Letter: Tryptophan plus a pyrrolase inhibitor for depression

Tryptophan Metabolism: A Versatile Area Providing Multiple Targets for Pharmacological Intervention

The essential amino acid L-tryptophan (Trp) undergoes extensive metabolism along several pathways, resulting in production of many biologically active metabolites which exert profound effects on physiological processes. The disturbance in Trp metabolism and disposition in many disease states provides a basis for exploring multiple targets for pharmaco-therapeutic interventions. In particular, the kynurenine pathway of Trp degradation is currently at the forefront of immunological research and immunotherapy. In this review, I shall consider mammalian Trp metabolism in health and disease and outline the intervention targets. It is hoped that this account will provide a stimulus for pharmacologists and others to conduct further studies in this rich area of biomedical research and therapeutics.

Tryptophan: the key to boosting brain serotonin synthesis in depressive illness

It has been proposed that focusing on brain serotonin synthesis can advance antidepressant drug development. Biochemical aspects of the serotonin deficiency in major depressive disorder (MDD) are discussed here in detail. The deficiency is caused by a decreased availability of the serotonin precursor tryptophan (Trp) to the brain. This decrease is caused by accelerated Trp degradation, most likely induced by enhancement of the hepatic enzyme tryptophan 2,3-dioxygenase (TDO) by glucocorticoids and/or catecholamines. Induction of the extrahepatic Trp-degrading enzyme indolylamine 2,3-dioxygenase (IDO) by the modest immune activation in MDD has not been demonstrated and, if it occurs, is unlikely to make a significant contribution. Liver TDO appears to be a target of many antidepressants, the mood stabilisers Li(+) and carbamazepine and possibly other adjuncts to antidepressant therapy. The poor, variable and modest antidepressant efficacy of Trp is due to accelerated hepatic Trp degradation, and efficacy can be restored or enhanced by combination with antidepressants or other existing or new TDO inhibitors. Enhancing Trp availability to the brain is thus the key to normalisation of serotonin synthesis and could form the basis for future antidepressant drug development.

Pharmacologic Use of Niacin

Niacin is required for a host of critical redox and adenosine diphosphate-ribosylation reactions in metabolism. Niacin deficiency leads to the distinctive signs and symptoms of pellagra, but these can happen in an unpredictable progression and can be altered in patients with polymorphisms in any of the hundreds of niacin-dependent enzymes. The symptomatology of niacin deficiency is becoming a forgotten knowledge base, and niacin deficiency is likely underdiagnosed. Additionally, high levels of niacin and niacinamide have pharmacological effects distinct from their role as sources of vitamin B3, allowing a wide range of effects on processes such as blood flow and lipid metabolism, which can be used to treat or prevent a variety of disease processes.

Effects of the haem precursor 5-aminolaevulinate on tryptophan metabolism and disposition in the rat

5-Aminolaevulinate administration to rats inhibits cerebral 5-hydroxytryptamine synthesis by decreasing tryptophan availability to the brain secondarily to activation of hepatic tryptophan pyrrolase. The results show that tryptophan metabolism and disposition can be influenced by changes in liver haem concentration, and are discussed briefly in relation to mood disorders in the hepatic porphyrias.

Metabolism of an oral tryptophan load. II: Effect of pretreatment with the putative tryptophan pyrrolase inhibitors nicotinamide or allopurinol

1 The effect of seven days administration of either allopurinol (300 mg daily) or nicotinamide (500 mg twice daily) on the metabolism of an oral L-tryptophan load (50 mg/kg) has been investigated. 2 Administration of either drug failed to alter the plasma total or free tryptophan or plasma kynurenine curve. Nor was the urinary excretion of tryptophan, kynurenine, 5-hydroxyindoleacetic acid or indole acetic acid influenced. 3 Allopurinol pretreatment did increase the volume of distribution of tryptophan. 4 These data suggest that allopurinol and nicotinamide are unlikely to be of value as tryptophan pyrrolase inhibitors in vivo and therefore would not increase the therapeutic effect of L-tryptophan when it is given to treat depressive illness.

Inhibition of rat brain tryptophan metabolism by ethanol withdrawal and possible involvement of the enhanced liver tryptophan pyrrolase activity

1. Chronic ethanol administration to rats was previously shown to enhance brain 5-hydroxytryptamine synthesis by increasing the availability of circulating tryptophan to the brain secondarily to the NAD(P)H-mediated inhibition of liver tryptophan pyrrolase activity. 2. At 24h after ethanol withdrawal, all the above effects were observed because liver [NAD(P)H] was still increased. By contrast, all aspects of liver and brain tryptophan metabolism were normal at 12 days after withdrawal. 3. At 7--9 days after withdrawal, brain 5-hydroxytryptamine synthesis was decreased, as was tryptophan availability to the brain. Liver tryptophan pyrrolase activity at these time-intervals was maximally enhanced. 4. Administration of nicotinamide during the withdrawal phase not only abolished the withdrawal-induced enhancement of tryptophan pyrrolase activity on day 8, but also maintained the inhibition previously caused by ethanol. Under these conditions, the withdrawal-induced decreases in brain 5-hydroxytryptamine synthesis and tryptophan availability to the brain were abolished, and both functions were enhanced. Nicotinamide alone exerted similar effects in control rats. 5. It is suggested that ethanol withdrawal inhibits brain 5-hydroxytryptamine synthesis by decreasing tryptophan availability to the brain secondarily to the enhanced liver tryptophan pyrrolase activity. 6. The results are discussed in relation to the possible involvement of 5-hydroxytryptamine in dependence on ethanol and other drugs.

Unsuitability of control sucrose or glucose in studies of the effects of chronic ethanol administration on brain 5-hydroxytryptamine metabolism

Chronic sucrose or glucose administration enhances 5-hydroxytryptamine synthesis in rat brain, as does ethanol. This suggests that the above sugars are not suitable for use as control treatments in studies of the chronic effects of ethanol on brain 5-hydroxytryptamine metabolism.

Enhancement of rat brain metabolism of a tryptophan load by chronic ethanol administration

We have previously shown that chronic ethanol administration enhances brain 5-hydroxytryptamine synthesis by increasing the availability of circulating tryptophan to the brain secondary to the decreased liver tryptophan pyrrolase activity. We now find that ethanol enhances the brain metabolism of a tryptophan load by the same mechanism. The results are discussed in relation to ethanol preference and the need for further clinical work on the effects of alcoholism on tryptophan metabolism.

Tryptophan antidepressant 'physiological sedative': fact or fancy?

Some recent publications relating to the allegedly antidepressive and sedative effects of L-tryptophan the precursor of 5-HT have been reviewed. The evidence to date suggests that the amino acid is as effective as standard tricyclic drugs in alleviating the symptoms of depression, especially those cases presenting with mainly psychomotor retardation, and is synergistic with MAOIs. L-Tryptophan would also appear to be a 'physiological sedative'. This action, however, appears to be related to the time of administration and at present has only been demonstrated at night. when endogenous levels of 5-HT are at their peak. In terms of practical therapeutics L-tryptophan would appear to have greater potential as an 'antidepressant' than as a 'sedative'.

The acute effects of ethanol on liver and brain tryptophan metabolism

1. The effects of acute ethanol administration of liver and brain tryptophan metabolism are reviewed. 2. Ethanol enhances the activity of rat liver tryptophan pyrrolase by increasing the availability of circulating free tryptophan to the liver by catecholamine-mediated lipolysis followed by displacement of protein-bound serum tryptophan. 3. The response of the mouse liver enzyme to ethanol is strain-dependent. Ethanol activates the enzyme in CBA/CA but not in C57/BL mice. 4. Ethanol exerts a biphasic effect on the concentrations of rat brain tryptophan, 5-hydroxytryptamine and 5-hydroxyindol-3-ylacetic acid. 5. Both aspects of this biphasic effect are associated with an altered availability of circulating free tryptophan. 6. The initial enhancement by ethanol of brain tryptophan metabolism may be due to the above-mentioned lipolytic mechanism, whereas the subsequent decrease in brain indoles may be caused by the enhanced tryptophan pyrrolase activity. 7. Brain tryptophan metabolism is decreased by ethanol in CBA/CA whereas no change is observed in that in C57/BL mice. 8. These results are discussed in relation to previous work on the acute effects of ethanol on rat and mouse brain 5-hydroxytryptamine metabolism.

The role of free serum tryptophan in the biphasic effect of acute ethanol administration on the concentrations of rat brain tryptophan, 5-hydroxytryptamine and 5-hydroxyindol-3-ylacetic acid

1. Acute administration of ethanol exerts a biphasic effect on the concentrations of rat brain tryptophan, 5-hydroxytryptamine and 5-hydroxyindol-3-ylacetic acid. Both effects are associated with corresponding changes in the availability of circulating free tryptophan. 2. The initial increases in the above concentrations are prevented by ergotamine, are unaltered by allopurinol and are potentiated by theophylline, whereas the later decreases are prevented by both ergotamine and allopurinol. 3. It is suggested that the initial enhancement by ethanol of brain tryptophan metabolism is caused by catecholamine-mediated lipolysis followed by displacement of protein-bound serum tryptophan, whereas the activation of liver tryptophaan pyrrolase, which is produced by the same mechanism, leads to the later decreases in the brain concentrations of tryptophan and its metabolites. 4. The initial effects of ethanol can be reproduced by an equicaloric dose of sucrose, and a comparison of the two treatments alone could therefore be misleading. 5. The effects of ethanol on liver and brain tryptophan metabolism have also been examined in mice, and a comparison of the results with those previously reported suggests that the ethanol effects are strain-dependent.