Thermodynamic study of the interaction between terbutaline and salbutamol with an immobilized beta(2)-adrenoceptor by...

Investigation of the interaction between drugs and receptors is very important in revealing the biologic basis and mechanism of the drug, and designing new bioactive compounds. The beta(2)-adrenoceptor (beta(2)-AR) was purified from rabbit lung tissues and immobilized on the surface of macro-pore silica gel through covalent bonds to prepare the stationary phase. Binding isotherms of terbutaline and salbutamol were determined by frontal analysis and the perturbation method, respectively. On the basis of the model of binding isotherm assumed, zonal elution was used to investigate the binding interaction of the receptor with terbutaline and salbutamol. The two drugs had one type of common binding site on immobilized beta(2)-AR. Salbutamol had at least one other major binding region. The association constant for terbutaline was (9.76+/-0.67)x10(4)/M, and the concentration of the binding sites was (9.37+/-1.32)x10(-6)M. Under identical conditions, association constants for salbutamol at the two types of binding site were (1.11+/-0.08)x10(4)/M and (1.34+/-0.13)x10(3)/M, and the concentration of the binding sites was (5.46+/-0.35)x10(-6)M. Entropy increase was the main driving force for terbutaline and salbutamol to bind with beta(2)-AR. The associating reaction of terbutaline and beta(2)-AR was an exothermal process primarily from electrostatic interactions; hydrophobic force was the major factor contributing to the process for salbutamol.