Tryptophan (TRP), an essential amino acid in mammals, is involved in several physiological processes including neuronal function, immunity, and gut homeostasis. In humans, TRP is metabolized via the kynurenine and serotonin pathways, leading to the generation of biologically active compounds, such as serotonin, melatonin and niacin. In addition to endogenous TRP metabolism, resident gut microbiota also contributes to the production of specific TRP metabolites and indirectly influences host physiology. The variety of physiologic functions regulated by TRP reflects the complex pattern of diseases associated with altered homeostasis. Indeed, an imbalance in the synthesis of TRP metabolites has been associated with pathophysiologic mechanisms occurring in neurologic and psychiatric disorders, in chronic immune activation and in the immune escape of cancer. In this chapter, the role of TRP metabolism in health and disease is presented. Disorders involving the central nervous system, malignancy, inflammatory bowel and cardiovascular disease are discussed.

Previously, we showed that a transient early-in-life interference with the expression of multiple genes by mithramycin (MTR) followed by later-in-life exposure to chronic stress, leads to a "daring" and novelty seeking behavior in rats. In this study we searched for molecular changes that contribute to this behavioral alteration. We applied a non-hypothesis driven strategy using whole genome cDNA array analysis (WGA) followed by Genome Scale Metabolic modeling analysis (GSMM). Gene expression validation was performed by qRT-PCR and immunoblotting. Brain and serum amino acids levels were measured by HPLC. WGA data directed us towards metabolic pathways and GSMM pointed at branched chain amino acids (BCAA) pathway. Out of 21 amino acids analyzed in the prefrontal cortex of MTR+Stress rats only tryptophan, whose brain levels depend on serum BCAA levels, showed a significant decrease. No change was observed in serotonin or kynurenine levels. However, a significant reduction in mRNA and protein levels of the large neutral amino acid transporter (LAT1), which transports BCAA and tryptophan into the brain, as well as in serum levels of tryptophan/BCAA ratio were observed. The latter may be attributed to the failure to increase serum insulin, following stress, in rats pre-exposed to mithramycin. Finally, significant correlations were observed between the anxiety index and tryptophan and between T-maze errors and LAT1. This study shows a specific behavioral pattern, which is linked to modulations in fluxes of amino acids both peripheral and central, which converge and reciprocally interact, and may thus be equally important targets for therapeutic intervention.

Quinine is a major drug of choice for the treatment of malaria. However, the primary mode of quinine action is unclear, and its efficacy is marred by adverse reactions among patients. To help address these issues, a genome-wide screen for quinine sensitivity was carried out using the yeast deletion strain collection. Quinine-sensitive mutants identified in the screen included several that were defective for tryptophan biosynthesis (trp strains). This sensitivity was confirmed in independent assays and was suppressible with exogenous Trp, suggesting that quinine caused Trp starvation. Accordingly, quinine was found to inhibit [(3)H]Trp uptake by cells, and the quinine sensitivity of a trp1Delta mutant could be rescued by overexpression of Trp permeases, encoded by TAT1 and TAT2. The site of quinine action was identified specifically as the high affinity Trp/Tyr permease, Tat2p, with which quinine associated in a Trp-suppressible manner. A resultant action also on Tyr levels was reflected by the Tyr-suppressible quinine hypersensitivity of an aro7Delta deletion strain, which is auxotrophic for Tyr (and Phe). The present genome-wide dataset provides an important resource for discovering modes of quinine toxicity. That potential was validated with our demonstration that Trp and Tyr uptake via Tat2p is a major target of cellular quinine toxicity. The results also suggest that dietary tryptophan supplements could help to avert the toxic effects of quinine.

Total plasma tryptophan was analysed during a 24-h period in 27 control persons and 42 patients with a diagnosis of endogenous depression. Total plasma tryptophan was unchanged in endogenously depressed patients compared with controls. The effect of one night's sleep reversal was investigated in control persons. During the following day, when the persons were asleep, plasma tryptophan concentration fell below control values. The normal diurnal variation with low values in the night and high values in the daytime disappeared when the persons were kept fasting.

One hundred and fifteen patients from 5 general practices participated in a 12-week, double-blind study comparing L-tryptophan, amitriptyline, L-tryptophan-amitriptyline combination and placebo in the treatment of depression. Analysis of total score on the Hamilton Depression Scale and a global rating of depression showed that all 3 active treatments were more effective than placebo. Significantly more patients were withdrawn as treatment failures in the placebo group compared with the active treatment groups. Side-effects necessitated withdrawal of more patients from the amitriptyline group than from the other active treatment groups, but this difference was not significant. Plasma amitriptyline and nortriptyline levels were similar in the amitriptyline and combined treatment groups. Standard haematological and biochemical profiles did not alter significantly in any group, but mean heart rate was significantly increased in patients receiving amitriptyline. There was no change in free or total plasma tryptophan concentration with treatment or on remission of symptoms.

The concentration of free and total tryptophan and kynurenine in plasma from 49 female depressives and 26 female controls was measured following oral loading with L-tryptophan, 100 mg/kg body weight. There was no significant difference between five depressives and six controls in the area under curve for free or total tryptophan or kynurenine in plasma. The peak concentration of kynurenine occurred 4 h after loading and in correlated significantly with the area under curve for kynurenine. There was no significant correlation between the L-tryptophan dose (g) and the plasma concentration of kynurenine a 4 h in the 49 depressives or 26 controls. The mean plasma levels of tryptophan and kynurenine at 4 h in the depressives were not significantly different from control levels. There was no clear relationship between the plasma levels of tryptophan or kynurenine at 4 h and the therapeutic response in 13 depressives treated with L-tryptophan for 15 days. It is concluded that the absorption, the plasma clearance, and the degradation to kynurenine of loading doses of L-tryptophan are normal in depressed patients. Results furthermore suggest that the plasma levels of tryptophan and kynurenine at 4 h are poor predictors of the response to L-tryptophan treatment in depressives.

Changes in adrenoceptor and serotonin receptor activities, and platelet serotonin uptake, in a group of moderately depressed patients before and during treatment have been monitored. alpha 2 and beta-adrenoceptor densities on platelets and lymphocytes were markedly higher before treatment commenced and decreased to approximately control values following clinical response to drug treatment; patients not responding to such treatment failed to show any attenuation of the receptor hypersensitivity. Responders and non-responders showed differing receptor affinity for the ligands at the conclusion of the treatment period. Platelet serotonin receptor activity and uptake were lower before the start of treatment than in those patients who responded to therapy. There was no difference between plasma free and bound tryptophan concentrations of patients and controls, either before or during treatment. The observed changes in receptor function may correlate with changes in the clinical status of depressed patients.

The equilibrium binding constants of the interaction of tryptophan with albumin have been determined in plasma samples from both controls and patients suffering from affective disorders. The data corresponded with literature values for both the albumin concentration in plasma and the equilibrium constant for tryptophan binding to albumin. No differences in the equilibrium constants were detected between patients and controls but a small increase in the albumin concentration was noted in the depressives. It was agreed that this rise would be inadequate to lead to a significant effect on free tryptophan levels. Non-esterified fatty acid analysis indicated no differences between patient groups.

Biochemical variates that may be related to depressive illness have been shown to vary significantly according to the season. This factor must therefore be carefully considered in biochemical and therapeutic investigations into depression, and may account for the seasonal variation in depression and suicide that have been reported by many investigators.

Patients suffering from unipolar illness and anticipating a complex investigation, or the first of a series of ECTs, had lower total tryptophan levels than control subjects under the former circumstances. This suggested a qualitative or quantitative difference in response to stress, associated with a fall in the amino acid level. It was hypothesized that this might selectively decrease synthesis of both protein and 5-HT in serotoninergic neurones.

Plasma cortisol, free and total tryptophan were determined in 71 subjects on 8 occasions between 36 weeks gestation and 6 weeks post-partum. Affect was measured by rating scales and clinical interview. Twenty-eight subjects were judged to have experienced post-partum 'blues'. Seasonal variation occurred in the incidence of 'blues' and in cortisol and free tryptophan levels. Puerperally-depressed mood was correlated with high cortisol at 38 weeks irrespective of season. Free tryptophan was reduced in 'blues' subjects but only at the time of year when free tryptophan was normally high. Total tryptophan was low antenatally; a rapid rise on days 1 and 2 post-partum was superimposed on a slower return to normal. This initial peak was clearly absent in 37 per cent of subjects. Its absence was significantly related to occurrence of post-partum 'blues' and of complaints of depression in the ensuing 6 months. This finding is discussed in relation to the possible occurrence of an occult disturbance of tryptophan handling in subjects susceptible to depression.

The concentrations of total and free plasma tryptophan were measured in 12 unipolar depressed patients before and after a course of electroconvulsive therapy (ECT) and before and after single ECTs during the course of treatment. Eleven patients undergoing diagnostic cystoscopy served and controls to examine the acute effect of anaesthesia. Total and free plasma tryptophan concentrations in the depressed patients were not significantly different from control values and were not changed by course of ECT. Free plasma tryptophan varied considerably within individual patients. Total plasma tryptophan was reduced acutely by ECT/anaesthesia in the depressed patients (P less than 0.05) and by anaesthesia in the cystoscopy controls (P less than 0.01). Free plasma tryptophan was not significantly altered. This reduction in total plasma tryptophan could be secondary to an effect of thiopentone on albumin binding of tryptophan.

The free and total plasma tryptophan concentrations were measured in 41 female depressive. Fifteen patients were subsequently treated with L-tryptophan. The free and total tryptophan were normal in the drug-free depressives and in the depressives on lithium. No significant in free and total plasma tryptophan were found between unipolar and bipolar depressed subjects, or between patients who recovered from depression following L-tryptophan therapy and patients who were resistant. The results suggest that the basal free tryptophan concentration in the plasma of depressed subjects is normal. They furthermore indicate that neither free nor total plasma tryptophan concentrations are valid predictors for the course of a treatment of depressed subjects with L-tryptophan.

The combination of electroconvulsive treatment (ECT) and i.v. L-tryptophan (T) was compared with ECT and saline in double-blind study comprising 20 patients with endogenous depression. No significant difference was found with regard to the number of ECT given nor in the rate of reduction of depressive symptoms. Except for a slight decrease of plasma total tryptophan in the palcepo group, no difference were found in plasma total and free tryptophan nor in the concentration of total tryptophan in the cerebrospinal fluid.

Plasma free tryptophan is significantly decreased in monopolar, depressed patients. No evidence was found to suggest that poor nutritional history prior to hospital admission was responsible for these low levels. Factors known to influence tryptophan-albumin binding in plasma, e.g. concentration of plasma proteins, albumin and non-esterified fatty acids, did not account for the low levels of free tryptophan in depressed patients. A significant decrease in plasma free tryptophan levels was found in perimenopausal but not in pre- or post-menopausal female controls. This mirrors the decrease in circulating oestrogens. Although exogenously administered oestrogens do not have any therapeutic efficacy in relieving mild residual depressive symptoms of lithium treated patients, they increased the levels of plasma free tryptophan. Clofibrate also displaces tryptophan from plasma protein binding sites in both depressed patients and controls. Utilization of the increased levels of plasma free tryptophan is reduced in depressed patients. A situation therefore exists in depressed patients where the plasma free tryptophan is not only reduced but also leaves the plasma less readily than in control subjects.

The Geoffrey Knight Psychosurgical Unit admits patients on a regular basis and thus offers special opportunities for studying severely ill psychiatric cases, all having one particular treatment under relatively controlled conditions. The opportunity has been taken to repeat various metabolic studies previously reported to be abnormal in some psychiatric illnesses. In the present investigation several measures of endocrinological activity were studied, as was plasma tryptophan, both free and bound. None of these data confirmed reports of abnormalities and neither did the values found at operation help to predict clinical outcome 1 year later, which was another possibility. Urinary catecholamines were also measured and 2 weeks after operation. Male patients, regardless of diagnosis, showed a mean increase in adrenaline output after operation compared with the pre-operative value and this was significantly different from the females, who showed a small mean decrease. The depressed patients showed a significant reduction in noradrenaline excretion after operation compared with before operation and this trend was enhanced in those of good outcome at 1 year, the difference from those who responded poorly being significant. It could be that the ventromedial lesion that is produced alters noradrenaline metabolism or autonomic activity in depression and this possibility merits further study.

Recently, the Ultrafiltrator has been described, in the literature, which allows the ultrafiltration of small molecules in minute blood samples under anaerobic conditions with filters of any pore size. This apparatus has been tested for the separation of free tryptophan in heparinised and EDTA-plasma and in whole blood. At the same time, free tryptophan has been analysed after ultrafiltration with CF-50 A and CF-25 membrane cones (Centriflo). The results obtained by these two techniques are compared. Finally, a comprehensive review of other methods reported in the literature, concerning the isolation of free tryptophan, is presented.