title: Activation by beta-carbolines of G-proteins in HL-60 membranes and the bovine retinal G-protein transducin in a receptor-independent manner.
abstract: Naturally occurring beta-carbolines are lipophilic compounds which show psychotropic and physiological effects in mammals. They bind to distinct high-affinity binding sites in various mammalian tissues. However, the mechanism by which the beta-carbolines affect transmembrane signal transduction processes is still unknown. Since beta-carbolines are cationic-amphiphilic substances and since such substances are known to activate heterotrimeric regulatory guanine nucleotide binding proteins (G-proteins) in a receptor-independent manner, we put forward the hypothesis that beta-carbolines act directly on G-proteins. Therefore, we investigated the ability of beta-carbolines to stimulate high-affinity GTP hydrolysis in membranes of dibutyryl-cAMP differentiated HL-60 cells and of the purified bovine G-protein, transducin (TD). The beta-carbolines norharman and harman, stimulated the GTPase in HL-60 membranes with an EC50 of 410 microM and 450 microM, respectively, and a maximum effect at 1 mM each. Norharman and harman stimulated the GTPase of TD with an EC50 of 60 microM and 300 microM, and a maximum at 1 mM for both compounds. The stimulatory effect of norharman in HL-60 membranes was pertussis toxin-sensitive. Structure/activity characteristics of the beta-carbolines showed a specificity of norharman to stimulate the GTPase of TD, because norharman activated GTP hydrolysis in HL-60 membranes approximately 7 times less potently than that of TD. Norharman was a five-fold more potent activator of TD than tetrahydronorharman. Hydroxylation of the beta-carboline molecule in position 6 led to a loss of GTPase-activating properties. Our data suggest that naturally occurring beta-carbolines are a novel class of receptor-independent G-protein activating substances. This mechanism could contribute to their diverse biological effects.
authors.affiliation: Howard Hughes Medical Institute, Beckman Center for Molecular and Genetic Medicine, Stanford University Medical Center, CA 94305, USA.
authors.forename: J F
authors.initials: JF
authors.lastname: Klinker
authors.forename: R
authors.initials: R
authors.lastname: Seifert
authors.forename: H
authors.initials: H
authors.lastname: Damm
authors.forename: H
authors.initials: H
authors.lastname: Rommelspacher
mesh_terms: D002243:Carbolines / Q000737:chemistry / Q000494:pharmacology*; D002462:Cell Membrane / Q000187:drug effects*; D004789:Enzyme Activation; D020558:GTP Phosphohydrolases / Q000378:metabolism; D019204:GTP-Binding Proteins / Q000378:metabolism*; D018922:HL-60 Cells / Q000187:drug effects; D006247:Harmine / Q000031:analogs & derivatives / Q000494:pharmacology; D006801:Humans; D036341:Intercellular Signaling Peptides and Proteins; D010455:Peptides; D013329:Structure-Activity Relationship; D015293:Transducin / Q000378:metabolism*; D014862:Wasp Venoms / Q000494:pharmacology
chemical_list: D002243:Carbolines; D036341:Intercellular Signaling Peptides and Proteins; D010455:Peptides; D014862:Wasp Venoms; D006247:Harmine; C037500:mastoparan; C005010:harman; C010262:norharman; D020558:GTP Phosphohydrolases; D019204:GTP-Binding Proteins; D015293:Transducin