Mobility of nucleoside transporter of human erythrocytes differs greatly when loaded with different nucleosides

Time courses of transmembrane equilibration of 2-chloroadenosine, 2'-deoxyadenosine, 3'-deoxyadenosine, cytidine and 2'-deoxycytidine were measured by rapid kinetic techniques in human erythrocytes under equilibrium exchange and zero-trans conditions. The kinetic parameters for transport were computed by fitting appropriate integrated rate equations to the data pooled for seven concentrations and compared to the kinetic parameters for uridine, adenosine, thymidine and formycin B transport determined previously for human erythrocytes under comparable experimental conditions. The transport of all nucleosides conformed to the simple carrier model and was directionally symmetric. The Michaelis-Menten constants for equilibrium exchange (Kee) ranged from 22 microM for 2-chloroadenosine to about 4 mM for cytidine and the maximum velocities (Vee) differed in a similar manner, so that the first-order rate constants (Vee/Kee) were similar for all nucleosides. The kinetic parameters for 2'-deoxyadenosine transport were similar to those for adenosine transport, whereas the lack of the 3'-OH group greatly reduced the affinity of 3'-deoxyadenosine (cordycepin) for the carrier. 2', 3'-Dideoxynucleosides were transported less than 1% as efficiently as 2'- and 3'-deoxynucleosides. Thus, the 2'- and 3'-OH groups play an important role in nucleoside transport. The mobility of the carrier when loaded with pyrimidine nucleosides (reflected by Vee) was 5-10-times greater than that of the empty carrier, whereas the mobility of the adenosine-loaded or 2'-deoxyadenosine-loaded carrier was about equal to that of the empty carrier. Loading the carrier with 2-chloroadenosine or 3'-deoxyadenosine actually decreased its mobility. Thus, the differential mobility of the loaded and empty carrier differs greatly with the nucleoside substrate. The mobility of the loaded carrier as well as Kee increased with a decrease in lipid solubility of the nucleoside substrate, but the relationship was complex.