Activation by β-carbolines of G-proteins in HL-60 membranes and the bovine retinal G-protein transducin in a receptor-independent manner

Abstract

Naturally occurring β-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 β-carbolines affect transmembrane signal transduction processes is still unknown. Since β-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 β-carbolines act directly on G-proteins. Therefore, we investigated the ability of β-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 β-carbolines norharman and barman, stimulated the GTPase in HL-60 membranes with an ec 50 of 410 μM and 450 μM, respectively, and a maximum effect at 1 mM each. Norharman and barman stimulated the GTPase of TD with an ec 50 of 60 μM and 300 μM, 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 β-carbolines showed a specifity 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 β-carboline molecule in position 6 led to a loss of GTPase-activating properties. Our data suggest that naturally occurring β-carbolines are a novel class of receptor-independent G-protein activating substances. This mechanism could contribute to their diverse biological effects.