β-Carboline Alkaloids in Soy Sauce and Inhibition of Monoamine Oxidase (MAO)

Monoamine oxidase (MAO) oxidizes neurotransmitters and xenobiotic amines, including vasopressor and neurotoxic amines such as the MPTP neurotoxin. Its inhibitors are useful as antidepressants and neuroprotectants. This work shows that diluted soy sauce (1/3) and soy sauce extracts inhibited human MAO-A and -B isozymes in vitro, which were measured with a chromatographic assay to avoid interferences, and it suggests the presence of MAO inhibitors. Chromatographic and spectrometric studies showed the occurrence of the β-carboline alkaloids harman and norharman in soy sauce extracts inhibiting MAO-A. Harman was isolated from soy sauce, and it was a potent and competitive inhibitor of MAO-A (0.4 µM, 44 % inhibition). The concentrations of harman and norharman were determined in commercial soy sauces, reaching 243 and 52 μg/L, respectively. Subsequently, the alkaloids 1,2,3,4-tetrahydro-β-carboline-3-carboxylic acid (THCA) and 1-methyl-1,2,3,4-tetrahydro-β-carboline-3-carboxylic acid (MTCA) were identified and analyzed in soy sauces reaching concentrations of 69 and 448 mg/L, respectively. The results show that MTCA was a precursor of harman under oxidative and heating conditions, and soy sauces increased the amount of harman under those conditions. This work shows that soy sauce contains bioactive β-carbolines and constitutes a dietary source of MAO-A and -B inhibitors.


Introduction
Monoamine oxidase (MAO) is an enzyme located at the outer membranes of mitochondria in the brain, liver, intestinal mucosa, and other organs that catalyzes the oxidative deamination of biogenic amines, neurotransmitters, vasoactive dietary amines and xenobiotic amines, including dopamine, serotonin, norepinephrine, tyramine, tryptamine and the MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) neurotoxin. MAO appears as two isozymes, MAO-A and B, which are distinguished by substrate and inhibitor selectivity [1]. MAO-A preferentially catalyzes the oxidation of serotonin and norepinephrine and is inhibited by clorgyline, whereas MAO-B selectively catalyzes the oxidation of phenylethylamine and benzylamine and is inhibited by (R)-deprenyl. Tyramine, dopamine, and tryptamine are substrates for both enzymes. MAO isozymes play a significant role in the metabolism and regulation of neurotransmitters and biogenic amines and are involved in several diseases [1,2]. Thus, MAO-B has been implicated in neurological disorders and diseases and MAO-A in psychiatric conditions and depression [3]. An increased number of inhibitors of MAO, both of synthetic and natural product origin, are currently a matter of interest in drug discovery [1,4], both as antidepressants (MAO-A inhibitors) [3,5] or neuroprotectants (MAO-B inhibitors) [6][7][8][9][10]. The oxidation of biogenic amines and neurotransmitters by MAO produces hydrogen peroxide (H 2 O 2 ), ammonia and aldehydes, which represent risk factors for oxidative cell injury and toxicity [11,12]. MAO also bioactivates proneurotoxins such as MPTP [13][14][15]. The inhibition of MAO could reduce reactive oxygen species or toxins [16,17]. On the other hand, the inhibition of peripheral MAO (e.g., intestinal and liver) has been linked to hypertensive crisis produced by a reduced metabolism of vasopressor dietary amines (e.g., tyramine) [1,18]. In this regard, the inhibition of MAO may potentiate the physiological effects of dietary vasoactive amines and exert possible food-drug interactions. The available inhibitors are sometimes irreversible inhibitors that may produce hypertensive crises, and in this regard, new inhibitors devoid of those undesirable effects are currently needed.
Dietary and environmental factors such as smoking, foods, herbal preparations or drugs might affect the action of MAO. MAO is reduced in smokers compared to nonsmokers [19,20], and cigarette smoke inhibits MAO-A and -B isozymes [21]. Naturally occurring β-carboline alkaloids have been identified in the inhibition of MAO by cigarette smoke [21,22], coffee [23] and raisins [24]. The inhibition of MAO in smokers and coffee drinkers might be linked with some biological actions, such as the addictive properties of cigarettes and depression [25,26] and the lowest incidence of Parkinson's disease (PD) [27][28][29][30]. More research is currently needed to study the effects of diet and environmental factors on MAO enzymes. On the other hand, new inhibitors of MAO devoid of undesirable effects might arise from naturally occurring compounds and sources. β-Carbolines are naturally occurring bioactive alkaloids that exhibit an array of pharmacological effects such as the binding to benzodiazepine, imidazoline, serotonin and opiate receptors as well as inhibition of kinases and MAO [14,[31][32][33][34][35][36][37][38][39]. Some of these substances are also involved in toxicological effects [14,15]. β-Carbolines can be absorbed and accumulated in tissues contributing to their presence and actions in the biological systems [40][41][42]. These alkaloids have been found in foods and seasonings [22,24,[43][44][45][46][47][48]. In this regard, soy sauce is a special seasoning made from salt, water, soybeans and wheat that, after heating processes, is traditionally fermented in a process involving molds, lactic bacteria and yeast lasting months, and this process may result in an increase of β-carboline alkaloids [49]. This research aimed to study the presence of β-carboline alkaloids in soy sauce and to assess the activity and inhibition of MAO by soy sauce using a chromatographic assay. It is shown that MAO isozymes are inhibited in vitro by soy sauce and soy sauce extracts, suggesting the presence of inhibitors. The aromatic β-carboline alkaloids norharman and harman were identified and analyzed in soy sauce, and harman was subsequently isolated as a potent and competitive inhibitor of MAO-A. Finally, tetrahydro-β-carboline-3-carboxylic acid (THβC-3-COOH) alkaloids were analyzed in soy sauce, and it was found that 1-methyl-1,2,3,4-tetrahydro-β-carboline-3-carboxylic acid (MTCA) was the precursor of the MAO inhibitor harman.

Results
The enzymatic activity of human MAO-A and -B was assayed by chromatographic means following HPLC analysis of 4-hydroxyquinoline (4-HQ) formed by enzymatic deamination of kynuramine (kyn) substrate ( Figure 1A). This chromatographic analysis allows the determination of the inhibition of MAO without interferences that may occur when using inhibitors present in complex mixtures [17,50]. The activity of MAO was studied in the presence of increasing concentrations of soy sauce. Both MAO-A and -B isozymes were inhibited in the presence of soy sauce (Figure 2A). This inhibition was reversible for MAO-A (90%) and partly reversible for MAO-B (60%), as suggested from the activity recovered following incubation of MAO with soy sauce when compared with controls. The inhibition of MAO-A and -B obtained with different samples of soy sauces (1/3 diluted) ranged from 23-50% for MAO-A and 30-73% for MAO-B ( Figure 2B). In the assays, the standard inhibitors of MAO, clorgyline (1 µM) and R-deprenyl (1 µM) provided more than 90% inhibition of MAO-A and -B, respectively.    The inhibition of the deamination of kynuramine by MAO suggested the presence of inhibitors in soy sauces. Then, soy sauce with a relatively high inhibition of MAO was fractionated by solid phase extraction (SPE) using propylsulfonic (PRS) columns. The isolated extract inhibited MAO-A and, to a lesser extent, MAO-B ( Figure 2C), suggesting that it contained compounds contributing to the inhibition of MAO-A. When considering dilutions, a similar degree of inhibition of MAO-A was observed with both soy sauce and their PRS extracts, suggesting that PRS extracts contained compounds that were contributors to MAO-A inhibition in soy sauce. The analysis of those PRS-extracts evidenced the presence of two β-carbolines that were identified by co-injection with standards, UV-VIS spectra and HPLC-MS as norharman (m/z at 169 (M + H) + ) and harman (m/z at 183 (M + H) + ; Figure 3A). Harman that was the major β-carboline in the PRS extracts, was isolated following successive RP-HPLC chromatographic injections by collecting the chromatographic peak at the end of the Diode Array Detector (DAD) and then used to study inhibition ( Figures 1B and 4A). This β-carboline from soy sauce was a potent inhibitor of MAO-A and showed a competitive type of inhibition when determined at different concentrations of substrate and inhibitor ( Figure 4B). The inhibition of MAO-A by the harman isolated from soy sauce was in agreement with the concentration of harman included in the assays and its IC 50 [21]. Thus, harman from soy sauce (0.4 µM) gave a 44 % inhibition of MAO-A using kynuramine (250 µM).
Molecules 2023, 28, x FOR PEER REVIEW 4 of 14 their PRS extracts, suggesting that PRS extracts contained compounds that were contributors to MAO-A inhibition in soy sauce. The analysis of those PRS-extracts evidenced the presence of two β-carbolines that were identified by co-injection with standards, UV-VIS spectra and HPLC-MS as norharman (m/z at 169 (M + H) + ) and harman (m/z at 183 (M + H) + ; Figure 3A). Harman that was the major β-carboline in the PRS extracts, was isolated following successive RP-HPLC chromatographic injections by collecting the chromatographic peak at the end of the Diode Array Detector (DAD) and then used to study inhibition ( Figures 4A and 1B). This β-carboline from soy sauce was a potent inhibitor of MAO-A and showed a competitive type of inhibition when determined at different concentrations of substrate and inhibitor ( Figure 4B). The inhibition of MAO-A by the harman isolated from soy sauce was in agreement with the concentration of harman included in the assays and its IC50 [21]. Thus, harman from soy sauce (0.4 µM) gave a 44 % inhibition of MAO-A using kynuramine (250 µM). The aromatic β-carboline alkaloids norharman and harman were measured in commercial samples of soy sauces by SPE and HPLC-fluorescence detection (Table 1). Harman was a major aromatic β-carboline, and its concentration varied among different samples of soy sauces ranging from 46.7 to 243 µg/L, whereas norharman ranged from 11.6 to 52 µg/L. Subsequently, soy sauces were analyzed for the presence of tetrahydro-β-carboline-3-carboxylic acids (THβC-3-COOH) that are precursors of the aromatic β-carboline alkaloids [44,[51][52][53]. Soy sauces contained 1,2,3,4-tetrahydro-β-carboline-3-carboxylic acid (THCA) and 1-methyl-1,2,3,4-tetrahydro-β-carboline-3-carboxylic acid (MTCA; Figure 3B) that were identified by chromatographic and mass spectrometric analysis (i.e., m/z at 217 (M + H) + for THCA, and m/z at 231 (M + H) + for MTCA). THβC-3-COOHs were determined following SPE, and their concentration was up to 1000 times higher than that of aromatic βCs ( Table 1). The main THβC-3-COOH was MTCA which appeared as two diastereoisomers (1S,3S and 1R,3S) with the same spectra ( Figure 3B) and concentration reached 450 mg/L. The compound MTCA was isolated from soy sauce by successive HPLC injections, and it afforded harman when heated (90 °C, 6 h; results not shown), showing that it is a direct precursor of this aromatic β-carboline. The formation of harman from MTCA was studied, and it afforded harman when treated under conditions of heating and oxidation ( Figure 5A). Moreover, a sample of soy sauce increased the level of harman under conditions of heating and oxidation ( Figure 5B). Therefore, these results indicate that MTCA was the precursor of harman, a potent inhibitor of MAO-A, in soy sauce ( Figure  6).    The aromatic β-carboline alkaloids norharman and harman were measured in commercial samples of soy sauces by SPE and HPLC-fluorescence detection (Table 1). Harman was a major aromatic β-carboline, and its concentration varied among different samples of soy sauces ranging from 46.7 to 243 µg/L, whereas norharman ranged from 11.6 to 52 µg/L. Subsequently, soy sauces were analyzed for the presence of tetrahydro-β-carboline-3-carboxylic acids (THβC-3-COOH) that are precursors of the aromatic β-carboline alkaloids [44,[51][52][53]. Soy sauces contained 1,2,3,4-tetrahydro-β-carboline-3-carboxylic acid (THCA) and 1-methyl-1,2,3,4-tetrahydro-β-carboline-3-carboxylic acid (MTCA; Figure 3B) that were identified by chromatographic and mass spectrometric analysis (i.e., m/z at 217 (M + H) + for THCA, and m/z at 231 (M + H) + for MTCA). THβC-3-COOHs were determined following SPE, and their concentration was up to 1000 times higher than that of aromatic βCs ( Table 1). The main THβC-3-COOH was MTCA which appeared as two diastereoisomers (1S,3S and 1R,3S) with the same spectra ( Figure 3B) and concentration reached 450 mg/L. The compound MTCA was isolated from soy sauce by successive HPLC injections, and it afforded harman when heated (90 • C, 6 h; results not shown), showing that it is a direct precursor of this aromatic β-carboline. The formation of harman from MTCA was studied, and it afforded harman when treated under conditions of heating and oxidation ( Figure 5A). Moreover, a sample of soy sauce increased the level of harman under conditions of heating and oxidation ( Figure 5B). Therefore, these results indicate that MTCA was the precursor of harman, a potent inhibitor of MAO-A, in soy sauce ( Figure 6).

Discussion
The results of this work have shown that MAO isozymes (MAO-A and -B) are inhibited in vitro by soy sauce aliquots and soy sauce extracts suggesting the presence of inhibitors in soy sauce. They also have shown the occurrence of aromatic β-carboline alkaloids in soy sauce extracts inhibiting MAO. The β-carbolines, norharman and harman, were identified and analyzed, and among them, harman was a major compound. Subsequently, harman was isolated from soy sauce, and it was a potent inhibitor of MAO-A. The level of β-carboline alkaloids highly varied among the samples of soy sauces analyzed (46 to 243 µg/L for harman). The amount of β-carbolines in soy sauce is relatively high when compared with other foodstuffs [22]. Besides aromatic β-carboline alkaloids, the results in this work also show the presence of the tetrahydro-β-carboline-3-carboxylic acids (THβC-3-COOHs) THCA and MTCA in soy sauce. THβC-3-COOHs in foods form through the reaction of tryptophan with formaldehyde (THCA) or acetaldehyde (MTCA)

Discussion
The results of this work have shown that MAO isozymes (MAO-A and -B) are inhibited in vitro by soy sauce aliquots and soy sauce extracts suggesting the presence of inhibitors in soy sauce. They also have shown the occurrence of aromatic β-carboline alkaloids in soy sauce extracts inhibiting MAO. The β-carbolines, norharman and harman, were identified and analyzed, and among them, harman was a major compound. Subsequently, harman was isolated from soy sauce, and it was a potent inhibitor of MAO-A. The level of β-carboline alkaloids highly varied among the samples of soy sauces analyzed (46 to 243 µg/L for harman). The amount of β-carbolines in soy sauce is relatively high when compared with other foodstuffs [22]. Besides aromatic β-carboline alkaloids, the results in this work also show the presence of the tetrahydro-β-carboline-3-carboxylic acids (THβC-3-COOHs) THCA and MTCA in soy sauce. THβC-3-COOHs in foods form through the reaction of tryptophan with formaldehyde (THCA) or acetaldehyde (MTCA)

Discussion
The results of this work have shown that MAO isozymes (MAO-A and -B) are inhibited in vitro by soy sauce aliquots and soy sauce extracts suggesting the presence of inhibitors in soy sauce. They also have shown the occurrence of aromatic β-carboline alkaloids in soy sauce extracts inhibiting MAO. The β-carbolines, norharman and harman, were identified and analyzed, and among them, harman was a major compound. Subsequently, harman was isolated from soy sauce, and it was a potent inhibitor of MAO-A. The level of β-carboline alkaloids highly varied among the samples of soy sauces analyzed (46 to 243 µg/L for harman). The amount of β-carbolines in soy sauce is relatively high when compared with other foodstuffs [22]. Besides aromatic β-carboline alkaloids, the results in this work also show the presence of the tetrahydro-β-carboline-3-carboxylic acids (THβC-3-COOHs) THCA and MTCA in soy sauce. THβC-3-COOHs in foods form through the reaction of tryptophan with formaldehyde (THCA) or acetaldehyde (MTCA) [45]. Thus, MTCA forms during fermentation and storage as tryptophan reacts with acetaldehyde released ( Figure 6) [45,49,54,55]. Soy sauce is made from a mixture of salt, water, soybeans and wheat that, after heating processes, is fermented in a process successively involving molds (Aspergillus oryzae), lactic bacteria and yeast that may last months, and that is finally filtered and heat-treated for sterilization. Alternatively, elaboration methods may involve the chemical hydrolysis of soybeans or chemical hydrolysis and fermentation. Then, factors such as the fermentation process, time of storage and heating processes could determine the content of THβC-3-COOH. For instance, soy sauces contain a high level of THβC-3-COOHs [45], and MTCA appeared up to 1000 times higher in concentration than harman (Table 1). Results here have shown that MTCA is the precursor of harman in soy sauce (Figures 5 and 6). Harman increased in soy sauce under heating and oxidative conditions. Therefore, technological processes such as high temperatures and long storage time or oxidation could increase the levels of the MAO inhibitor harman in soy sauce. These results agree with previous results on the formation of β-carbolines from THβC-3-COOHs [44,51,53,56]. Previously, MTCA received attention as a possible precursor of mutagens when reacted with nitrite [57]. The reaction of MTCA with nitrite affords harman among other compounds, as proved during nitrosation of THβCs [56].
Further studies are needed to fully delineate the implications of dietary, environmental, and endogenous β-carboline alkaloids in human health. Some of the effects attributed to β-carboline alkaloids could be produced by their interaction with MAO enzymes [21,23,68]. The primary role of MAO isozymes in the CNS lies in the metabolism of amines and the regulation of neurotransmitter levels and intracellular amine stores. In the gastrointestinal tract, the circulatory system and the liver, MAO regulates the levels of exogenous dietary amines that exert vasopressor effects serving a protective function. The use of MAO inhibitors, particularly irreversibly inhibitors, may cause a hypertensive crisis when the patients consume tyramine-containing foods (the so-called "cheese effect") [1,18,69,70]. MAO also metabolizes toxic xenobiotic amines such as the neurotoxin MPTP and, in this regard, the inhibitors of MAO can be protective agents [13,14,71,72]. Moreover, the oxidation of amine substrates by MAO results in the production of hydrogen peroxide, ammonia, and aldehydes, which are risk factors for cell oxidative injury [12,17,73]. Therefore, the use of MAO-inhibiting substances can protect against toxicants and oxidative stress [11,17,71,74]. Currently, MAO inhibitors are being developed against Parkinsonism and neurodegeneration and as antidepressant drugs [1,[3][4][5][6][7][8][9]. Inhibition of MAO could help to spare neurotransmitters such as dopamine, serotonin and others while reducing reactive oxygen species. Alternatively, the inhibition of peripheral MAO could eventually affect the metabolism of exogenous and dietary amines like tyramine or tryptamine, potentiating vasopressor effects [1,3]. This work has shown the presence of β-carboline alkaloids in soy sauce, which are inhibitors of MAO. Based on concentration, other sub-stances can contribute to MAO inhibition in soy sauce in addition to β-carbolines. However, β-carbolines could be absorbed and accumulated in tissues exhibiting MAO inhibition [62]. Soy sauce products might be beneficial against neurodegenerative diseases [75,76]. This study shows that soy sauce contains MAO inhibitors, and the inhibition of MAO is a target in neuroprotection.
(b) The aromatic β-carbolines were isolated for subsequent chromatographic analysis, identification and MAO inhibition studies by using a solid phase extraction procedure reported before that affords good performance and reliability with recoveries higher than 90% [22,46]. Briefly, soy sauce diluted with 0.6 M HClO 4 and added with 125 µL 1-ethylβ-carboline (0.2 mg/L) as internal standard was passed through a propylsulfonic acid (PRS)-derivatized silica column that was eluted with water (6 mL), 0.4 M phosphate buffer, pH 9 (3 mL) and 0.2 M buffer phosphate-methanol (1:1), pH 9 (3 mL). The eluates of 0.2 M buffer phosphate-methanol (1:1), pH 9 (3 mL) containing the β-carbolines, were analyzed by HPLC and norharman and harman detected by fluorescence (300 nm, excitation and 433 nm, emission). Quantitation was obtained from calibration curves of standards isolated under the same procedure. The same PRS-SPE procedure but without an internal standard was used to isolate the fractions containing β-carboline for subsequent MAO inhibition studies. In this case, the eluting fractions of 0.4 M phosphate buffer, pH 9 (3 mL) and 0.2 M buffer phosphate-methanol (1:1), pH 9 (3 mL) were mixed and used for MAO inhibition. In order to isolate the β-carboline harman, PRS fractions of 0.2 M buffer phosphate-methanol (1:1) pH 9 were evaporated under vacuum and extracted with dichloromethane (Merck). The organic phase was evaporated, redissolved and injected into HPLC as mentioned below, with the peak corresponding to the β-carboline harman collected from successive HPLC injections. After removing acetonitrile, harman was extracted with dichloromethane in pH 9, concentrated to dryness, and redissolved in phosphate buffer containing 30% DMSO, and used for inhibition of MAO. Corresponding blanks following the same procedures but without β-carbolines were used in the assays.

Monoamine Oxidase (MAO-A and B) Assay and Inhibition by Soy Sauce and β-Carbolines Isolated from Soy Sauce
A chromatographic assay was carried out to determine MAO activity [17,21,50]. It was performed with membrane protein fractions containing MAO-A or MAO-B that were diluted to the desired concentrations in 100 mM potassium phosphate buffer (pH 7.4) [21,50]. A 0.2 mL reaction mixture containing 0.01 mg/mL protein and 0.25 mM kynuramine in 100 mM potassium phosphate (pH 7.4) was incubated at 37 • C for 40 min. After incubation, the reaction was stopped by the addition of 2N NaOH (75 µL), followed by the addition of 70% HClO 4 (25 µL), and the sample was centrifuged (10,000× g) for 5 min. Under these conditions, kynuramine deaminated by MAO spontaneously cyclizes to give 4-hydroxyquinoline. An aliquot of the supernatant (20 µL) was injected into the HPLC, and the deamination product of kynuramine (i.e., 4-hydroxyquinoline) formed during the enzymatic reaction determined by RP-HPLC-DAD at 320 nm. A response curve of area versus concentration was constructed to calculate the concentration of 4-hydroxyquinoline. The standard inhibitors of MAO, clorgyline (MAO-A inhibitor) and deprenyl (MAO-B inhibitor) were used as reference inhibitors in the assays (1 µM).
The inhibition assays were performed with: (a) aliquots of diluted soy sauce (diluted 1/3 in 100 mM phosphate buffer pH 7.4 with 10% of DMSO), (b) soy sauce extracts and fractions prepared from SPE containing the β-carbolines (i.e., buffer-methanol PRS fractions), and (c) samples of isolated harman from soy sauces. For that, aliquots of those samples were added to reaction mixtures containing kynuramine (0.25 mM) and MAO enzyme (A or -B; 0.01 mg/mL membrane protein) in 100 mM potassium phosphate buffer (pH 7.4), as above. Corresponding blanks and controls were used. The MAO kinetic and the mechanism of inhibition were assessed by analyzing the corresponding Michaelis-Menten curves and double reciprocal Lineweaver-Burk plots obtained at different concentrations of the substrate kynuramine. To determine MAO-binding reversibility, membrane proteins of MAO-A and B (0.12 mg/mL) in 100 mM phosphate buffer (pH 7.4) were preincubated (37 • C, 40 min) with diluted soy sauce aliquots. The mixtures were centrifuged (15,000× g) for 15 min to pellet membrane proteins, washed twice with 100 mM phosphate buffer, and finally, the pellet was resuspended in 100 mM phosphate buffer + 10% DMSO (0.1 mL). An aliquot (40 µL) was used to measure MAO activity and compared with corresponding controls.

RP-HPLC chromatographic Analysis and Chemical Identification by MS
The analysis of 4-hydroxyquinoline (kynuramine deamination product), β-carbolines and tetrahydro-β-carbolines was performed by RP-HPLC with uv-DAD and fluorescence detection using an HPLC 1050 (Agilent Technologies, Santa Clara, CA, USA) with a Diode Array Detector (DAD) and a 1046A-fluorescence detector. A 150 mm × 3.9 mm, 4 µm, Novapak C18 column (Waters, Milford, MA, USA) was used for chromatographic separation. Chromatographic conditions were 50 mM ammonium phosphate buffer (pH 3) (buffer A) and 20% of A in acetonitrile (buffer B). Gradient programmed from 0% (100% A) to 32% B in 8 min, and 90% B in 15 min. The flow rate was 1 mL/min, the column temperature was 40 • C, and the injection volume was 20 µL. Absorbance detection was set at 320 nm (analysis of 4-hydroxyquinoline), whereas fluorescence detection was used for tetrahydroβ-carbolines (270 nm, excitation and 343 nm, emission) and norharman and harman (300 nm, excitation and 433 nm, emission). Identification of compounds was done by UV, fluorescence and mass spectrometry. The identification by HPLC-ESI-mass spectrometry of 4-hydroxyquinoline in MAO assays was carried out as previously [50]. Identification of the β-carbolines norharman and harman in soy sauce was carried out in the PRS extracts, and the THβC-3-COOH in SCX extracts was obtained as mentioned above. They were analyzed with a 2.1 × 150 mm Zorbax SB-C18, 5 µm, column (Agilent Technologies, Santa Clara, CA, USA) by using an HPLC-MSD series 1100 (Agilent) (electrospray-positive ion mode). Eluent A: formic acid (0.5%); B: formic acid (0.5%) in acetonitrile; 80% B in 30 min, flow rate 0.25 mL/min.; T: 40 • C; mass range 50-700 u and cone voltage 100 V.

Conclusions
This work has shown that soy sauce inhibits MAO-A and -B isozymes in vitro, as evidenced by an assay of kynuramine deamination performed by chromatographic means, suggesting the occurrence of MAO inhibitors in soy sauces. The inhibition of MAO was reversible for MAO-A and partly reversible for MAO-B. Soy sauces were analyzed, and the β-carboline alkaloids, norharman and harman, were identified. Subsequently, harman was isolated from soy sauce, and it potently inhibited MAO-A in a competitive mode. This βcarboline was analyzed in several soy sauces, and it reached a concentration of 243 µg/L. In addition, the THβC-3-COOH compounds, THCA and MTCA, were identified and analyzed in soy sauces. MTCA isolated from soy sauce afforded harman under conditions of heating and oxidation. Also, under heating and oxidative conditions, soy sauces increased the levels of harman, showing that MTCA was the precursor of harman in soy sauces. Then, soy sauce contained β-carboline alkaloids that are considered bioactive compounds, and among them, harman is a potent inhibitor of MAO-A, as shown here. Results suggest that soy sauce constitutes a dietary source of MAO inhibitors.