Blood Harmane (1-Methyl-9H-Pyrido[3,4-b]indole) and Mercury in Essential Tremor: A Population-Based, Environmental Epidemiology Study in the Faroe Islands.

BACKGROUND: Essential tremor (ET) is among the most prevalent neurological diseases. Its environmental determinants are poorly understood. Harmane (1-methyl-9H-pyrido[3, 4-b]indole), a dietary tremor-producing neurotoxin, has been linked to ET in a few studies in New York and Madrid. Mercury, also a tremor-producing neurotoxin, has not been studied in ET. The Faroe Islands have been the focus of epidemiological investigations of numerous neurological disorders. OBJECTIVE: In this population-based, case-control study, we directly measured blood harmane concentrations (HA) and blood mercury concentrations (Hg) in ET cases and controls. METHODS: In total, 1,328 Faroese adults were screened; 26 ET cases were identified whose (HA) and (Hg) were compared to 197 controls. RESULTS: Although there were no statistically significant differences between diagnostic groups, median (HA) was 2.7× higher in definite ET (4.13 g-10/mL) and 1.5× higher in probable ET (2.28 g-10/mL) than controls (1.53 g-10/mL). Small sample size was a limitation. For definite ET versus controls, p = 0.126. (Hg) were similar between groups. CONCLUSIONS: We demonstrated marginally elevated (HA) in definite and probable ET. These data are similar to those previously published and possibly extend etiological links between this neurotoxin and ET to a third locale. The study did not support a link between mercury and ET.

Specific binding of [(18)F]fluoroethyl-harmol to monoamine oxidase A in rat brain cryostat sections, and compartmental analysis of binding in living brain.

We investigated [(18)F]fluoroethyl-harmol ([(18)F]FEH) as a reversible and selective ligand for positron emission tomography (PET) studies of monoamine oxidase A (MAO-A). Binding of [(18)F]FEH in rat brain cryostat sections indicated high affinity (KD = 3 nM), and density (Bmax; 600 pmol/g). The plasma free fraction was 45%, and untransformed parent constituted only 13% of plasma radioactivity at 10 min after injection. Compartmental analysis of PET recordings in pargyline-treated rats showed high permeability to brain (K1; 0.32 mL/g/min) and slow washout (k2; 0.024/min), resulting in a uniformly high equilibrium distribution volume (VD; 20 mL/g). Using this VD to estimate unbound ligand in brain of untreated rats, the binding potential ranged from 4.2 in cerebellum to 7.2 in thalamus. We also calculated maps of rats receiving [(18)F]FEH at a range of specific activities, and then estimated saturation binding parameters in the living brain. In thalamus, striatum and frontal cortex KD was globally close to 300 nM and Bmax was close to 1600 pmol/g; the 100-fold discrepancy in affinity suggests a very low free fraction for [(18)F]FEH in the living brain. Based on a synthesis of findings, we calculate the endogenous dopamine concentration to be 0.4 µM in the striatal compartment containing MAO-A, thus unlikely to exert competition against [(18)F]FEH binding in vivo. In summary, [(18)F]FEH has good properties for the detection of MAO-A in the rat brain by PET, and may present logistic advantages for clinical research at centers lacking a medical cyclotron. We made a compartmental analysis of [(18)F]fluoroethylharmol ([(18)F]FEH) binding to monoamine oxidase A (MAO-A) in living rat brain and estimated the saturation binding parameters from the binding potential (BPND). The Bmax was of comparable magnitude to that in vitro, but with apparent affinity (300 nM), it was 100-fold lower in vivo. PET imaging with [(18) F]FEH is well suited for quantitation of MAO-A in living brain.

Blood harmane (1-methyl-9H-pyrido[3,4-b]indole) concentrations in essential tremor: repeat observation in cases and controls in New York.

Essential tremor (ET) is a widespread late-life neurological disease. Genetic and environmental factors are likely to play important etiological roles. Harmane (1-methyl-9H-pyrido[3,4-b]indole) is a potent tremor-producing neurotoxin. Previously, elevated blood harmane concentrations were demonstrated in ET cases compared to controls, but these observations have all been cross-sectional, assessing each subject at only one time point. Thus, no one has ever repeat-assayed blood harmane in the same subjects twice. Whether the observed case-control difference persists at a second time point, years later, is unknown. The current goal was to reassess a sample of our ET cases and controls to determine whether blood harmane concentration remained elevated in ET at a second time point. Blood harmane concentrations were quantified by a well-established high-performance liquid chromatography method in 63 ET cases and 70 controls. A mean of approximately 6 yr elapsed between the initial and this subsequent blood harmane determination. The mean log blood harmane concentration was significantly higher in cases than controls (0.30 ± 0.61 g(-10)/ml versus 0.08 ± 0.55 g(-10)/ml), and the median value in cases was double that of controls: 0.22 g(-10)/ml versus 0.11 g(-10)/ml. The log blood harmane concentration was highest in cases with a family history of ET. Blood harmane concentration was elevated in ET cases compared to controls when reassessed at a second time point several years later, indicating what seems to be a stable association between this environmental toxin and ET.

Beta-carboline alkaloids and essential tremor: exploring the environmental determinants of one of the most prevalent neurological diseases.

Essential tremor (ET) is among the most prevalent neurological diseases, yet its etiology is not well understood. Susceptibility genotypes undoubtedly underlie many ET cases, although no genes have been identified thus far. Environmental factors are also likely to contribute to the etiology of ET. Harmane (1-methyl-9H-pyrido[3,4-beta]indole) is a potent, tremor-producing beta-carboline alkaloid, and emerging literature has provided initial links between this neurotoxin and ET. In this report, we review this literature. Two studies, both in New York, have demonstrated higher blood harmane levels in ET cases than controls and, in one study, especially high levels in familial ET cases. Replication studies of populations outside of New York and studies of brain harmane levels in ET have yet to be undertaken. A small number of studies have explored several of the biological correlates of exposure to harmane in ET patients. Studies of the mechanisms of this putative elevation of harmane in ET have explored the role of increased dietary consumption, finding weak evidence of increased exogenous intake in male ET cases, and other studies have found initial evidence that the elevated harmane in ET might be due to a hereditarily reduced capacity to metabolize harmane to harmine (7-methoxy-1-methyl-9H-pyrido[3,4-beta]-indole). Studies of harmane and its possible association with ET have been intriguing. Additional studies are needed to establish more definitively whether these toxic exposures are associated with ET and are of etiological importance.

Meat Consumption and Meat Cooking Practices in Essential Tremor: A Population-Based Study in the Faroe Islands.

Background: Elevated tissue levels of the tremor-producing neurotoxin, harmane, have been detected in patients with essential tremor (ET) in the USA and Spain. Recently, a study in the Faroe Islands similarly noted an elevation in blood harmane concentrations in probable and definite ET cases. The underlying mechanism is not understood. Possible mechanisms include increased dietary consumption (esp. through cooked meats), impaired metabolism, or increased endogenous production of harmane. To investigate this issue further, we conducted a population-based study in the Faroe Islands to examine meat consumption and meat cooking practices in ET cases and controls. Methods: 1,328 Faroese adults were screened for tremor and 27 ET cases were identified. Meat consumption and meat cooking practices were compared to 200 controls. Detailed data were collected via questionnaires regarding current meat consumption for 14 meat types and meat cooking doneness for 8 meat types. Data were also available on blood harmane concentrations. Results: Current meat consumption was similar in ET cases and controls in 12 out of 14 meat types, with no differences observed after a Bonferroni correction in any meat type; no difference was observed when stratified by gender. No difference was observed in meat doneness between ET cases and controls. Blood harmane concentrations were not correlated with dietary data. Discussion: This is the first population-based study of harmane-linked dietary factors in ET. The study suggests the observed difference in blood harmane in ET is not driven by dietary differences and is likely due to other mechanisms (e.g., impaired metabolism).

Cancer and blood concentrations of the comutagen harmane in essential tremor.

Blood concentrations of harmane, a tremor-producing neurotoxin, are elevated in essential tremor (ET). Harmane is also a comutagen. Using a case-control design, we compared the prevalence of cancer in ET cases vs. controls, and determined whether blood harmane concentrations are elevated among ET cases with cancer. 66/267 (24.7%) ET cases vs. 55/331 (16.6%) controls had cancer (adjusted OR 1.52, 95% CI 1.01-2.30, P = 0.04). Among specific cancer types, colon cancer was more prevalent in ET cases than controls (2.6% vs. 0.6%, P = 0.04). Log blood harmane concentration was higher in ET cases vs. controls (P = 0.02) and in participants with vs. without cancer (P = 0.02). Log blood harmane concentration was highest in ET cases with cancer when compared with other groups (P = 0.009). These links between cancer and ET and between high blood harmane and cancer in ET deserve further study.

Elevated blood harmane (1-methyl-9H-pyrido[3,4-b]indole) concentrations in essential tremor.

Essential tremor (ET) is a widespread late-life neurological disease. Genetic and environmental factors likely play an etiological role. Harmane (1-methyl-9H-pyrido[3,4-b]indole) is a potent tremor-producing neurotoxin. In 2002, we demonstrated elevated blood harmane concentrations in an initial sample of 100 ET cases compared to 100 controls. Between 2002 and 2007, we assembled a new and larger sample of ET cases and controls. We now attempt to replicate our previous findings. Cases and controls were frequency-matched on age, gender, and race. Blood harmane concentrations were quantified by high-performance liquid chromatography. Subjects comprised 150 ET cases and 135 controls (mean age 65.3+/-15.5 vs. 65.5+/-14.2 years, p=0.94). Mean log blood harmane concentration was approximately 50% higher in cases than controls (0.50+/-0.54g(-10)/ml vs. 0.35+/-0.62g(-10)/ml, p=0.038). In a logistic regression analysis, log blood harmane concentration was associated with ET (OR(adjusted) 1.56, 95% CI 1.01-2.42, p=0.04), and odds of ET was 1.90 (95% CI 1.07-3.39, p=0.029) in the highest versus lowest log blood harmane tertile. Log blood harmane was highest in ET cases with familial ET (0.53+/-0.57g(-10)/ml), intermediate in cases with sporadic ET (0.43+/-0.45g(-10)/ml) and lowest in controls (0.35+/-0.62g(-10)/ml) (test for trend, p=0.026). Blood harmane appears to be elevated in ET. The higher concentrations in familial ET suggests that the mechanism may involve genetic factors.

Higher blood harmane (1-methyl-9H-pyrido[3,4-b]indole) concentrations correlate with lower olfactory scores in essential tremor.

BACKGROUND: Harmane (1-methyl-9H-pyrido[3,4-b]indole), a neurotoxin, may be an environmental risk factor for essential tremor (ET). Harmane and related chemicals are toxic to the cerebellum. Whether it is through this mechanism (cerebellar toxicity) that harmane leads to ET is unknown. Impaired olfaction may be a feature of cerebellar disease. OBJECTIVE: To determine whether blood harmane concentrations correlate with olfactory test scores in patients with ET. METHODS: Blood harmane concentrations were quantified using high performance liquid chromatography. Odor identification testing was performed with the University of Pennsylvania Smell Identification Test (UPSIT). RESULTS: In 83 ET cases, higher log blood harmane concentration was correlated with lower UPSIT score (rho=-0.46, p<0.001). 25/40 (62.5%) cases with high log blood harmane concentration (based on a median split) had low UPSIT scores (based on a median split) vs. 12/43 (27.9%) ET cases with low log blood harmane concentration (adjusted odd ratios (OR) 4.04, 95% confidence intervals (CI) 1.42-11.50, p=0.009). When compared with the low log blood harmane tertile, the odds of olfactory dysfunction were 2.64 times higher in cases in the middle tertile and 10.95 times higher in cases in the high tertile. In 69 control subjects, higher log blood harmane concentration was not correlated with lower UPSIT score (rho=0.12, p=0.32). CONCLUSIONS: Blood harmane concentrations were correlated with UPSIT scores in ET cases but not controls. These analyses set the stage for postmortem studies to further explore the role of harmane as a cerebellar toxin in ET.

Role of acetaldehyde in tobacco smoke addiction.

This review evaluates the presumed contribution of acetaldehyde to tobacco smoke addiction. In rodents, acetaldehyde induces reinforcing effects, and acts in concert with nicotine. Harman and salsolinol, condensation products of acetaldehyde and biogenic amines, may be responsible for the observed reinforcing effect of acetaldehyde. Harman and salsolinol inhibit monoamine oxidase (MAO), and some MAO-inhibitors are known to increase nicotine self-administration and maintain behavioural sensitization to nicotine. Harman is formed in cigarette smoke, and blood harman levels appear to be 2-10 times higher compared to non-smokers. Since harman readily passes the blood-brain barrier and has sufficient MAO-inhibiting potency, it may contribute to the lower MAO-activity observed in the brain of smokers. In contrast, the minor amounts of salsolinol that can be formed in vivo most likely do not contribute to tobacco addiction. Thus, acetaldehyde may increase the addictive potential of tobacco products via the formation of acetaldehyde-biogenic amine adducts in cigarette smoke and/or in vivo, but further research is necessary to substantiate this hypothesis.

Positron emission tomography quantification of [11C]-harmine binding to monoamine oxidase-A in the human brain.

This article describes the kinetic modeling of [(11)C]-harmine binding to monoamine oxidase A (MAO-A) binding sites in the human brain using positron emission tomography (PET). Positron emission tomography studies were performed in healthy volunteers at placebo conditions and after treatment with clinical doses of moclobemide. In either condition, a two-tissue compartment model (2CM) provided better fits to the data than a one-tissue model. Estimates of k(3)/k(4) values from an unconstrained 2CM were highly variable. In contrast, estimates of the specifically bound radioligand distribution volume (DV(B)) from an unconstrained 2CM were exceptionally stable, correlated well with the known distribution of MAO-A in the brain (cerebellum

Fast and slow metabolizers of Hoasca.

Harmine, a major alkaloid in ayahuasca (hoasca), is a selective and reversible inhibitor of the enzyme monoamine oxidase-A (MAO-A). It is also a selective inhibitor of the human cytochrome P450 isozyme 2D6 (CYP 2D6), which metabolizes harmine to a more hydrophilic derivative for eventual excretion. CYP 2D6 exhibits a wide range of polymorphisms in human populations, and variations in this enzymatic activity could account for differences in effects between individuals who use hoasca. This report broadly describes two subgroups of CYP 2D6 phenotypes--i.e., fast and slow metabolizers of harmine-in 14 experienced male members of the União do Vegetal (UDV) who received a standardized dosage of hoasca. To compensate for metabolic variations in their normal religious practice, the administered dose of hoasca is always determined by the presiding mestre, who is responsible for deciding the actual amount for each individual. This age-old method compensates for metabolic variations between individuals and variations in both the alkaloid profile and strength of the hoasca.

Elevation of blood beta-carboline alkaloids in essential tremor.

BACKGROUND: beta-Carboline alkaloids are normal body constituents but are also potent tremor-producing chemicals that are naturally present in the food chain. OBJECTIVE: To explore the hypothesis that high concentrations of beta-carboline alkaloids are associated with essential tremor (ET). METHODS: One hundred cases and 100 controls were frequency matched on age, sex, and ethnicity. Blood concentrations of harmane and harmine were quantified by high-performance liquid chromatography, blinded to clinical information. RESULTS: The mean log blood concentration of harmane was higher in cases than controls (0.72 +/- 0.53 vs 0.51 +/- 0.64 g(-10)/mL; p = 0.01). A nonparametric test on nontransformed data (median harmane = 5.21 g(-10)/mL in cases and 2.28 g(-10)/mL in controls) confirmed this difference (p = 0.005). The mean log blood concentration of harmine was 0.20 +/- 0.48 g(-10)/mL in cases and 0.10 +/- 0.65 g (-10)/mL in controls (p = 0.20). Log harmane concentrations were stratified based on the median value; 62% of cases vs 39% of controls had a high log harmane concentration (p = 0.001). Mean log harmane concentration was similar in the cases with (0.74 +/- 0.58 g(-10)/mL) and without (0.71 +/- 0.50 g(-10)/mL) an affected relative (p = 0.83). CONCLUSIONS: Blood concentrations of harmane were measured in ET cases compared with controls. Concentrations were elevated in cases with and without a family history of ET.

Long-term changes of markers of alcoholism after orthotopic liver transplantation (OLT).

Both physical rehabilitation and the course of the alcoholism improve after orthotopic liver transplantation (OLT) in patients with end-stage alcoholic liver cirrhosis. In the present study including 17 alcoholics and 14 nonalcoholics, after OLT, three of the alcoholic patients resumed their pre-OLT alcohol drinking habits, 4 consumed alcohol occasionally, 10 remained abstinent over the observation period of 13 to 36 months. The laboratory parameters before OLT did not discriminate alcoholics from nonalcoholic patients. Furthermore, the blood levels of two so-called alcogens (harman and norharman) were determined to investigate whether they discriminate between the two groups. Alcogens are natural compounds that are presumed to induce alcohol abuse in predisposed individuals. Both alcogens measured were elevated in plasma from nonalcoholics and alcoholics before OLT, suggesting a disturbance in inactivation in end-stage liver disease. Following OLT, the alcogens normalized but in the alcoholics this process was slower with respect to harman. The present exploratory study suggests that the normalized metabolic capacity of the liver after OLT causes a normalization of the levels of alcogens, for which harman and norharman are representative. These changes could contribute to the observed benefit to the outcome in alcoholics with respect to the alcohol dependence.

Secretion of the organic anion harmol sulfate from liver into blood. Evidence for a carrier-mediated mechanism.

In the liver, drugs with phenolic groups can be converted to sulfate or glucuronide conjugates and are then transported into bile or back into the bloodstream. The mechanism for transport of drugs and drug conjugates from the hepatocytes into the blood at the sinusoidal side of the cell are not well defined. In the case of carrier-mediated transport of these strongly polar conjugates, saturability of transport and mutual competition between structurally related compounds would be anticipated. This competitive aspect was investigated by using two organic anions, dibromosulfophthalein (DBSP) and harmol sulfate. The latter compound was generated by the hepatocytes from harmol, which was continuously infused into the rat in vivo and in isolated perfused rat livers. In addition we loaded the perfused rat livers with preformed harmol sulfate and studied its efflux rate to the perfusate as influenced by DBSP. In steady state, about 80% of harmol was sulfated and 20% was glucuronidated. Harmol sulfate was mainly excreted in the urine, the glucuronide was equally excreted in urine and bile. DBSP lowered the urinary excretion of harmol sulfate by about 30% which was due to a decrease in plasma concentration. However, renal clearance of harmol sulfate (3.2 +/- 0.2 ml/min) was unchanged. At the same time DBSP doubled the biliary clearance of harmol sulfate (1.0 +/- 0.1 and 2.2 +/- 0.2 ml/min in controls and DBSP studies respectively). DBSP decreased glucuronide excretion both in urine and bile. The increase in biliary output and decrease in urinary excretion of harmol sulfate is explained by competitive interaction between the organic anion DBSP and harmol sulfate at the sinusoidal level. Efflux experiments in single pass perfused isolated livers showed a clear decrease of harmol sulfate transport from liver into plasma by DBSP and provided evidence for such an inhibitory phenomenon (t 1/2 of efflux was 3.58 +/- 0.21 compared with 2.46 +/- 0.07 min for controls). These results indicate that organic anion transport from the hepatocyte into the blood stream is very likely carrier-mediated. A decrease in renal output of drug conjugates therefore may not only be due to a decrease in the conjugation process but also to a lower liver to blood transport rate which at the same time may produce a higher biliary output.

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.

Antibiotic-mediated release of tumour necrosis factor alpha and norharman in patients with hospital-acquired pneumonia and septic encephalopathy.

OBJECTIVE: To investigate antibiotic-mediated release of tumour necrosis factor (TNF)-alpha and norharman in patients with hospital-acquired pneumonia with and without additional septic encephalopathy. DESIGN: Prospective observational study with a retrospective post hoc analysis. SETTING: Surgical intensive care unit (ICU) at a university hospital. PATIENTS: Thirty-seven patients were consecutively included (9 patients with hospital-acquired pneumonia, 11 patients with hospital-acquired pneumonia and septic encephalopathy, 17 control patients) in the study. Pneumonia was defined according to the criteria of the American Thoracic Society. INTERVENTIONS: Patients received cephalosporins for antibiotic treatment of hospital-acquired pneumonia. Blood samples were taken before, immediately after and 4 h after application of cephalosporins. MEASUREMENTS AND RESULTS: Of the pneumonia patients, 55% developed septic encephalopathy. ICU stay, complications and mortality were significantly increased. An increased release of TNF-alpha was immediately seen in all pneumonia patients after antibiotics compared to controls, whereas the level did not differ between patients with and without septic encephalopathy. Norharman was significantly increased in pneumonia patients 4 h after antibiotic treatment, in tendency more enhanced in the pneumonia patients without encephalopathy. CONCLUSIONS: Patients with hospital-acquired pneumonia and septic encephalopathy had a significantly longer ICU stay with higher mortality rate compared to patients with hospital-acquired pneumonia alone. Antibiotic-mediated TNF-alpha release may induce the kynurenine pathway. TNF-alpha activates indolamine-2,3-dioxygenase with neurotoxic quinolinic acid as the end product. Norharman seems to counteract this mechanism and seems to play a role in neuroprotection. The worse outcome of patients with encephalopathy expresses the need to investigate protective factors and mechanisms.

The levels of norharman are high enough after smoking to affect monoamineoxidase B in platelets.

Epidemiological studies suggest that smoking reduces the risk for Parkinson's disease. It has been hypothesized that inhibition of monoamineoxidase contributes to this action. The present study examined the contribution of the beta-carbolines norharman, an inhibitor of monoamineoxidase B, and harman, an inhibitor of monoamineoxidase A, which are present in high concentrations in tobacco smoke to the protective action. Nineteen active smokers and five nonsmokers smoked one and two cigarettes. The levels of norharman and harman increased in plasma from smokers and nonsmokers. Ex vivo saturation kinetic experiments revealed that the baseline affinity constant of monoamineoxidase in platelets from smokers was higher than that of nonsmokers in contrast to the maximum turnover rate, which did not differ. Acute smoking affected the monoamineoxidase in nonsmokers only. It is discussed that inhibition of both isoforms of monoamineoxidase is necessary for the neuroprotection and that both norharman and harman play an important role.

Harman (1-methyl-beta-carboline) is a natural inhibitor of monoamine oxidase type A in rats.

Harman (1-methyl-beta-carboline) displaces [3H]pargyline in vitro from high affinity binding sites on membranes from cerebral cortex, provided that experimental conditions are chosen under which [3H]pargyline labels selectively monoamine oxidase type A. Norharman (beta-carboline) is a much weaker displacing compound. It is well known that the type A enzyme can be blocked irreversibly in vivo by treatment of rats with clorgyline. Under these conditions no specific binding of [3H]harman and [3H]pargyline to monoamine oxidase type A was detected in brain, whereas the specific binding was reduced to 5% in liver tissue. The in vitro and ex vivo experiments suggest that there is a specific binding site for harman on monoamine oxidase type A, thereby extending earlier in vitro findings. It has been postulated that harman operates as a natural inhibitor of monoamine oxidase type A in mammals. The present study demonstrates that harman and norharman occur in rat brain, blood plasma, heart, kidney and liver. It further shows that pretreatment with clorgyline induces a time-dependent increase in the blood plasma levels of harman, suggesting the displacement of harman from the enzyme in tissue with its subsequent delivery into the blood. These findings strongly support the hypothesis based on in vitro experiments, that harman binds reversibly to the active site of monoamine oxidase type A in vivo. Dietary sources for mammalian harman play probably only a minor role, because the concentrations in beer and wine as well as other foodstuffs are too low to contribute substantially to endogenous levels of harman.

Tryptophan: a precursor for the endogenous synthesis of norharman in man.

We investigated whether in healthy subjects L-tryptophan may serve as a precursor for the endogenous synthesis of the beta-carboline norharman. For this purpose subjects, smokers as well as non-smokers, received 0 or 1.2 g of an oral dose of tryptophan. Smokers started the experiment 2 h after cessation of smoking. Plasma levels of tryptophan and norharman were measured 100 and 125 min after the start of the experiment. The levels of both compounds were significantly higher in the group receiving tryptophan. Norharman concentrations in the plasma of smokers were significantly higher than in the non-smoking subjects under both experimental conditions. These results add some proof to the hypothesis that in humans tryptophan may serve as a precursor for the synthesis of norharman.

The impact of smoking and drinking on plasma levels of norharman.

The hypothesized role of the beta-carboline norharman in processes of drug dependence forms the basis for several studies on plasma levels of norharman among substance-using populations, particularly among alcoholics and smokers. However, it is not clear whether norharman is implicated in processes of dependence to both substances, or only to tobacco smoke. In the present study plasma concentrations of norharman were measured among four groups of participants regarding heavy smokers who do or do not drink alcohol excessively and nonsmokers who do or do not drink alcohol excessively. All measurements were conducted on three different days with an interval of 2 months in between and at three times during the day to account for possible circadian or seasonal variations. Results showed that elevated plasma levels of norharman appear only in heavy smokers regardless of their drinking profile. The norharman plasma levels of nonsmoking excessive drinkers showed a similar pattern to that of the control group. The findings indicate that elevated plasma levels of norharman are due to heavy smoking and not to excessive drinking.