[3H]norharman ([3H]beta-carboline) binds reversibly and with high affinity to a specific binding site in rat liver

The strong inhibition of triosephosphate isomerase by the natural beta-carbolines may explain their neurotoxic actions

The natural beta-carbolines (BC) closely resemble the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in structure. The N-methylated beta-carbolinium ions (BC+) are potent inhibitors of mitochondrial respiration and are nigrostriatal neurotoxins. Utilizing [3H]BC, we have identified several proteins to which BC binds with high affinity (e.g. the chaperone member glucose regulated protein 78, the enzyme carboxylesterase, the cytochrome P450 2E1, the enzyme monoamine oxidase B and a small G-protein of the Rho subfamily). In the present study we isolated a protein from bovine brain to which [3H]BC binds with high affinity and identified it being the enzyme triosephosphate isomerase (TPI; EC 5.3.1.1.). 2,9-Dimethyl-BC+ was the most potent inhibitor of TPI, clearly more potent than the known inhibitors. TPI deficiency is a rare disorder in humans characterized by a severe progressive extrapyramidal course. Thus, TPI inhibition could contribute to neurodegeneration observed after injection of BCs into substantia nigra. Furthermore, the findings fit into the hypothesis of BCs as endogenous toxins responsible for neurodegeneration.

Possibility of the involvement of 9H-pyrido[3,4-b]indole (norharman) in carcinogenesis via inhibition of cytochrome P450-related activities and intercalation to DNA

This study investigated the inhibitory effect of 9H-pyrido[3,4-b]indole (norharman), one of the naturally occurring beta-carbolines, on cytochrome P450 (CYP)-related activities and the relationship between its inhibitory effect, its intercalation to DNA, and its comutagenic effect. Norharman reduced the mutagenicities of heterocyclic amines (HCAs) containing 2-amino-6-methyldipyrido[1,2-a:3',2'-d]imidazole (Glu-P-1), aflatoxin B1, benzo[a]pyrene (BP), and some nitrosamines in the presence of 10 microl liver S9 (20.9 microg protein/ml) from polychlorinated biphenyl-treated rats. Norharman inhibited microsomal CYP-related enzyme activities and CO-binding to the CYP heme (50% inhibitory concentration (IC50), 0.07-6.4 microg/ml). It also inhibited the formation of 3-hydroxyamino-6-methyldipyrido[1,2-a:3',2'-d]imidazole (N-OH-Glu-P-1) and was a noncompetitive-inhibitor of CYP1A-related activities, while it enhanced the direct mutagenicity of N-OH-Glu-P-1 (50% effective concentration, 25.0 microg/ml) and inhibited topo I activity (IC50, 31.0 microg/ml). In the presence of norharman, S9 up to 100 microl incrementally enhanced the mutagenicities of HCAs, BP and dimethylnitrosamine. These data clarified that norharman acts as an inhibitor of the CYP-mediated biotransformation of Glu-P-1 via inhibition of O2-binding to CYP heme, and its inhibition of CYP enzymes occurs at much lower concentration than that for its intercalation to DNA. It is indicated that norharman's inhibitory effect on CYP results in the inhibition of excess metabolism by S9 and this is more likely the mechanism for comutagenic action than the intercalation. Norharman's inhibition of CYP and its enhancement of the N-OH-Glu-P-1 mutagenicity suggest that beta-carbolines modulate chemical carcinogenesis by controlling the xenobiotic metabolism and by intercalating to DNA.

Isolation and identification of two [(3)H]norharman- ([(3)H]beta-carboline)-binding proteins from rat liver

Norharman (9H-pyrido-[3,4-b]indol) represents a member of the mammalian alkaloids with the group name beta-carbolines. In mammals, it exhibits psychotropic and co-mutagenic actions. Highly specific [(3)H]norharman binding sites have been detected in the liver of rats (B(max): 11 pmol mg(-1) protein; K(D): lower nanomolar range). Two [(3)H]norharman binding proteins with apparent molecular masses of 60 and 80 kDa (SDS-PAGE) were isolated from rat liver crude membrane fraction and identified as the enzyme carboxylesterase (EC 3.1.1.1; 60 kDa) and the stress protein glucose-regulated protein 78 (GRP78; 78 kDa). Possible functional consequences of the interaction of norharman with these two proteins are discussed.

The high-affinity binding of [3H]norharman ([3H]beta-carboline) to the ethanol-inducible cytochrome P450 2E1 in rat liver

High-affinity binding sites of [3H]norharman (synonymous: [3H]beta-carboline) were characterized in microsomal membranes from rat liver utilizing various beta-carboline (BC) derivatives and substances binding to enzymes of the cytochrome P450 (CYP) superfamily (EC 1.14.14.1). Saturation experiments demonstrated that [3H]norharman binds with high-affinity (dissociation constant 20.86 nM; maximum binding 21.40 pmol/mg protein). Displacement experiments with the beta-carboline derivatives 6-methyl-BC and 6-hydroxy-BC revealed a better adaptation to the two-site model, indicating that [3H]norharman binds to at least two sites, with an affinity of the high-affinity site in the low nM range. Substances binding with relative preference to isozymes of the CYP superfamily displaced [3H]norharman with a lesser potency than unlabeled norharman. Imidazole, pyrazole, and 4-methylpyrazole, known as inducers of the ethanol-inducible CYP2E1, displaced [3H]norharman with relative high potency. Furthermore, binding experiments with microsomes from human lymphoblast-expressed rat CYP2E1 revealed a high-affinity binding site [inhibition constant (Ki) 13.21 nM] comparable to that of microsomal membranes for norharman. It was displaceable by ethanol (Ki 14.25 microM), indicating that norharman and ethanol bind to the same binding site on CYP2E1. In vivo experiments with rats which had ingested ethanol for two weeks revealed that norharman blood plasma levels were significantly elevated at the end of this period, supporting the notion of an interaction of norharman and ethanol metabolism. Since it has been demonstrated in the Ames test that norharman's comutagenic action is connected with microsomal membranes (containing CYP isozymes), the present findings suggest that the observed increase in the levels of norharman in alcoholics leads to further CYP enzyme induction and thereby contributes to the increased risk of carcinomas in these patients.

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

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.