Melatonin: deacetylation to 5-methoxytryptamine by liver but not brain aryl acylamidase

The Engagement of Cytochrome P450 Enzymes in Tryptophan Metabolism

Tryptophan is metabolized along three main metabolic pathways, namely the kynurenine, serotonin and indole pathways. The majority of tryptophan is transformed via the kynurenine pathway, catalyzed by tryptophan-2,3-dioxygenase or indoleamine-2,3-dioxygenase, leading to neuroprotective kynurenic acid or neurotoxic quinolinic acid. Serotonin synthesized by tryptophan hydroxylase, and aromatic L-amino acid decarboxylase enters the metabolic cycle: serotonin → N-acetylserotonin → melatonin → 5-methoxytryptamine→serotonin. Recent studies indicate that serotonin can also be synthesized by cytochrome P450 (CYP), via the CYP2D6-mediated 5-methoxytryptamine O-demethylation, while melatonin is catabolized by CYP1A2, CYP1A1 and CYP1B1 via aromatic 6-hydroxylation and by CYP2C19 and CYP1A2 via O-demethylation. In gut microbes, tryptophan is metabolized to indole and indole derivatives. Some of those metabolites act as activators or inhibitors of the aryl hydrocarbon receptor, thus regulating the expression of CYP1 family enzymes, xenobiotic metabolism and tumorigenesis. The indole formed in this way is further oxidized to indoxyl and indigoid pigments by CYP2A6, CYP2C19 and CYP2E1. The products of gut-microbial tryptophan metabolism can also inhibit the steroid-hormone-synthesizing CYP11A1. In plants, CYP79B2 and CYP79B3 were found to catalyze N-hydroxylation of tryptophan to form indole-3-acetaldoxime while CYP83B1 was reported to form indole-3-acetaldoxime N-oxide in the biosynthetic pathway of indole glucosinolates, considered to be defense compounds and intermediates in the biosynthesis of phytohormones. Thus, cytochrome P450 is engaged in the metabolism of tryptophan and its indole derivatives in humans, animals, plants and microbes, producing biologically active metabolites which exert positive or negative actions on living organisms. Some tryptophan-derived metabolites may influence cytochrome P450 expression, affecting cellular homeostasis and xenobiotic metabolism.

In vitro deacetylation of N-acetylserotonin by arylacetamide deacetylase

Arylacetamide deacetylase (AADAC) is a deacetylation enzyme present in the mammalian liver, gastrointestinal tract, and brain. During our search for mammalian enzymes capable of metabolizing N-acetylserotonin (NAS), AADAC was identified as having the ability to convert NAS to serotonin. Both human and rodent recombinant AADAC proteins can deacetylate NAS in vitro, although the human AADAC shows markedly higher activity compared with rodent enzyme. The AADAC-mediated deacetylation reaction can be potently inhibited by eserine in vitro. In addition to NAS, recombinant hAADAC can deacetylate melatonin (to form 5-methoxytryptamine) and N-acetyltryptamine (NAT) (to form tryptamine). In addition to the in vitro deacetylation of NAS by the recombinant AADAC proteins, liver (mouse and human) and brain (human) extracts were able to deacetylate NAS; these activities were sensitive to eserine. Taken together, these results demonstrate a new role for AADAC and suggest a novel pathway for the AADAC-mediated metabolism of pineal indoles in mammals.

Protective Role of Melatonin and Its Metabolites in Skin Aging

The skin, being the largest organ in the human body, is exposed to the environment and suffers from both intrinsic and extrinsic aging factors. The skin aging process is characterized by several clinical features such as wrinkling, loss of elasticity, and rough-textured appearance. This complex process is accompanied with phenotypic and functional changes in cutaneous and immune cells, as well as structural and functional disturbances in extracellular matrix components such as collagens and elastin. Because skin health is considered one of the principal factors representing overall “well-being” and the perception of “health” in humans, several anti-aging strategies have recently been developed. Thus, while the fundamental mechanisms regarding skin aging are known, new substances should be considered for introduction into dermatological treatments. Herein, we describe melatonin and its metabolites as potential “aging neutralizers”. Melatonin, an evolutionarily ancient derivative of serotonin with hormonal properties, is the main neuroendocrine secretory product of the pineal gland. It regulates circadian rhythmicity and also exerts anti-oxidative, anti-inflammatory, immunomodulatory, and anti-tumor capacities. The intention of this review is to summarize changes within skin aging, research advances on the molecular mechanisms leading to these changes, and the impact of the melatoninergic anti-oxidative system controlled by melatonin and its metabolites, targeting the prevention or reversal of skin aging.

Identification of potential biomarkers in cholestasis and the therapeutic effect of melatonin by metabolomics, multivariate data and pathway analyses

The present study investigated the anti‑cholestatic effect of melatonin (MT) against α‑naphthyl isothiocyanate (ANIT)‑induced liver injury in rats and screened for potential biomarkers of cholestasis. Rats were administered ANIT by intraperitoneal injection and then sacrificed 36 h later. Serum biochemical parameters were measured and liver tissue samples were subjected to histological analysis. Active components in the serum were identified by gas chromatography‑mass spectrometry, while biomarkers and biochemical pathways were identified by multivariate data analysis. The results revealed that the serum levels of alanine aminotransferase, aspartate aminotransferase, total bilirubin, direct bilirubin, γ‑glutamyl transpeptidase, and alkaline phosphatase were reduced in rats with ANIT‑induced cholestasis that were treated with MT. The histological observations indicated that MT had a protective effect against ANIT‑induced hepatic tissue damage. Metabolomics analysis revealed that this effect was likely to be associated with the regulation of compounds related to MT synthesis and catabolism, and amino acid metabolism, including 5‑aminopentanoate, 5‑methoxytryptamine, L‑tryptophan, threonine, glutathione, L‑methionine, and indolelactate. In addition, principal component analysis demonstrated that the levels of these metabolites differed significantly between the MT and control groups, providing further evidence that they may be responsible for the effects induced by MT. These results provide an insight into the mechanisms underlying cholestasis development and highlight potential biomarkers for disease diagnosis.

Taxon- and Site-Specific Melatonin Catabolism

Melatonin is catabolized both enzymatically and nonenzymatically. Nonenzymatic processes mediated by free radicals, singlet oxygen, other reactive intermediates such as HOCl and peroxynitrite, or pseudoenzymatic mechanisms are not species- or tissue-specific, but vary considerably in their extent. Higher rates of nonenzymatic melatonin metabolism can be expected upon UV exposure, e.g., in plants and in the human skin. Additionally, melatonin is more strongly nonenzymatically degraded at sites of inflammation. Typical products are several hydroxylated derivatives of melatonin and N¹-acetyl-N²-formyl-5-methoxykynuramine (AFMK). Most of these products are also formed by enzymatic catalysis. Considerable taxon- and site-specific differences are observed in the main enzymatic routes of catabolism. Formation of 6-hydroxymelatonin by cytochrome P₄₅₀ subforms are prevailing in vertebrates, predominantly in the liver, but also in the brain. In pineal gland and non-mammalian retina, deacetylation to 5-methoxytryptamine (5-MT) plays a certain role. This pathway is quantitatively prevalent in dinoflagellates, in which 5-MT induces cyst formation and is further converted to 5-methoxyindole-3-acetic acid, an end product released to the water. In plants, the major route is catalyzed by melatonin 2-hydroxylase, whose product is tautomerized to 3-acetamidoethyl-3-hydroxy-5-methoxyindolin-2-one (AMIO), which exceeds the levels of melatonin. Formation and properties of various secondary products are discussed.

Melatonin as an Anti-Inflammatory Agent Modulating Inflammasome Activation

Inflammation may be defined as the innate response to harmful stimuli such as pathogens, injury, and metabolic stress; its ultimate function is to restore the physiological homeostatic state. The exact aetiology leading to the development of inflammation is not known, but a combination of genetic, epigenetic, and environmental factors seems to play an important role in the pathogenesis of many inflammation-related clinical conditions. Recent studies suggest that the pathogenesis of different inflammatory diseases also involves the inflammasomes, intracellular multiprotein complexes that mediate activation of inflammatory caspases thereby inducing the secretion of proinflammatory cytokines. Melatonin, an endogenous indoleamine, is considered an important multitasking molecule with fundamental clinical applications. It is involved in mood modulation, sexual behavior, vasomotor control, and immunomodulation and influences energy metabolism; moreover, it acts as an oncostatic and antiaging molecule. Melatonin is an important antioxidant and also a widespread anti-inflammatory molecule, modulating both pro- and anti-inflammatory cytokines in different pathophysiological conditions. This review, first, gives an overview concerning the growing importance of melatonin in the inflammatory-mediated pathological conditions and, then, focuses on its roles and its protective effects against the activation of the inflammasomes and, in particular, of the NLRP3 inflammasome.

On the significance of an alternate pathway of melatonin synthesis via 5-methoxytryptamine: comparisons across species

Melatonin is a phylogenetically ancient molecule. It is ubiquitously present in almost all organisms from primitive photosynthetic bacteria to humans. Its original primary function is presumable to be that of an antioxidant with other functions of this molecule having been acquired during evolution. The synthetic pathway of melatonin in vertebrates has been extensively studied. It is common knowledge that serotonin is acetylated to form N-acetylserotonin by arylalkylamine N-acetyltransferase (AANAT) or arylamine N-acetyltransferase (SNAT or NAT) and N-acetylserotonin is, subsequently, methylated to melatonin by N-acetylserotonin O-methyltransferase (ASMT; also known as hydroxyindole-O-methyltransferase, HIOMT). This is referred to as a classic melatonin synthetic pathway. Based on new evidence, we feel that this classic melatonin pathway is not generally the prevailing route of melatonin production. An alternate pathway is known to exist, in which serotonin is first O-methylated to 5-methoxytryptamine (5-MT) and, thereafter, 5-MT is N-acetylated to melatonin. Here, we hypothesize that the alternate melatonin synthetic pathway may be more important in certain organisms and under certain conditions. Evidence strongly supports that this alternate pathway prevails in some plants, bacteria, and, perhaps, yeast and may also occur in animals.

Sulfation of 6-hydroxymelatonin, N-acetylserotonin and 4-hydroxyramelteon by the human cytosolic sulfotransferases (SULTs)

1. This study aimed to investigate the involvement of sulfation in the metabolism of 6-hydroxymelatonin (6-OH-Mel), N-acetylserotonin (NAS) and 4-hydroxyramelteon (4-OH-Ram), and to identify and characterize the human cytosolic sulfotransferases (SULTs) capable of sulfating these drug compounds. 2. A systematic analysis using 13 known human SULTs revealed that SULT1A1 displayed the strongest activity in catalyzing the sulfation of 6-OH-Mel and 4-OH-Ram, whereas SULT1C4 exhibited the strongest sulfating-activity towards NAS. pH-dependence and kinetic parameters of these SULT enzymes in mediating the sulfation of respective drug compounds were determined. A metabolic labeling study showed the generation and release of [³⁵S]sulfated 6-OH-Mel, NAS and 4-OH-Ram by HepG2 human hepatoma cells and Caco-2 human colon adenocarcinoma cells labeled with [³⁵S]sulfate in the presence of these drug compounds. Cytosols of human lung, liver, kidney and small intestine were examined to verify the presence of 6-OH-Mel-, NAS- and 4-OH-Ram-sulfating activity in vivo. Of the four human organ samples tested, small intestine and liver cytosols displayed considerably higher 6-OH-Mel-, NAS- and 4-OH-Ram-sulfating activities than those of lung and kidney. 3. Collectively, these results provided a molecular basis for the metabolism of 6-OH-Mel, NAS and 4-OH-Ram through sulfation.

Melatonin and its metabolites accumulate in the human epidermis in vivo and inhibit proliferation and tyrosinase activity in epidermal melanocytes in vitro

Melatonin and its metabolites including 6-hydroxymelatonin (6(OH)M), N(1)-acetyl-N(2)-formyl-5-methoxykynuramine (AFMK) and 5-methoxytryptamine (5MT) are endogenously produced in human epidermis. This production depends on race, gender and age. The highest melatonin levels are in African-Americans. In each racial group they are highest in young African-Americans [30-50 years old (yo)], old Caucasians (60-90 yo) and Caucasian females. AFMK levels are the highest in African-Americans, while 6(OH)M and 5MT levels are similar in all groups. Testing of their phenotypic effects in normal human melanocytes show that melatonin and its metabolites (10(-5) M) inhibit tyrosinase activity and cell growth, and inhibit DNA synthesis in a dose dependent manner with 10(-9) M being the lowest effective concentration. In melanoma cells, they inhibited cell growth but had no effect on melanogenesis, except for 5MT which enhanced L-tyrosine induced melanogenesis. In conclusion, melatonin and its metabolites [6(OH)M, AFMK and 5MT] are produced endogenously in human epidermis and can affect melanocyte and melanoma behavior.

Local melatoninergic system as the protector of skin integrity

The human skin is not only a target for the protective actions of melatonin, but also a site of melatonin synthesis and metabolism, suggesting an important role for a local melatoninergic system in protection against ultraviolet radiation (UVR) induced damages. While melatonin exerts many effects on cell physiology and tissue homeostasis via membrane bound melatonin receptors, the strong protective effects of melatonin against the UVR-induced skin damage including DNA repair/protection seen at its high (pharmocological) concentrations indicate that these are mainly mediated through receptor-independent mechanisms or perhaps through activation of putative melatonin nuclear receptors. The destructive effects of the UVR are significantly counteracted or modulated by melatonin in the context of a complex intracutaneous melatoninergic anti-oxidative system with UVR-enhanced or UVR-independent melatonin metabolites. Therefore, endogenous intracutaneous melatonin production, together with topically-applied exogenous melatonin or metabolites would be expected to represent one of the most potent anti-oxidative defense systems against the UV-induced damage to the skin. In summary, we propose that melatonin can be exploited therapeutically as a protective agent or as a survival factor with anti-genotoxic properties or as a “guardian” of the genome and cellular integrity with clinical applications in UVR-induced pathology that includes carcinogenesis and skin aging.

Physiological and pharmacological properties of 5-methoxytryptophol

5-methoxytryptophol (5-ML) is a pineal indoleamine derived from serotonin shown to be biologically active in a number of species. This indolamine exhibits a circadian pattern synchronized with the day-night cycle with significant increases during daylight, already recognized in vertebrates. The multiplicity of physiological and endocrine functions of 5-ML is remarkable; it is involved in circadian rhythms, reproduction and sexual processes. Furthermore, a number of pharmacological benefits of 5-ML have been reported, including immunomodulatory, antitumor and antioxidative activities. However, the molecular mechanisms of these pharmacological effects remain unclear. The purpose of this article is to provide an overview on the general properties and physiological functions of 5-ML. An attempt has been made to fully document all studies performed using 5-ML. In addition, this article aims to gain insight into the current state of knowledge regarding pharmacological and therapeutic effects of this indoleamine.

Metabolism of melatonin and biological activity of intermediates of melatoninergic pathway in human skin cells

Indolic and kynuric pathways of skin melatonin metabolism were monitored by liquid chromatography mass spectrometry in human keratinocytes, melanocytes, dermal fibroblasts, and melanoma cells. Production of 6-hydroxymelatonin [6(OH)M], N(1)-acetyl-N(2)-formyl-5-methoxykynuramine (AFMK) and 5-methoxytryptamine (5-MT) was detected in a cell type-dependent fashion. The major metabolites, 6(OH)M and AFMK, were produced in all cells. Thus, in immortalized epidermal (HaCaT) keratinocytes, 6(OH)M was the major product with Vmax = 63.7 ng/10(6) cells and Km = 10.2 μM, with lower production of AFMK and 5-MT. Melanocytes, keratinocytes, and fibroblasts transformed melatonin primarily into 6(OH)M and AFMK. In melanoma cells, 6(OH)M and AFMK were produced endogenously, a process accelerated by exogenous melatonin in the case of AFMK. In addition, N-acetylserotonin was endogenously produced by normal and malignant melanocytes. Metabolites showed selective antiproliferative effects on human primary epidermal keratinocytes in vitro. In ex vivo human skin, both melatonin and AFMK-stimulated expression of involucrin and keratins-10 and keratins-14 in the epidermis, indicating their stimulatory role in building and maintaining the epidermal barrier. In summary, the metabolism of melatonin and its endogenous production is cell type-dependent and expressed in all three main cell populations of human skin. Furthermore, melatonin and its metabolite AFMK stimulate differentiation in human epidermis, indicating their key role in building the skin barrier.

Melatonin: an underappreciated player in retinal physiology and pathophysiology

In the vertebrate retina, melatonin is synthesized by the photoreceptors with high levels of melatonin at night and lower levels during the day. Melatonin exerts its influence by interacting with a family of G-protein-coupled receptors that are negatively coupled with adenylyl cyclase. Melatonin receptors belonging to the subtypes MT(1) and MT(2) have been identified in the mammalian retina. MT(1) and MT(2) receptors are found in all layers of the neural retina and in the retinal pigmented epithelium. Melatonin in the eye is believed to be involved in the modulation of many important retinal functions; it can modulate the electroretinogram (ERG), and administration of exogenous melatonin increases light-induced photoreceptor degeneration. Melatonin may also have protective effects on retinal pigment epithelial cells, photoreceptors and ganglion cells. A series of studies have implicated melatonin in the pathogenesis of age-related macular degeneration, and melatonin administration may represent a useful approach to prevent and treat glaucoma. Melatonin is used by millions of people around the world to retard aging, improve sleep performance, mitigate jet lag symptoms, and treat depression. Administration of exogenous melatonin at night may also be beneficial for ocular health, but additional investigation is needed to establish its potential.

Melatonin membrane receptors in peripheral tissues: distribution and functions

Many of melatonin's actions are mediated through interaction with the G-protein coupled membrane bound melatonin receptors type 1 and type 2 (MT1 and MT2, respectively) or, indirectly with nuclear orphan receptors from the RORα/RZR family. Melatonin also binds to the quinone reductase II enzyme, previously defined the MT3 receptor. Melatonin receptors are widely distributed in the body; herein we summarize their expression and actions in non-neural tissues. Several controversies still exist regarding, for example, whether melatonin binds the RORα/RZR family. Studies of the peripheral distribution of melatonin receptors are important since they are attractive targets for immunomodulation, regulation of endocrine, reproductive and cardiovascular functions, modulation of skin pigmentation, hair growth, cancerogenesis, and aging. Melatonin receptor agonists and antagonists have an exciting future since they could define multiple mechanisms by which melatonin modulates the complexity of such a wide variety of physiological and pathological processes.

Melatonin metabolism in the central nervous system

The metabolism of melatonin in the central nervous system is of interest for several reasons. Melatonin enters the brain either via the pineal recess or by uptake from the blood. It has been assumed to be also formed in some brain areas. Neuroprotection by melatonin has been demonstrated in numerous model systems, and various attempts have been undertaken to counteract neurodegeneration by melatonin treatment. Several concurrent pathways lead to different products. Cytochrome P₄₅₀ subforms have been demonstrated in the brain. They either demethylate melatonin to N-acetylserotonin, or produce 6-hydroxymelatonin, which is mostly sulfated already in the CNS. Melatonin is deacetylated, at least in pineal gland and retina, to 5-methoxytryptamine. N¹-acetyl-N²-formyl-5-methoxykynuramine is formed by pyrrole-ring cleavage, by myeloperoxidase, indoleamine 2,3-dioxygenase and various non-enzymatic oxidants. Its product, N¹-acetyl-5-methoxykynuramine, is of interest as a scavenger of reactive oxygen and nitrogen species, mitochondrial modulator, downregulator of cyclooxygenase-2, inhibitor of cyclooxygenase, neuronal and inducible NO synthases. Contrary to other nitrosated aromates, the nitrosated kynuramine metabolite, 3-acetamidomethyl-6-methoxycinnolinone, does not re-donate NO. Various other products are formed from melatonin and its metabolites by interaction with reactive oxygen and nitrogen species. The relative contribution of the various pathways to melatonin catabolism seems to be influenced by microglia activation, oxidative stress and brain levels of melatonin, which may be strongly changed in experiments on neuroprotection. Many of the melatonin metabolites, which may appear in elevated concentrations after melatonin administration, possess biological or pharmacological properties, including N-acetylserotonin, 5-methoxytryptamine and some of its derivatives, and especially the 5-methoxylated kynuramines.

Selective inhibition of butyrylcholinesterase by singlet oxygen-generated melatonin derivatives

The inhibition of cholinesterases plays a crucial role in a therapy of neurodegenerative diseases, including Alzheimer's disease. Especially, butyrylcholinesterase (BChE) has recently gained special interest. On the other hand, compounds having antioxidative properties may have a beneficial role in slowing down neurodegeneration processes. To combine these two effects, we synthesized a series of new derivatives of melatonin, which is a strong antioxidant, possessing structural elements essential for the inhibitory activity against cholinesterase. The structure of the new compounds was confirmed by NMR spectroscopy and mass spectrometry, and their activity against cholinesterases was measured in vitro using modified Ellman's method. The compounds obtained showed a high inhibitory activity, together with a strong selectivity against BChE. These results may point at new area of interest in a research on cholinesterase inhibitors.

Gender-related differences in circadian rhythm of rat plasma acetyl- and butyrylcholinesterase: effects of sex hormone withdrawal

The role of acetylcholinesterase (AChE) in the termination of the cholinergic response through acetylcholine (ACh) hydrolysis and the involvement of plasma butyrylcholinesterase (BuChE), mainly of hepatic origin, in the metabolism of xenobiotics with ester bonds is well known. Besides, BuChE has a crucial role in ACh hydrolysis, especially when selective anticholinesterases inhibit AChE. Herein, we analyzed the gender-related differences and the circadian changes of rat plasma cholinesterases. Plasma and liver cholinesterase activities were evaluated in control or 2-30-day castrated adult male and female rats. Plasma and liver AChE activities did not differ between genders and were not influenced by sex hormone deprivation. BuChE plasma activity was 7 times greater in female, reflecting gender differences in liver enzyme expression. Castration increased liver and plasma BuChE activity in male, while reduced it in female, abolishing gender differences in enzyme activity. Interestingly, female AChE and BuChE plasma activities varied throughout the day, reaching values 27% and 42% lower, respectively, between 2 p.m. and 6 p.m. when compared to the morning peaks at 8 a.m. Castration attenuated daily female BuChE oscillation. On the other hand, male plasma enzymes remained constant throughout the day. In summary, our results show that liver and plasma BuChE, but not AChE, expression is influenced by sex hormones, leading to high levels of blood BuChE in females. The fluctuation of female plasma BuChE during the day should be taken into account to adjust the bioavailability and the therapeutic effects of cholinesterase inhibitors used in cholinergic-based conditions such Alzheimer's disease.

Melatonin Inhibits Benzene-Induced Lipid Peroxidation in Rat Liver

We studied the antioxidative role of melatonin against benzene toxicity in rat liver. The inhibition of mitochondrial and microsomal lipid peroxidation differed between 24-hour (single-dose), 15-day, and 30-day treatments. Inhibition of mitochondrial lipid peroxidation was the highest after the single dose of melatonin, whereas highest microsomal inhibition was recorded after 30 days of melatonin treatment. No significant difference was recorded between 15-day and 30-day treatments. Cytochrome P 4502E1 (CYP 4502E1) activity declined after the single-dose and 15-day melatonin treatment in the benzene-treated group, but it rose again, though not significantly after 30 days of treatment. Liver histopathology generally supported these findings. Phenol concentration in the urine samples declined in melatonin and benzene-treated rats. Our results show that melatonin affects CYP 4502E1, which is responsible for benzene metabolism. Inhibition of its metabolism correlated with lower lipid peroxidation. In conclusion, melatonin was found to be protective against lipid peroxidation induced by benzene.

Metabolism of melatonin by cytochrome P450s in rat liver mitochondria and microsomes

In the present study we provide direct evidence for the involvement of rat microsomal cytochrome P450s in melatonin O-demethylation and hydroxylation at two different positions: 2 and 6, as well as generation of N(1)-acetyl-N(2)-formyl-5-methoxy-kynuramine (AFMK) and two unknown products. Moreover, we found that mitochondrial cytochrome P450s also converts melatonin into AFMK, N-acetylserotonin, 2-hydroxymelatonin, 6-hydroxymelatonin and the same two unknown products. Eadie-Hofstee plots for 6-hydroxylation and O-demethylation reactions were curvilinear for all tested fractions, suggestive of involvement of at least two components, one with a high affinity and low capacity, and another with a low affinity and high capacity. Mitochondrial cytochrome P450s exhibited higher affinity (suggesting lower K(m) value) and higher V(max) for melatonin 6-hydroxylation and O-demethylation for both high-affinity and low-affinity components as compared with microsomal enzymes. The intrinsic clearance for melatonin hydroxylation by high- and low-affinity components displayed the highest values in all tested fractions, indicating that both mitochondrial and microsomal cytochrome P450s metabolize melatonin principally by 6-hydroxylation, with O-demethylation representing a minor metabolic pathway.

The cutaneous serotoninergic/melatoninergic system: securing a place under the sun

It was recently discovered that mammalian skin can produce serotonin and transform it into melatonin. Pathways for the biosynthesis and biodegradation of serotonin and melatonin have been characterized in human and rodent skin and in their major cellular populations. Moreover, receptors for serotonin and melatonin receptors are expressed in keratinocytes, melanocytes, and fibroblasts and these mediate phenotypic actions on cellular proliferation and differentiation. Melatonin exerts receptor-independent effects, including activation of pathways protective of oxidative stress and the modification of cellular metabolism. While serotonin is known to have several roles in skin-e.g., pro-edema, vasodilatory, proinflammatory, and pruritogenic-melatonin has been experimentally implicated in hair growth cycling, pigmentation physiology, and melanoma control. Thus, the widespread expression of a cutaneous seorotoninergic/melatoninergic syste,m(s) indicates considerable selectivity of action to facilitate intra-, auto-, or paracrine mechanisms that define and influence skin function in a highly compartmentalized manner. Notably, the cutaneous melatoninergic system is organized to respond to continuous stimulation in contrast to the pineal gland, which (being insulated from the external environment) responds to discontinuous activation by the circadian clock. Overall, the cutaneous serotoninergic/melatoninergic system could counteract or buffer external (environmental) or internal stresses to preserve the biological integrity of the organ and to maintain its homeostasis.-Slominski, A. J., Wortsman, J., Tobin, D. J. The cutaneous serotoninergic/melatoninergic system: securing a place under the sun.

Polymorphic cytochrome P450 2D6: humanized mouse model and endogenous substrates

Cytochrome P450 2D6 (CYP2D6) is the first well-characterized polymorphic phase I drug-metabolizing enzyme, and more than 80 allelic variants have been identified for the CYP2D6 gene, located on human chromosome 22q13.1. Human debrisoquine and sparteine metabolism is subdivided into two principal phenotypes--extensive metabolizer and poor metabolizer--that arise from variant CYP2D6 genotypes. It has been estimated that CYP2D6 is involved in the metabolism and disposition of more than 20% of prescribed drugs, and most of them act in the central nervous system or on the heart. These drug substrates are characterized as organic bases containing one nitrogen atom with a distance about 5, 7, or 10 A from the oxidation site. Aspartic acid 301 and glutamic acid 216 were determined as the key acidic residues for substrate-enzyme binding through electrostatic interactions. CYP2D6 transgenic mice, generated using a lambda phage clone containing the complete wild-type CYP2D6 gene, exhibits enhanced metabolism and disposition of debrisoquine. This transgenic mouse line and its wild-type control are models for human extensive metabolizers and poor metabolizers, respectively, and would have broad application in the study of CYP2D6 polymorphism in drug discovery and development, and in clinical practice toward individualized drug therapy. Endogenous 5-methoxyindole- thylamines derived from 5-hydroxytryptamine were identified as high-affinity substrates of CYP2D6 that catalyzes their O-demethylations with high enzymatic capacity and specificity. Thus, polymorphic CYP2D6 may play an important role in the interconversions of these psychoactive tryptamines, including a crucial step in a serotonin-melatonin cycle.

Melatonin circadian rhythm in the retina of mammals

Melatonin has been traditionally considered to be derived principally from the pineal gland. However, several investigations have now demonstrated that melatonin synthesis occurs also in the retina (and in other organs as well) of several vertebrate classes, including mammals. As in the pineal, melatonin synthesis in the retina is elevated at night and reduced during the day. Since melatonin receptors are present in the retina and retinal melatonin does not contribute to the circulating levels, retinal melatonin probably acts locally as a neuromodulator. Melatonin synthesis in the retinas of mammals is under control of a circadian oscillator located within the retina itself, and circadian rhythms in melatonin synthesis and/or release have been described for several species of rodents. These rhythms are present in vivo, persist in vitro, are entrained by light, and are temperature compensated. The recent cloning of the gene responsible for the synthesis of the enzyme arylalkylamine N-acetyltransferase (the only enzyme unique to the melatonin synthetic pathway) will facilitate localizing the cellular site of melatonin synthesis in the retina and investigating the molecular mechanism responsible for the generation of retinal melatonin rhythmicity. Melatonin has been implicated in many retinal functions, and the levels of melatonin and dopamine appear to regulate several aspects of retinal physiology that relate to light and dark adaptation. In conclusion, it seems that retinal melatonin is involved in several functions, but its precise role is yet to be understood.

Cellular mechanisms of melatonin action

The pineal hormone melatonin is involved in photic regulations of various kinds, including adaptation to light intensity, daily changes of light and darkness, and seasonal changes of photoperiod lengths. The melatonin effects are mediated by the specific high-affinity receptors localized on plasma membrane and coupled to GTP-binding protein. Two different G proteins coupled to the melatonin receptors have been described, one sensitive to pertussis toxin and the other sensitive to cholera toxin. On the basis of the molecular structure, three subtypes of the melatonin receptors have been described: Mel1A, Mel1B, and Mel1C. The first two subtypes are found in mammals and may be distinguished pharmacologically using selective antagonists. Melatonin receptor regulates several second messengers: cAMP, cGMP, diacylglycerol, inositol trisphosphate, arachidonic acid, and intracellular Ca2+ concentration ([Ca2+]i). In many cases, its effect is inhibitory and requires previous activation of the cell by a stimulatory agent. Melatonin inhibits cAMP accumulation in most of the cells examined, but the indole effects on other messengers have been often observed only in one type of the cells or tissue, until now. Melatonin also regulates the transcription factors, namely, phosphorylation of cAMP-responsive element binding protein and expression of c-Fos. Molecular mechanisms of the melatonin effects are not clear but may involve at least two parallel transduction pathways, one inhibiting adenylyl cyclase and the other regulating phospholipide metabolism and [Ca2+]i.

Metabolism of serotonin to N-acetylserotonin, melatonin, and 5-methoxytryptamine in hamster skin culture

Biotransformation of [3H]serotonin by cultured hamster skin to 3H-metabolites corresponding to N-acetylserotonin (NAS), melatonin, and 5-methoxytryptamine (5-MT) was demonstrated. This process was time-dependent, with the highest production of radioactive NAS and melatonin metabolites after 3 and 5 h of incubation followed by a decrease in the rate of metabolite release into the media. Conversely, the formation of radioactive metabolite corresponding to 5-MT increased gradually during skin culture, reaching the highest level after 24 h of incubation. The production of 3H-metabolites, corresponding to NAS, melatonin, and 5-MT, was stimulated by forskolin with a maximum effect of forskolin at 10 microM concentration. The gas chromatographic/mass spectroscopy analysis of the fraction eluting at the retention time of NAS standard material showed that it contained NAS, further confirming production and release of NAS into the media by hamster skin. Therefore, we conclude that mammalian skin can acetylate serotonin to NAS and postulate that the NAS is further metabolized by the skin to form melatonin which is subsequently transformed to 5-MT.

Melatonin deacetylase activity in the pineal gland and brain of the lizards Anolis carolinensis and Sceloporus jarrovi

Melatonin modulates a variety of rhythmic processes in vertebrates, and is synthesized in both the retina and pineal gland. We have shown previously that retinal melatonin is deacetylated generating 5-methoxytryptamine, which is then deaminated by monoamine oxidase, producing 5-methoxyindoleacetic acid and 5-methoxytryptophol. This process occurs within the eyes of a variety of vertebrates including the iguanid lizard Anolis carolinensis. To determine whether melatonin deacetylase activity also occurs in the pineal organ or in other parts of the lizard brain, pineals and brains of Anolis carolinensis and Sceloporus jarrovi were cultured in the presence of [3H-methoxy]-melatonin. High-performance liquid chromatography of the resulting culture media and tissues revealed the generation of radiolabeled 5-methoxytryptamine and 5-methoxyindoleacetic acid. These two methoxyindoles were the only radiolabeled metabolites detectable, and together accounted for all melatonin lost. Both the loss of melatonin and the production of melatonin metabolites were inhibited by inclusion of 100 microM eserine, an inhibitor of the melatonin deacetylase. Pargyline, a monoamine oxidase inhibitor, reduced the production of 5-methoxyindoleacetic acid and increased the production of 5-methoxytryptamine relative to control incubations. Similar effects of eserine and pargyline were seen in eyecup, brain and pineal gland, but the specific activity of melatonin deacetylation in cultured pineal glands was much greater than in either brains or eyecups. These results indicate that pineal glands of both Anolis carolinensis and Sceloporus jarrovi can rapidly catabolize melatonin by a mechanism very similar to that in the eye, that the melatonin deacetylation pathway exists elsewhere in the iguanid brain, and also extend our previous observations of ocular melatonin deacetylation to an additional species.

The significance of the metabolism of the neurohormone melatonin: antioxidative protection and formation of bioactive substances

Recent findings suggest that the ability of melatonin to enter all body tissues and to be metabolized, enzymatically or nonenzymatically, in any of them results in a spectrum of effects, which exceed substantially those transduced by membrane receptors. These actions comprise the formation of various bioactive compounds such as N-acetylserotonin, 5-methoxytryptamine, N,N-dimethyl-5-methoxytryptamine, 5-methoxytryptophol, cyclic 2-hydroxymelatonin, pinoline, and 5-methoxylated kynuramines. Apart from enzymatic metabolism, nonenzymatic reactions with free radicals, in particular the superoxide anion and the hydroxyl radical, represent a new and significant aspect of melatonin's biological role. Melatonin represents the most potent physiological scavenger of hydroxyl radicals found to date, and recent findings suggest an essential role of this indoleamine for protection from hydroxyl radical-induced carcinogenesis and neurodegeneration.

Solubilization and biochemical characterization of the melatonin deacetylase from Xenopus laevis retina

Melatonin deacetylase, an enzyme activity recently discovered in the Xenopus laevis retina, regulates local melatonin levels. The deacetylase occurs in retina, retinal pigment epithelium, and skin, all sites of melatonin action, and is widely distributed among vertebrates. We have solubilized the enzyme from Xenopus retina and pigment epithelium using nonionic detergents, and have developed a specific enzyme assay. We have characterized the enzyme and now report that the deacetylase is relatively specific for melatonin and is inhibited by the melatonin precursor N-acetylserotonin and the product of the deacetylase, 5-methoxytryptamine. Inhibition of deacetylase activity by eserine (physostigmine) suggests a relationship between deacetylase and cholinesterase activities. However, among a variety of cholinesterase inhibitors tested, only eserine inhibits the deacetylase. Furthermore, eserine is much less potent as an inhibitor of the deacetylase than the cholinesterases, and purified cholinesterases failed to deacetylate melatonin. We also show that melatonin deacetylase and aryl acylamidase (an enzyme related to cholinesterases) activities are differentially extractable from Xenopus ocular tissues, and that they exhibit different pH optima and inhibition profiles. Our results provide an initial characterization of the Xenopus retinal melatonin deacetylase, and indicate that deacetylase activity is distinct from cholinesterase and aryl acylamidase activities.

Rhythmic regulation of retinal melatonin: metabolic pathways, neurochemical mechanisms, and the ocular circadian clock

1. Current knowledge of the mechanisms of circadian and photic regulation of retinal melatonin in vertebrates is reviewed, with a focus on recent progress and unanswered questions. 2. Retinal melatonin synthesis is elevated at night, as a result of acute suppression by light and rhythmic regulation by a circadian oscillator, or clock, which has been localized to the eye in some species. 3. The development of suitable in vitro retinal preparations, particularly the eyecup from the African clawed frog, Xenopus laevis, has enabled identification of neural, cellular, and molecular mechanisms of retinal melatonin regulation. 4. Recent findings indicate that retinal melatonin levels can be regulated at multiple points in indoleamine metabolic pathways, including synthesis and availability of the precursor serotonin, activity of the enzyme serotonin N-acetyltransferase, and a novel pathway for degradation of melatonin within the retina. 5. Retinal dopamine appears to act through D2 receptors as a signal for light in this system, both in the acute suppression of melatonin synthesis and in the entrainment of the ocular circadian oscillator. 6. A recently developed in vitro system that enables high-resolution measurement of retinal circadian rhythmicity for mechanistic analysis of the circadian oscillator is described, along with preliminary results that suggest its potential for elucidating general circadian mechanisms. 7. A model describing hypothesized interactions among circadian, neurochemical, and cellular mechanisms in regulation of retinal melatonin is presented.

Effects of different doses and durations of melatonin infusions on plasma melatonin concentrations in pinealectomized Syrian hamsters: consequences at the level of sexual activity

The effect of different doses and durations of melatonin infusions on plasma melatonin concentrations has been studied in pinealectomized Syrian hamsters maintained under short photoperiod at either 7 degrees C or 18 degrees C. The effects of the infusions on plasma melatonin concentrations and on gonadal activity were compared. The results show that the minimal effective quantity of infused melatonin that induced gonadal atrophy was 40 ng/h at 7 degrees C and 20 ng/h at 18 degrees C. An infusion of 8 hr duration per day is necessary to inhibit sexual activity, while an infusion of 6 hr duration was ineffective. This finding suggests that the critical duration of melatonin infusion is between 6 and 8 hr. Despite the various doses of melatonin infused, plasma melatonin concentrations measured in the middle of the infusion period did not differ significantly from concentrations measured in intact animals. This finding suggests that the metabolism of infused melatonin increases as the dose of melatonin increases. Moreover, the different physiological effects observed after the various melatonin infusions cannot be explained by variations in plasma melatonin concentrations.

Melatonin deacetylation: retinal vertebrate class distribution and Xenopus laevis tissue distribution

Deacetylation is a rapid clearance mechanism for ocular melatonin. We have studied the distribution of retinal melatonin deacetylase activity among vertebrate classes. Exogenous radiolabeled melatonin is metabolized by ocular tissue prepared from the amphibian Xenopus laevis, the reptile Anolis carolinensis, the teleost fish Carassius auratus, and the bird Gallus domesticus. In contrast, we were unable to detect ocular melatonin breakdown in rat or pig. In each species exhibiting ocular melatonin breakdown, melatonin is first deacetylated to 5-methoxytryptamine, which is deaminated, producing 5-methoxyindoleacetic acid and 5-methoxytryptophol. Deacetylation of melatonin is inhibited by eserine (physostigmine), causing a reduction in the levels of all 3 metabolites. Deamination of 5-methoxytryptamine is inhibited by the monoamine oxidase inhibitor pargyline, such that 5-methoxyindoleacetic acid and 5-methoxytryptophol levels are decreased while levels of 5-methoxytryptamine are increased. Incubation with the deacetylase inhibitor eserine increases endogenous melatonin levels in Xenopus and Carassius eyecups, indicating that endogenous melatonin is metabolized via the deacetylase. We also studied the tissue distribution of the deacetylase in Xenopus laevis. Melatonin deacetylation occurs in retina, retinal pigment epithelium, and skin, all of which are sites of melatonin action. These results indicate that among non-mammalian vertebrates, deacetylation is a common clearance mechanism for ocular melatonin, and may degrade melatonin at other sites of action as well. Melatonin deacetylation may help regulate local melatonin concentration, and generates other biologically active methoxyindoles.

Plasma concentrations of 5-methoxytryptamine, 5-methoxytryptophol and melatonin after 5-methoxytryptamine administration of golden hamsters: physiological implications

5-Methoxytryptamine (5-MT), 5-methoxytryptophol (5-ML) and melatonin (Mel) were measured in the plasma after 2, 5, and 8 weeks administration of 25 micrograms 5-MT to golden hamsters kept under long photoperiod. 5-MT showed a one compartment kinetic profile in the plasma with half lives of 14.8 min after 2 weeks, 15 min after 5 weeks and 19.1 min after 8 weeks. A rapid metabolism of 5-MT was shown, Mel and 5-ML being detected in the plasma following 5-MT administration. However it was also shown that the gonadal atrophy observed after 5-MT administration cannot be due to its metabolism into these 2 compounds. Indeed when exogenously administered at a dose generating the same plasma concentration as that observed after 5-MT, the gonadal regression observed after the association of 5-ML and Mel is much less than that observed after 5-MT. 5-MT is thus a compound of great physiological interest.

Retinal melatonin is metabolized within the eye of xenopus laevis

Retinal synthesis of melatonin, a potent modulator of rhythmic retinal processes, is elevated at night as a result of regulation by a circadian clock. Despite high nocturnal synthetic capacity, both melatonin content and release are low in the retina of the frog Xenopus laevis. We report here that cultured eyecups from Xenopus have the capacity for rapid metabolic breakdown of melatonin. Pharmacological analysis indicates that the initial step in this degradation pathway is deacetylation of melatonin by the enzyme aryl acylamidase (aryl-acylamide amidohydrolase, EC 3.5.1.13). This produces 5-methoxytryptamine, which is then deaminated by monoamine oxidase [amine:oxygen oxidoreductase (deaminating) (flavin-containing), EC 1.4.3.4], producing 5-methoxyindoleacetic acid and 5-methoxytryptophol. Inhibition of aryl acylamidase with eserine dramatically increases the release of endogenous melatonin by eyecups cultured at night, indicating that this pathway is the normal fate of retinal melatonin. Metabolism within the eye suggests a local neuromodulatory role for retinal melatonin, in contrast to the hormonal role of pineal melatonin.

In vitro uptake and metabolism of [3H]indole compounds in the pineal organ of the pike. I. A radiochromatographic study

Thin layer chromatography analysis of [3H]serotonin and [3H]melatonin metabolites synthetized in vitro by the pineal organ of the pike was performed. After a 10-min pulse, [3H]serotonin was mainly converted into [3H]-5-hydroxyindoleacetic acid (37%), [3H]-5-hydroxytryptophan and [3H]-5-methoxytryptophan (12 to 14%), and [3H]-5-hydroxytryptophol and [3H]-5-methoxytryptophol (3.5 and 9%) at the onset of darkness. When the pulse was followed by postincubations (in a cold medium) of increasing duration (15, 30, and 60 min), it appeared that the amount of [3H]-5-hydroxyindoleacetic acid decreased, that of [3H]-5-hydroxytryptophol decreased faster than that of [3H]-5-methoxytryptophol, and the amounts of [3H]-5-hydroxy- and [3H]-5-methoxytryptophan increased. [3H]-N-acetylserotonin, [3H]melatonin, and [3H]-5-methoxytryptamine were found in very low amounts. At the beginning of the photophase or at the onset of darkness, the uptake and metabolism of [3H]melatonin (after a 10-min pulse followed by a 10-min incubation in cold medium) resulted mainly in the formation of [3H]-5-methoxytryptophol (23 to 43%) and of [3H]-5-methoxytryptamine (6 to 12%). These results show that the pike pineal organ can synthesize all indoles that are known in the pineal gland of higher vertebrates. Usual, but also unusual, pathways of the indole metabolism were found that will need further clarification. Among these are the possible carboxylation of serotonin and deacetylation of melatonin (leading to the synthesis of 5-methoxytryptophol). Altogether, the results obtained suggest that the indole metabolism might be more complex than what has already been described in vertebrates.

Relative efficacy of melatonin and 5-methoxytryptamine in terms of their antigonadotrophic and counterantigonadotrophic actions in male Syrian hamsters

Antigonadotrophic and counterantigonadotrophic activities of melatonin and 5-methoxytryptamine (5-MT) were quantitatively compared in male Syrian hamsters. In long day conditions, the daily afternoon administration of either 5, 15, 25, 50, 100 or 200 micrograms melatonin induced testicular regression within 10 wk; under the same circumstances, only the 200-microgram dosage of 5-MT was able to suppress testicular weights. Thus, 5-MT appears to have about one-tenth the antigonadotrophic action of melatonin in the male Syrian hamster. In short days, the subcutaneous implantation of either 50 or 100 micrograms melatonin (every 2 wk in beeswax) prevented testicular regression whereas it required much larger doses of 5-MT (1 mg every 2 wk in beeswax) to achieve the same counterantigonadotrophic action. In terms of both their antigonadotrophic and counterantigonadotrophic effects, hamsters seem to be more sensitive to melatonin than to 5-MT.

Ability of tryptophan derivatives to mimic melatonin's action upon the Syrian hamster reproductive system

The ability of 18 different tryptophan derivatives to induce gonadal regression in Syrian hamsters when injected daily at either midday or late evening has been examined. The compounds chosen have either been identified within mammalian pineal glands or are thought to be possible metabolic derivatives of melatonin. Of the compounds tested, only melatonin and 5-methoxytryptamine were found to possess antigonadotropic activity. 5-Methoxy-tryptamine's potency, however, was 1/10th that of melatonin and, like melatonin, 5-methoxytryptamine was effective when injected in the evening but not when injected during midday. In addition to the general survey of tryptophan derivatives for antigonadal capability when injected, 5-methoxytryptamine, melatonin, and 5-methoxytryptophol were compared relative to their abilities to prevent photo-induced gonadal regression when administered within beeswax implants. Again 5-methoxytryptamine and melatonin, but not 5-methoxytryptophol, were effective with 5-methoxytryptamine's potency being less than that of melatonin. These results indirectly support the contention that melatonin is the pineal product which mediates photoperiodic effects upon the Syrian hamster reproductive system.

Serotonin-sensitive aryl acylamidase activity of platelet acetylcholinesterase

Serotonin-sensitive aryl acylamidase (AAA, EC 3.5.1.13) was purified to apparent homogeneity from sheep platelets by affinity chromatography and it was shown to be associated with the platelet acetylcholinesterase (AChE, EC 3.1.1.7). The basis for the association of the two enzymes was the following. Both enzyme activities co-eluted from the affinity columns with constant ratios of specific activities and percentage recoveries. Both enzymes co-migrated on gel electrophoresis. Both enzymes co-eluted during sepharose 6B gel filtration. Potent inhibitors of AChE such as bis(4-allyldimethyl ammoniumphenyl) pentan-3-one dibromide (BW 284C51), neostigmine and eserine also inhibited AAA potently. Both enzymes lost significant activity on treatment with deoxycholate or taurodeoxycholate and the loss could be partly restored by a mixture of phospholipids. The platelet AAA was specifically inhibited by serotonin and to a lesser extent by tryptamine but not by several other amines. It was also inhibited by acetylcholine and several of its analogues and homologues. It is suggested that in the platelets the two enzymes (AAA and AChE) are probably identical.

Brain aryl acylamidase

1. The serotonin (5-HT) sensitive brain aryl acylamidase (AAA) has received considerable attention due to its potential involvement in 5-HT action mechanism in CNS. 2. Multiple forms, AAA-1 and 2, have been separated by ammonium sulfate precipitation of brain extract and subsequent gel filtration. 3. Their chemical properties have been characterized and differentiated by effects of several classes of drugs including d-LSD, 5-HT, 5-HT related compounds and tetrahydro-beta-carbolines on their enzyme activities. 4. In the rat brain, AAA-1 shows highest specific activity in corpus striatum and lowest activity in cerebellum whereas AAA-2 shows highest specific activity in cerebellum and lowest activity in corpus striatum. 5. Subcellularly, AAA-1 exhibits highest specific activity in synaptosomal fraction of rat corpus striatum, lowest activity in mitochondrial fraction and no activity in nuclear fraction while AAA-2 exhibits highest specific activity in microsomal fraction and lowest activity in nuclear fraction. 6. Triton X-100 treatment altered the subcellular distribution pattern of both AAA-1 and AAA-2. 7. AAA-2 is possibly associated with true acetylcholinesterase (AChE) in brain based on its inhibition by neostigmine but its identity with AChE needs further elucidation. 8. To determine the physiological role(s) for brain AAA, naturally occurring aromatic alkylamines other than melatonin need to be tested as possible substrate(s) for the enzyme activity.

In vivo formation of 5-methoxytryptamine from melatonin in rat

Deacetylation of melatonin to 5-methoxytryptamine (5-MT) in vitro and in vivo was investigated in rat liver and brain tissue, using a gas chromatographic--mass spectrometric 5-MT assay method. In vitro incubation of liver but not brain (hypothalamic, Mesencephalic) slices with melatonin led to a concentration-dependent formation of small amounts of 5-MT; the conversion being 0.3--0.8%. In vivo administration of melatonin resulted in a dose-dependent formation of 5-MT in small quantities in the liver. The time course showed a peak maximum within 0.5 h, with a rapid decline; the half-life being about 1 h. 5-MT could be detected in both the blood and the hypothalamus after in vivo injection of melatonin. The time course of 5-MT in the blood was similar to that in the liver, but 5-MT could only be detected in the hypothalamus after large doses shortly after the melatonin injection. MAO had to be inhibited both in the in vitro and in vivo experiments in order to recover 5-MT, indicating that formed 5-MT is normally rapidly metabolised by MAO. It is concluded that a small fraction of melatonin can be converted to 5-MT by deacetylation (by aryl acylamidase) in the liver in vivo, constituting a minor pathway. Such a pathway could not be demonstrated in the brain. Trace amounts of 5-MT previously reported to be present in various tissues could originate from deacetylation of melatonin in the liver and possibly some other peripheral organs known to contain the deacetylating enzyme. The present results indicate that peripherally formed 5-MT, a psychoactive compound, is unlikely to have any effect on brain function under normal circumstances.

The identity of the serotonin-sensitive aryl acylamidase with acetylcholinesterase from human erythrocytes, sheep basal ganglia and electric eel

The identity of the serotonin-sensitive aryl acylamidase with acetylcholinesterase from three diverse sources, namely sheep basal ganglia, human erythrocyte membrane and electric eel, was examined. Both the enzymes co-purified with constant ratios of specific activity from all the three sources by different affinity chromatographic techniques. The ratio of acetylcholinesterase to aryl acylamidase activity was highest for basal ganglia, less for erythrocyte and lowest for eel enzymes. Both the purified enzymes co-migrated on polyacrylamide gel electrophoresis either as a single species or multiple species under different conditions. Gel density gradient electrophoresis indicated identical migration rates of both the enzymes. Extraction of the enzymes from the three sources by different techniques of membrane disruption and subsequent gel filtration on Sepharose 6B showed multiple peaks of enzyme activity. Both the enzymes had identical elution profiles on Sepharose 6B gel filtration. All the enzyme peaks from Sepharose 6B on gel electrophoresis showed co-migration of the enzyme activities. Apart from inhibition by serotonin and acetylcholine the purified aryl acylamidases from all the three sources were potently inhibited by neostygmine, eserine and BW 284C51, all strong inhibitors of acetylcholinesterase. It is suggested that the serotonin-sensitive aryl acylamidase is identical with acetylcholinesterase.