypoxanthine (Km approximately 90-120 microM) but maximum fluxes at 25 degrees C were generally 5-10-fold lower for adenine (Vmax approximately 0.6-1.4 pmol/microliters per s) than for hypoxanthine (Vmax approximately 9-11 pmol/microliters per s). The carrier behaved symmetrically with respect to influx and efflux for both substrates. Adenine, but not hypoxanthine reduced carrier mobility more than 10-fold. The mobility of the unloaded carrier, calculated from the kinetic data of either hypoxanthine or adenine transport, was the same thus providing further evidence that these substrates share a common transporter and that their membrane transport is adequately described by the alternating conformation model of carrier-mediated transport.