Reduction of a disulfide bond by beta-adrenergic agonists: evidence in support of a general "reductive activation" hypothesis for the mechanism of action of adrenergic agents

Zinc inhibition of electron transfer: mechanism of beta receptor inhibition?

Zinc has been shown to inhibit β-receptor activation of adenylate cyclase at a post receptor site. We have postulated that the β-receptor is one of several receptors activated by reduction, followed by transmembrane elector transfer accelerated by GTP. GTP accelerates electron transfer in a model system and this accelerated electron transfer is inhibited by zinc. This could explain the mechanism of the post receptor inhibition by zinc of the adenylate cyclase stimulation which follows β-receptor activation.

Evolving concepts of partial agonism

The exact mechanism of receptor activation at the molecular level are still not known, nor do we completely understand the precise factors that distinguish agonist- and partial agonist-induced activation. Nevertheless, recent years have brought forth an explosion of new information regarding beta-adrenergic receptor structure and ligand-induced activation. Partial agonists are likely intermediate in their ability to interact with crucial serine residues (Ser204 and Ser207) on the beta-adrenergic receptor; these interactions allow either incomplete stimulation of the entire receptor population, or full stimulation of only a portion of the entire receptor population. From the work presented by Tota and Schimerlik for the muscarinic cholinergic receptor (another G-protein coupled receptor), it is likely that partial agonists induce or stabilize receptor conformations that have a lower affinity for their G protein compared to receptors stimulated by a full agonist. Molecular cloning of beta-adrenergic receptors and analyses of mutated and chimeric receptors expressed in transfected systems have indicated that domains of the receptor that bind agonists may be different from those with which antagonists interact. Thus, the ability of a partial agonist to interact with these two different domains may be a determinant of efficacy. Agonists alter the sulfhydryl redox status of the beta-adrenergic receptors in the presence of Gs. Disulfide rearrangement has been postulated to provide a structural constraint which biases G-protein-linked receptors in the "ground state" and may be important for stabilizing the active state of the receptor and holding the agonist/receptor/Gs ternary complex in the high-affinity state. Partial agonists induce this state less efficaciously or are less capable of holding the receptor in the active conformation to allow disulfide exchange to take place. The extent of receptor stimulation may dictate which G proteins are activated by a particular receptor, and thus which cellular effectors are stimulated. Alternatively, the level of activation of a receptor may translate into varying states of activation of a particular G protein (stabilized in part by disulfide bonds). Techniques such as fluorescence energy transfer in reconstitution systems or nuclear magnetic resonance spectroscopy should prove useful in distinguishing among these possible mechanisms. Ultimately, as a long-term goal, X-ray crystallography of unoccupied receptors and receptors liganded by partial or full agonists may provide definitive insights. Although definitive answers are not yet possible, the rapid progress in understanding aspects of receptor structure allows a reformulation of ideas regarding the molecular basis of efficacy and partial agonism.(ABSTRACT TRUNCATED AT 400 WORDS)

Enhanced electron transfer by unsaturated fatty acids and superoxide dismutase

We have previously shown that unsaturated fatty acids (UFA) facilitate electron transfer between iron centers such as ferrous iron and ferricytochrome C. Extending this concept to a more physiologic model of fatty acids associated with proteins, we find that electron transfer is also enhanced in this model. While investigating whether free superoxide was involved in this electron transfer, we discovered that superoxide dismutase (SOD) enhanced the electron transfer. While the mechanism of electron transfer is unknown, the above findings are consistent with UFA and SOD participating in membrane redox systems.

Enhanced electron transfer by GTP: cross-membrane electron signaling by G-proteins?

Activation of several receptor types is followed by their binding to a G-protein. Prior to transmission of the agonist signal, the G-protein which had affinity for guanosine 5-diphosphate (GDP) binds guanosine 5-triphosphate (GTP) instead. Because evidence exists that several agonist groups activate their receptors by reduction, we evaluated whether the nucleotide associated with G-proteins could enhance electron flow. Using a model system of ferrous iron and ferric cytochrome c, it was determined that substitution of GTP for GDP led to an enhanced reduction of ferric cytochrome c. These results support the concept that cellular activation by certain receptors may involve reductive activation with the participation of GTP and G-proteins. We speculate that GTP, when bound to G-protein, can facilitate electron transfer perhaps from the receptor or the G-protein to the catalytic subunit of the adenylate cyclase enzyme.

Electron transfer facilitated by superoxide dismutase: a model for membrane redox systems?

Membranes, which are an amalgam of proteins and lipids, effect electron transfer through largely unknown mechanisms. Using albumin with bound fatty acids as a model, we have investigated the possible role of these two membrane constituents in electron transfer. In the presence of albumin: fatty acid, there is substantial enhancement of the reduction of ferricytochrome C by ferrous iron. To assess the possible role of free superoxide in cytochrome C reduction, we added mammalian copper/zinc containing superoxide dismutase (Cu/Zn SOD), which catalyzes the transfer of electrons between superoxide anion radicals, forming oxygen and hydrogen peroxide. Surprisingly, in the presence of either albumin or fatty acid free albumin, Cu/Zn SOD actually accelerates electron transfer from ferrous iron to ferricytochrome C. By contrast, neither inactive Cu/Zn SOD nor active manganese SOD facilitates the ferrous iron-dependent reduction of cytochrome C. These results suggest that, in some circumstances, Cu/Zn SOD may transfer electrons to alternative acceptors and that such transfer depends upon the unique reduction/oxidation reaction mechanism of Cu/Zn SOD. If so, this ubiquitous enzyme could be involved in regulating cellular electron transfer reactions as well as acting as a superoxide 'detoxify-ing' agent.