title: Kinetic and thermodynamic studies on nitrobenzylthioinosine binding to the nucleoside transporter of Chinese hamster ovary cells.
abstract: The binding of [G-3H]nitrobenzylthioinosine to intact Chinese hamster ovary cells has been studied kinetically and thermodynamically. The association of nitrobenzylthioinosine with cells is a second-order process which proceeds at 24 degrees C with a rate constant of 2 X 10(7) M-1 X S-1. Dissociation of the complex was characterized as a simple first-order process with rate constant on the order of 7 X 10(-3)S-1. The quotient of these is comparable to the dissociation constant as measured in equilibrium binding studies, 2.2 X 10(-10) M. The temperature dependence of the rate of association indicated an Arrhenius activation energy of 8.4 kcal X mol-1, while that of the equilibrium constant for dissociation indicated a standard enthalpy change of 8.8 kcal X mol-1. The large increase in affinity of nitrobenzylthioinosine as compared to natural nucleosides is attributable to an entropy-driven interaction with the binding site. Thymidine, dipyridamole and papaverine each decrease the apparent dissociation constant for the nitrobenzylthioinosine-cell complex; the latter, inhibitors of nucleoside transport, decrease the rate of dissociation of the complex.
mesh_terms: D000818:Animals; D002352:Carrier Proteins / Q000378:metabolism*; D002460:Cell Line; D006224:Cricetinae; D003412:Cricetulus; D004176:Dipyridamole / Q000494:pharmacology; D005260:Female; D007288:Inosine / Q000031:analogs & derivatives*; D007700:Kinetics; D009705:Nucleosides / Q000378:metabolism*; D010053:Ovary; D010208:Papaverine / Q000494:pharmacology; D013696:Temperature; D013816:Thermodynamics; D013868:Thioinosine / Q000031:analogs & derivatives* / Q000378:metabolism; D013936:Thymidine / Q000494:pharmacology
chemical_list: D002352:Carrier Proteins; D009705:Nucleosides; D013868:Thioinosine; D007288:Inosine; D004176:Dipyridamole; D010208:Papaverine; C001789:4-nitrobenzylthioinosine; D013936:Thymidine