Comparison of the effects of chemical and isotopic dilution on the dissociation of bound labeled ligands

Dissociation kinetics of the thyrotropin-receptor complex. Characterization of a slowly dissociable component

The kinetics of the dissociation of thyrotropin (TSH) from human thyroid plasma membranes were studied in an attempt to further understand the molecular dynamics of the TSH--receptor interaction. Dissociation of bound [125I]TSH from thyroid plasma membranes was a biphasic process consisting of rapidly and slowly dissociable components, RDC and SDC, respectively. The dilution induced dissociation of bound [125I]TSH was enhanced by the addition of excess TSH (DEC). DEC was proportional to the dose of unlabeled TSH and its magnitude increased linearly with temperature. These results are in contrast to those reported for the kinetics of [125I]insulin dissociation. The functional significance of DEC remains largely unexplained. It was found that the fraction of SDC was dependent upon time of association in a temperature-dependent and apparently saturable process. It could not be attributed to alterations in the electrophoretic, immunologic or binding properties of [125I]TSH. Furthermore, no correlation was observed between generation of SDC and change in the Scatchard profile of TSH binding, in contrast to studies on growth hormone. These data suggest that, like some other polypeptide hormones, binding of TSH to its receptor does not proceed according to laws describing simple, rapidly reversible, bimolecular reactions. Furthermore, bound TSH undergoes a receptor-mediated conversion from a rapidly to a slowly dissociable state with time of incubation.

Glucocorticoid receptor in human embryo fibroblasts. I. Kinetic and physicochemical properties

The kinetics of dexamethasone binding to L 809 E cell line cytosol have been investigated by means of the protamine sulfate precipitation assay. The KDeq for dexamethasone was 1.1--3.3 nM. Binding was specific for glucocorticoids. The mean association rate constant (k+1) was 8.5 x 10(5) M-1 x min-1 and the dissociation rate constant was 4.6 x 10(-5) min-1 at 0 degrees C. The concentration of binding sites was 0.3 pmol/mg of cytosol protein. Binding kinetics were compatible with a model of positive cooperativity. The receptor sedimented at 7.5--9 S in glycerol gradients. By a combination of calibrated ultracentrifugation and polyacrylamide gel electrophoresis, a Stokes radius of 8.5 nm, a molecular weight of 268 000 daltons and a frictional ratio of 1.8 were determined in low ionic strength conditions. When the cells were incubated with 10 nM [3H]dexamethasone for 1 h, a more than 90% depletion of cytosol receptor and an equivalent accumulation of nuclear dexamethasone--receptor complexes was observed.

Equilibrium and kinetic studies of the binding of Lens culinaris lectin to rabbit erythrocytes by a quantitative fluorometric method

The binding of Lens culinaris lectin to the receptors of rabbit erythrocytes was studied by a quantitative fluorometric method using the fluoresceinated conjugate. Equilibrium data showed the presence of 1.2 x 10(6) receptors/cell with an association constant varying from 8 to 3 x 10(6) M-1 over a temperature range of 5 to 37 degrees C. The binding reaction is exothermic with a deltaH = -4.6 kcal/mol and a deltaS = + 15 eu. It appears that most of the bound lectin molecules had only a single site occupied, the agglutination being due to a very low fraction (0.0003--0.0056) of lectin molecules with two sites occupied. The specificity of the reaction was demonstrated through its inhibition by nonfluoresceinated LL and by the specific sugars methyl alpha-D-glucoside and methyl alpha-D-mannoside. From the inhibition curves an association constant of 4 x 10(2) M-1 was calculated for methyl alpha-d-glucoside and 9 x 10(2) for methyl alpha-D-mannoside. The association and dissociation rate constants of the binding reaction are, respectively, in the range of 3--10 x 103 and 3--33 x 10(-4) M-1 s-1 in the temperature range of 5 to 37 degrees C. The activation energies of the forward and reverse reactions are 7 and 13 kcal/mol, respectively. The association constants and the binding enthalpy calculated from the activation energies were fully consistent with those obtained by equilibrium measurements.

The two-step model of ligand-receptor interaction

The theoretical properties of a "two-step" receptor, consisting of two independent molecular entities, a regulatory (R) and an effector (E) unit, which are supposed to diffuse freely in the plane of the fluid and mosaic membrane, are presented. The ligand (L)-hormone, drug, neurotransmitter-binds to the regulatory unit (first step), so that it then becomes able to interact with the effector unit (second step) which is then activated.