5-hydroxyindole acetaldehyde (5-HIAL), the product of oxidative deamination of serotonin can be metabolized to 5-HIAA or 5-hydroxytryptophol (5-HTOL). The former reaction is the major pathway under normal conditions. This reaction is catabolyzed by aldehyde dehydrogenase, employing NAD as coenzyme. Reduction of the aldehyde intermediate is catalysed by aldehyde reductase, which employs NADH as coenzyme.19 In case of a 5-HT overload, the known 5-HT catabolic pathways may become overloaded, allowing lesser-used pathways to convert more 5-HT thereby increasing levels of 5-HT metabolites that might not readily be observed under other conditions.
5-hydroxytryptophol is a minor serotonin metabolite under normal conditions, accounting for 1% of the total serotonin turnover. Alcohol consumption leads to an increased synthesis of 5-HTOL and a concomitant decrease in the synthesis of 5-HIAA, resulting in an increased 5-HTOL/5-HIAA ratio, which is a sensitive marker for detection of recent alcohol intake. During ethanol oxidation the conversion of serotonin shifts away from oxidation of the intermediate 5-HIAL producing 5-HIAA toward the reductive pathway forming 5-HTOL. This has been attributed to the competitive inhibition of aldehyde dehydrogenase by ethanol-derived acetaldehyde.19
Another proposed metabolic pathway of serotonin involves glucuronidation. Serotonin has been characterized as a highly selective substrate of human UDP-glucuronosyltransferase UGT1A6 in Caco-2 cells, an in vitro model of human intestinal epithelium. These findings suggest that UGT1A6 contributes to the homeostatic control of intestinal 5-HT metabolism.20 However, studies on the expression of UGT1A6 in human intestinal epithelium are not conclusive. Munzel et al. demonstrated the expression of UGT1A6 mRNA in human duodenum with large interindividual differences,21 while Radominska-Pandya et al.22 found that UGT1A6 was not present in the human intestine, suggesting a regulational control of intestinal 5-HT metabolism that remains to be elucidated.
Recently, the metabolite 5-hydroxyindole thiazolidine carboxylic acid (5-HITCA) has been identified in rodent CNS and ENS tissue samples. This is a condensation product of 5-hydroxyindole acetaldehyde and L-cysteine.23 5-HITCA can be detected natively in homogenizated rodent ENS samples. In 5-HT incubated central and enteric nervous system tissue samples, 5-HITCA forms at levels equivalent to 5-HIAA. An equilibrium between 5-HITCA and 5-hydroxyindole acetaldehyde coupled to the enzyme aldehyde dehydrogenase, in the CNS and ENS suggests equilibrium prevents this accumulation.23
Imbalances in 5-HT levels within the ENS and the intestine have been associated with various functional gastrointestinal disorders. The most attention perhaps has been given to irritable bowel syndrome, which has been documented extensively.1,24 Irritable bowel syndrome (IBS), affecting approximately 15–20% of the adult population, is a functional intestinal disorder characterized by abdominal pain or discomfort associated with altered bowel habits, without indications for an organic cause. An increased number of enterochromaffin cells and altered mucosal serotonin metabolism have been described in IBS and serotonergic compounds have been shown to beneficially influence intestinal motor and sensory function.1 A functional hallmark of IBS is visceral hypersensitivity, which is present in approximately 50% of patients with IBS.25 Previous research highlights the involvement of the 5-HT3 receptors on extrinsic primary afferent neurons of the ENS in mediating the pain sensation.1 It was also suggested previously that the increased visceral sensitivity observed in IBS patients may at least partly be caused by a decreased epithelial integrity, causing intraluminal compounds to cross the epithelial barrier and trigger the ENS and, thereby, nociception.26 Serotonin could therefore also play a role in regulating the permeability of the intestine.
Tight regulation of 5-HT levels in nervous tissue and intestinal mucosa is necessary and 5-HT catabolism plays an important role in this regulation. Understanding the catabolic pathways, and knowledge on the enzymes involved and the products of these conversions are particularly important because catabolism is vital to regulation of 5-HT levels. Elucidating 5-HT synthetic and catabolic pathways may provide novel approaches for the therapies designed to treat disorders associated with 5-HT homeostasis.
In adult young men, about 95% of dietary tryptophan is metabolized along the kynurenine pathway.3 The biological functions of the kynurenine pathway are: clearance of excess tryptophan and regulation of plasma tryptophan levels, maintenance of nicotinic acid levels, regulation of CNS function and enhancement of macrophage defence function.
Entering the kynurenine pathway, tryptophan is first oxidized by tryptophan 2,3-dioxygenase (TDO), which is almost entirely localized at hepatic cells. Tryptophan 2,3-dioxygenase is the rate limiting enzyme for kynurenine synthesis in the periphery. Tryptophan 2,3-dioxygenase expression and activity can be induced four- to tenfold by tryptophan loading within a period of a few hours.8 The principal branch of the kynurenine pathway generates quinolinic acid and nicotinamide, whereas the side chains generate kynurenic acid and xanthurenic acid (see Fig. 1). Several biological features of kynurenine metabolites have been described. Most attention has been given to the imbalance in neurotoxic and neuroprotective properties of these compounds, which have been associated with several CNS pathologies. Quinolinic acid is considered to be an excitotoxic N-methyl D-aspartate (NMDA) receptor agonist, whereas kynurenic acid is a neuroprotective NMDA antagonist and an α7 nicotinic cholinergic agonist.27 In mononuclear cells, including tissue macrophages, quinolinic acid is the main end product of the kynurenine pathway and plays a role in immunoregulatory processes.28
The kynurenine pathway also provides the precursors for the dietary supplement niacin, a collective term for nicotinamide and nicotinic acid. Under normal conditions, most of the tryptophan that enters the oxidative pathway is converted to CO2 and water in the glutarate pathway. Only if this branch of the pathway is saturated, NAD becomes a major product of metabolism.3 Although metabolites of the glutarate pathway are present in many tissues, including the intestine, NAD synthesis is only possible in the liver, because this is the only organ that possesses all the necessary enzymes.28
Another product of the kynurenine pathway is picolinic acid. Picolinic acid is only produced when the flux of metabolites through the glutarate pathway is high and enzymes of the glutarate pathway are saturated.3 Picolinic acid acts as a chelating agent of elements such as chromium, zinc, manganese, copper, iron, and molybdenum in the human body. It forms a complex with zinc that may facilitate the passage of zinc through the gastrointestinal wall and into the circulatory system.8 Several of the enzymes of the kynurenine pathway use vitamin B6 as cofactor and a key feature of the enzyme kynureninase is its exceptionally high sensitivity to pyridoxine deficiency. Lack of vitamin B6 leads to a large increase in xanthurenic acid excretion. This has been used for decades as a diagnostic test for vitamin B6 deficiency.8 Vitamin B6 deficiency also compromises serotonin synthesis, and hence can lead to competition between the two pathways for the co-factor.
Besides TDO, another enzyme initializing the kynurenine pathway is indoleamine 2,3-dioxygenase (IDO). Indoleamine 2,3-dioxygenase is widely distributed in peripheral tissues. The human intestine contains a relatively large amount of IDO.29 While TDO exclusively accepts tryptophan as substrate, IDO has a broader specificity and can also take 5-HTP, 5-HT and tryptamin.29 The expression of IDO increases in response to infection and inflammation, with interferon-γ being the strongest stimulator. Mononuclear cells that synthesize IDO reduce extracellular tryptophan concentration so that adjacent T-cells, which depend on tryptophan from the extracellular environment, are unable to activate and proliferate upon encountering antigens. Therefore, IDO might play a role in preventing the initiation of autoimmune disease by enforcing T-cell tolerance through suppressing their proliferation.28 Hence, high local expression of IDO by mononuclear cells may represent an anti-inflammatory and immunosuppressive mechanism tempting to counterbalance tissue damage.30 This mechanism could be involved in intestinal pathophysiology, as IDO expression is markedly induced in lesional colonic biopsies of inflammatory bowel disease (IBD) patients30 and increased IDO activity has been observed in patients with celiac disease31 and diverticulitis.32 A similar IDO-based intrinsic immunoescape mechanism is probably employed by colon tumour cells.33
Besides through the regulatory effect of IDO on T-cells and immune function, inflammatory responses in the ENS and the gastrointestinal tract related to the kynurenine pathway can also be based on a sensitive balance between the pro-inflammatory, excitotoxic quinolinic acid and the anti-inflammatory, neuroprotective kynurenic acid.34 This balance could have profound influence on the excitability of enteric neurons, which can affect intestinal motor and sensory function. Increased levels of the kynurenine pathway metabolites kynurenine and kynurenic acid have been observed in sera of patients with inflammatory bowel disease.35 The increased activity of the kynurenine pathway may represent either a compensatory response to elevated activation of enteric neurons or a primary abnormality which induces a compensatory increase in gut activity. In either case, the data may indicate a role for the kynurenine modulation of glutamate receptors in the symptoms of IBD.35
Recent evidence suggests the involvement of the kynurenine pathway in intestinal motility, although the exact roles of kynurenine metabolites in intestinal motor function remains unclear.34 Kynurenic acid acts as an antagonist on NMDA receptors on enteric glutamatergic neurons and may cause dysregulation of intestinal motility. Glutamate is likely to play an excitatory role and may modulate cholinergic transmission in the ENS.36 Glutamate immunoreactivity has been detected in submucosal and myenteric neurons in the guinea pig ileum, and NMDA receptors are present on enteric cholinergic neurons, and vagal and spinal primary afferent nerve endings. N-methyl D-aspartate receptor activation has been shown to stimulate acetylcholine release from myenteric neurons, thereby modulating smooth muscle contraction.36 Recent result have revealed a significant potential for kynurenic acid to decrease the facilitary pathways of colonic motility.34 Kynurenic acid might also play a role in intestinal mechanosensitivity, as it has been proven to act also as glutamate antagonist and inhibit mechanosensitivity of both mucosal and tension vagal afferents.37 Furthermore, kynurenic acid also exerts an anti-inflammatory effect due to inhibition of xanthine oxydase, resulting in less reactive oxygen species (ROS) production.34
Other products of the IDO and formamidases are kynuramin derivates. Formation of kynuramines has been described in various tissues, including the intestine and appears to be directly proportional to tryptophan concentrations. Kynuramines may be important as endogenous agonists or antagonists of 5-HT receptors in smooth muscle. Marked non-selective serotonergic agonist properties of 5-hydroxykynuramine at multiple 5-HT receptors were demonstrated in rat ileum. 5-hydroxykynuramine is formed from tryptophan to much lesser extent in vivo than 5-HT, but pathological conditions or situations in which tryptophan concentrations are increased may lead to an overproduction of kynuramines. Besides its effect on smooth muscle, 5-hydroxykynureamine is a potent inhibitor of the action of serotonin in promoting the aggregation of platelets. This may provide a measure of regulation in cases of over-synthesis of serotonin, not only as an alternative catabolic pathway of the amine, but also to inhibit one of its biological actions.38