RESUMO
Potassium channels are involved in modulating the excitability of neurones by regulating the membrane potential, or by affecting the amount of neurotransmitter released from nerve terminals. Potassium channels are highly diverse and can be activated by either voltage or increased intracellular calcium concentration. The potassium channel forms a highly selective membrane pore. Four subunits each with six membrane-spanning regions (S1-S6) are required to produce a functional pore. Molecular biologists have cloned more than 50 different potassium channel subtypes. Naturally occurring protein toxins have been used to pharmacologically characterize native and cloned potassium channels.
Assuntos
Canais de Potássio/efeitos dos fármacos , Canais de Potássio/fisiologia , Toxinas Biológicas/farmacologia , Canais de Cálcio/efeitos dos fármacos , Potenciais da Membrana/efeitos dos fármacosRESUMO
A peptide toxin, ShK, that blocks voltage-dependent potassium channels was isolated from the whole body extract of the Caribbean sea anemone Stichodactyla helianthus. It competes with dendrotoxin I and alpha-dendrotoxin for binding to synaptosomal membranes of rat brain, facilities acetylcholine release at an avian neuromuscular junction and suppresses K+ currents in rat dorsal root ganglion neurones in culture. Its amino acid sequence is R1SCIDTIPKS10RCTAFQCKHS20MKYRLSFCRK30TCGTC35. There is no homology with other K+ channel-blocking peptides, except for BgK from the sea anemone Bunodosoma granulifera. ShK and BgK appear to be in a different structural class from other toxins affecting K+ channels.
Assuntos
Venenos de Cnidários/química , Bloqueadores dos Canais de Potássio , Anêmonas-do-Mar/química , Sequência de Aminoácidos , Aminoácidos/análise , Animais , Venenos de Cnidários/isolamento & purificação , Venenos de Cnidários/toxicidade , Dados de Sequência Molecular , Canais de Potássio/análise , Ratos , Sinaptossomos/metabolismoRESUMO
Muscarinic toxins MTx1 and MTx2 are 7500 mol. wt polypeptides isolated from the venom of the green mamba snake Dendroaspis angusticeps. Previous competition binding studies indicate that the MTxs may be selective for the M1 subtype of muscarinic acetylcholine receptors. The present work was undertaken in order to clarify the muscarinic subtype specificity and functional effects of MTx1 and MTx2. Binding interactions were determined using 3H-N-methyl scopolamine (NMS) and cloned human muscarinic receptor subtypes m1, m2, m3 and m4. Some preliminary functional studies were performed on rabbit vas deferens preparations, which contain M1 cholinoceptors. MTx1 and MTx2 inhibited 3H-NMS binding to m1 and m3 receptors, with little effect on binding to m2 and m4 receptors. Affinity was higher for m1 receptors: Ki for MTx1 were 48 nM at m1 receptors and 72 nM at m3 receptors, and Ki for MTx2 were 364 nM at m1 and 1.2 microM at m3 receptors. At m1 receptors, about 90% of the binding of MTx1 and MTx2 appears to be irreversible. On rabbit vas deferens preparations, MTx1 and MTx2 at concentrations above 50 nM behaved in a similar way to the relatively selective M1-agonists McN-A-343 and CPCP (4-[N-(chlorophenyl)carbamoyloxy]-4-20-ynyl-trimethylammoniu m iodide) by reducing responses to nerve stimulation. The results confirm that MTx1 and MTx2 bind to m1 receptors rather than to m2 or m4 receptors, but they also reveal a slightly weaker effect at m3 receptors. The interaction at m1 receptors appears to be essentially irreversible, implying that the toxins could be useful tools in studies of the functional role of m1 muscarinic receptors.
Assuntos
Venenos Elapídicos/metabolismo , Neurotoxinas/metabolismo , Receptores Muscarínicos/metabolismo , Animais , Venenos Elapídicos/farmacologia , Masculino , Neurotoxinas/farmacologia , Coelhos , Ensaio Radioligante , Receptores Muscarínicos/genética , Proteínas Recombinantes/metabolismo , Proteínas de RépteisRESUMO
Muscarinic acetylcholine receptors exist as five subtypes that are widely distributed throughout the body. Conventional pharmacological agents are not highly selective for particular subtypes, making investigations on the functional significance of the subtypes difficult. Recent findings indicate that mamba snake venoms contain several small proteins ('muscarinic toxins') that are highly specific for muscarinic receptors, and are discussed in this review by Diana Jerusalinsky and Alan Harvey. Some of these toxins act selectively and irreversibly on individual subtypes of receptor, and some are antagonists, while others activate muscarinic receptors. The toxins should be useful tools in studies of the functions of individual receptor subtypes, and comparisons of their three-dimensional structures should give clues about how selective binding to muscarinic receptor subtypes can be obtained.
Assuntos
Venenos Elapídicos/química , Proteínas/química , Receptores Muscarínicos/química , Toxinas Biológicas/química , Sequência de Aminoácidos , Animais , Dados de Sequência MolecularRESUMO
A peptide toxin affecting potassium channels was isolated from the sea anemone Bunodosoma granulifera. It facilitates acetylcholine release at avian neuromuscular junctions, competes with dendrotoxin I, a probe for voltage-dependent potassium channels, for binding to synaptosomal membranes of rat brain with a Ki of 0.7 nM and suppresses K+ currents in rat dorsal root ganglion neurones in culture. It represents a new structural type of potassium channel toxin with the sequence V1RCDWFKETA10CRHAKSLGNC20RTSQKYRANC30AKTLQCC37 (M(r) 4275, three disulfides).