Hormone receptor mobility and catecholamine binding in membranes. A theoretical model.
[3H]-Catecholamine binding to intact cells, isolated cell membranes, and to several isolated macromolecules has been shown by several laboratories to be neither stereospecific nor inhibited by known beta-antagonists. Since additional evidence indicates that this binding is not an artifact (i.e. due neither to the binding of a catecholamine oxidation product nor hormone binding to a catabolic enzyme such as COMT), the question remains as to whether this represents binding to a bona fide membrane receptor. Because all ligands which bind strongly or compete for this binding possess a catechol group, one possible explanation is that the binding affinity is primarily determined by the catechol moiety, whereas the correct stereoisomer of the side chain is necessary to activate the receptor. Thus, although binding is a necessary condition for hormone action, the necessary and sufficient condition for activation of adenyl cyclase is both the catechol group and the correct stereoisomer of the side chain. A theoretical model is developed here to provide a quantitative basis for this hypothesis. This model extends the current concept of distinct subunits in the adenyl cyclase system by separating the receptors from the catalytic sites and placing them at separate locations within the membrane. Utilizing the spare receptor model of Furchgott, and the mobility of macromolecules within a "lipid sea," the appropriate equations to predict both hormone binding and enzyme activation are derived. Using the observed affinity constants from catecholamine binding studies, it is then shown that this model can predict the experimental observation and hence explain the apparent dichotomy arising from binding enzyme activation studies.