| Abstract: | The mode of coupling of the adenosine receptor to adenylate cyclase in turkey erythrocyte membranes was probed by two independent approaches. The progressive inactivation of the adenosine receptor by an adenosine receptor affinity label resulted in the proportional reduction in the adenosine plus GppNHp dependent specific activity. In contrast, the intrinsic rate constant (k3), characterizing the process of adenylate cyclase activation by the adenosine-adenosine receptor complex, is independent of the extent of receptor inactivation. This behavior favors the precoupled mechanism, A + R.E: formula: (see text), where the receptor R and the enzyme E are permanently coupled to each other and the adenosine A binds to the receptor and induces the first-order process of cyclase activation to its active form ARE'. The finding that adenosine receptor is permanently coupled to the cyclase catalytic unit is corroborated by the observation that the progressive increase in membrane fluidity has no effect on the rate constant (k3) of adenylate cyclase activation by the adenosine-adenosine receptor complex and that the dose-response curve for adenosine is noncooperative. |
