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.