TY - JOUR
T1 - Mode of Coupling between theβ- Adrenergic Receptor and Adenylate Cyclase in Turkey Erythrocytes
AU - Tolkovsky, Aviva M.
AU - Levitzki, Alexander
PY - 1978
Y1 - 1978
N2 - The mode of coupling of the β-adrenergic receptor to the enzyme adenylate cyclase in turkey erythrocyte membranes was analyzed in detail. A number of experimental techniques have been used: (1) measurement of the kinetics of cyclase activation to its permanently active state in the presence of guanylyl imidodiphosphate, as a function of hormone concentration; (2) measurement of antagonist and agonist binding to the β-adrenergic receptor prior and subsequent to the enzyme activation by hormone and guanylyl imidodiphosphate. On the basis of these two approaches, all the models of receptor to enzyme coupling which involve an equilibrium between the enzyme and the receptor can be rejected. The binding and the kinetic data, however, can be fitted by two diametrically opposed models of receptor to enzyme coupling: (a) the precoupled enzyme-receptor model where activation of the enzyme occurs, according to the following scheme: where H is the hormone, RE is the precoupled receptor-enzyme complex, k1 and k2 are the rate constants describing hormone binding, and k is the rate constant characterizing the formation of HRE' from the intermediate HRE. According to this model, the activated complex is composed of all of the interacting species, (b) The other model is the collision coupling mechanism: FORMULA OMMITTED where KH is the hormone-receptor dissociation constant, k1 is the bimolecular rate constant governing the formation of HRE, and k3 the rate constant governing the activation of the enzyme. In this case the intermediate never accumulates and constitutes only a small fraction of the total receptor and adenylate cyclase concentrations. In order to establish which of the two mechanisms governs the mode of adenylate cyclase activation by its receptor, a diagnostic experiment was performed: Progressive inactivation of the β receptor by a specific affinity label was found to cause a decrease in the maximal binding capacity of the receptor and a proportional decrease in the rate of activation, but no change in the maximum level of activity was attained. Progressive inactivation of the enzyme by p-hydroxymercuribenzoate was found not to change the rate of activation nor the capacity of the receptor to bind hormone. Only the maximal level of activation was found to be decreased. These results are not compatible with the precoupled model of receptor and cyclase nor with floating receptor models in which an intermediate of hormone, receptor, and cyclase is in equilibrium with its reactants. The data strongly suggest that the collision coupling is the mode of coupling between the β receptor and cyclase coupling in turkey erythrocyte membranes.
AB - The mode of coupling of the β-adrenergic receptor to the enzyme adenylate cyclase in turkey erythrocyte membranes was analyzed in detail. A number of experimental techniques have been used: (1) measurement of the kinetics of cyclase activation to its permanently active state in the presence of guanylyl imidodiphosphate, as a function of hormone concentration; (2) measurement of antagonist and agonist binding to the β-adrenergic receptor prior and subsequent to the enzyme activation by hormone and guanylyl imidodiphosphate. On the basis of these two approaches, all the models of receptor to enzyme coupling which involve an equilibrium between the enzyme and the receptor can be rejected. The binding and the kinetic data, however, can be fitted by two diametrically opposed models of receptor to enzyme coupling: (a) the precoupled enzyme-receptor model where activation of the enzyme occurs, according to the following scheme: where H is the hormone, RE is the precoupled receptor-enzyme complex, k1 and k2 are the rate constants describing hormone binding, and k is the rate constant characterizing the formation of HRE' from the intermediate HRE. According to this model, the activated complex is composed of all of the interacting species, (b) The other model is the collision coupling mechanism: FORMULA OMMITTED where KH is the hormone-receptor dissociation constant, k1 is the bimolecular rate constant governing the formation of HRE, and k3 the rate constant governing the activation of the enzyme. In this case the intermediate never accumulates and constitutes only a small fraction of the total receptor and adenylate cyclase concentrations. In order to establish which of the two mechanisms governs the mode of adenylate cyclase activation by its receptor, a diagnostic experiment was performed: Progressive inactivation of the β receptor by a specific affinity label was found to cause a decrease in the maximal binding capacity of the receptor and a proportional decrease in the rate of activation, but no change in the maximum level of activity was attained. Progressive inactivation of the enzyme by p-hydroxymercuribenzoate was found not to change the rate of activation nor the capacity of the receptor to bind hormone. Only the maximal level of activation was found to be decreased. These results are not compatible with the precoupled model of receptor and cyclase nor with floating receptor models in which an intermediate of hormone, receptor, and cyclase is in equilibrium with its reactants. The data strongly suggest that the collision coupling is the mode of coupling between the β receptor and cyclase coupling in turkey erythrocyte membranes.
UR - http://www.scopus.com/inward/record.url?scp=0018125541&partnerID=8YFLogxK
U2 - 10.1021/bi00611a020
DO - 10.1021/bi00611a020
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C2 - 212105
AN - SCOPUS:0018125541
SN - 0006-2960
VL - 17
SP - 3795
EP - 3810
JO - Biochemistry
JF - Biochemistry
IS - 18
ER -