Doses Of L-tryptophan Research Articles

Simultaneous detection of melatonin and six metabolites of L-tryptophan pathway in rat gastric mucosa.

Melatonin (N-acetyl-5-methoxytryptamine) is an indoleamine synthesized in vertebrates mainly in the pineal gland, and is known to be involved mainly in thermoregulation and control of the circadian rhythm. That indoleamine can affect the auto-, para- and endocrine pathways, regulating body functions and affecting the metabolism of animals and humans. In addition to the pineal gland, melatonin can be synthesized in many extra-pineal tissues, mainly in the gastrointestinal tract. Previous studies have shown that melatonin plays an important role in the defense system of the gastrointestinal mucosa, demonstrating a protective effect on the gastrointestinal tract and the acceleration of healing of chronic ulcers through the scavenging of reactive oxygen metabolites (ROS) and the activation of protective nitric oxide (NO) and vasodilator neuropeptides released from the sensory afferent neurons. The process of converting the melatonin precursor L-tryptophan into melatonin is already known, but not all aspects of this process for the synthesis of other metabolites of this pathway have been fully elucidated and this issue remains poorly understood. In this study, the conversion of L-tryptophan to melatonin and other metabolites was determined in gastric mucosa collected from rats with or without intragastric (i.g.) melatonin or L-tryptophan administration, both administered at a single dose of 50 mg/kg. For the determination of five metabolites of L-tryptophan: kynurenine, 5-hydroxytryptamine, 5-hydroxytryptophan, anthranilic acid, indole-3-acetic acid together with melatonin, we have modified the previously developed high-performance liquid chromatography (HPLC) method using a native fluorescence detection system and UV-VIS. The obtained results show that: 1) L-tryptophan is converted into melatonin in the gastric mucosa during the day, e.g. after eating a meal containing L-tryptophan, as it was imitated and confirmed by our study, in which this amino acid was administered directly to the stomach, 2) the gastric mucosa is capable of producing melatonin in much greater amounts than those recorded in the blood serum of rats given a single dose of L-tryptophan, and 3) apart from melatonin, the only serum levels of these five metabolites of the L-tryptophan metabolic pathway are detectable, while their level in the gastric mucosa is low and barely detectable under physiological conditions. Our present observations support the notion that the gastric mucosa is one of the main sources of melatonin production from L-tryptophan outside the pineal gland.

Research on the acute toxicity and teratogenicity of the feeding L-tryptophan

In order to evaluate the safety of the feeding L-tryptophan, we conducted acute toxicity test and teratogenicity test in rats. Rats were fed with L-tryptophan by gavage. The oral LD50 was more than 20.0g/kg b.w. In the teratogenicity test, the doses of L-tryptophan were 5.0g/Kg b.w., 2.5g/Kg b.w. and 1.25g/Kg b.w. We set up a normal control group. The rats were fed by gavage once a day from the 7th to 17th day of pregnancy. At the 20th day of pregnancy, we counted and measured the litter size, placenta weight, live baby weight, body length, tail length, fetus malformation rate, live fetus average malformation rate, maternal fetus malformation rate, fetus death rate, absorptive fetus rate and other indicators. Then we use SPSS12.0 software to carry out data statistical analysis, and One-Way ANOVA and t-test for comparison between groups. The results showed that, when each dose group compared with the control group, there was no significant difference in average body weight, body length, tail length, placenta weight, fetus live rate, fetus death rate and absorptive fetus rate when compared to the normal control group(P>0.05), and there was no significant difference in the common deformity indexes, such as fetus malformation rate, live fetus average malformation rate and maternal fetus malformation rate between each dose group and the normal control group(P>0.05). The oral LD50 of L-tryptophan to Wistar rats was more than 20.0g/kg b.w., and the traditional teratogenicity test was negative, indicating that the feed additive belongs to the actually non-toxic level and has no obvious teratogenicity.

Deep brain stimulation of the periaqueductal gray for treatment of nociceptive pain: tryptophan loading and the serotonergic circuit of pain

Background: Pain and pain-related diseases pose a major treatment challenge. Current medical treatments frequently provide inadequate pain control to patients, or cause dependence and other unwanted side-effects. Despite initial failures of open-label trials to show efficacy in the use of deep brain stimulation (DBS) for pain, several groups have reported utility of this modality on carefully selected patients. There are several characterized pain pathways in operation in the human brain. A common theme to the pathways thus far identified is their dependence on the neurotransmitter serotonin for modulation of pain. Deep brain stimulation for pain may well activate one of the serotonin-dependent pathways to stimulate relief. Greater experience, and improvements in both technology and patient selection, has paved the way for the potential use of electrical stimulation of subcortical structures to manage intractable and nociceptive pain syndromes and also for the mechanistic study of the pathways involved in the symptomatic relief of pain caused by stimulation. Methods: We preoperatively loaded four patients with 4 grams of L-tryptophan, and implanted DBS electrodes in the periaqueductal gray (PAG). Results: Upon loading the patients with 4 grams of L-tryptophan two weeks preoperatively and the continual use of tryptophan at doses greater than 2.5 grams, we note a significant improvement in the pain symptoms of all of our patients implanted at the PAG. Upon decrease in L-tryptophan dose below 2.5 grams, every patient experienced return of their pain symptoms despite PAG stimulation. The average length of followup is 9.75 months with a range of three to 14. Conclusions: Tryptophan loading of the patient prior to PAG electrode placement, and the continued use of tryptophan postoperatively correlates with better outcomes and more significant pain reduction. Tryptophan probably increases serotonin levels and results in activation of anti-nociceptive pain pathways at the PAG.

L-TRYPTOPHAN USE AND THE EOSINOPHILIA-MYALGIA SYNDROME

We present a case of the eosinophilia-myalgia syndrome in a patient taking large doses of L-tryptophan for sedation. He developed the now-classic findings of myalgias, skin changes, and marked eosinophilia. This syndrome was first recognized in late 1989, and epidemiologic studies have demonstrated an association of the clinical syndrome with ingestion of L-tryptophan. The mechanism of toxicity remains unknown. Casual use of L-tryptophan has been diminished by its rapid removal from the nutrition counter of health food stores.

The effect of L-tryptophan on hemorrhagic shock induced bacterial translocation 1

Hemorrhagic shock causes mucosal damage in intestine and it results in translocation of bacteria to distant organs. In this study, effects of various doses of L-Tryptophan on the prevention of bacterial translocation in hemorrhagic shock induced rabbits were investigated. This study was carried out on six groups, each was consisting of 10 rabbits. While any procedure was conducted on the rabbits in group 1 (as a control group), 1 x 10(10)Escherichia coli isolate were administered rabbits in the other groups by gavage. In groups 3, 4, 5, and 6, hemorrhagic shock was induced. After induction of hemorrhagic shock, 10, 50, and 200 mg/kg L-Tryptophan were intragastrically administered to animals in groups 4, 5, and 6, respectively. Blood and terminal ileum samples were taken to detect bacterial translocation by polymerase chain reaction and mucosal damage by histopathological examination at 24 h after hemorrhagic shock. The occurrence of bacterial translocation increased as well when intestinal bacterial intensity was increased (P < 0.05). The most intensive bacterial translocation was formed in group 3 as a result of the additive effect of hemorrhagic shock to bacterial augmentation. It was observed that bacterial translocation was significantly reduced in groups 5 and 6 that are 50 and 200 mg/kg L-Tryptophan were administered (P < 0.01). Histopathological changes on mucosa and submucosa support these results. As a result, we concluded that augmentation of intestinal bacterial intensity induces bacterial translocation, the addition of hemorrhagic shock to bacterial augmentation makes more excessive translocation and mucosal changes have effective roles in these events. L-Tryptophan decreased the intestinal mucosal damage and bacterial translocation induced by hemorrhagic shock, in a dose-dependent manner.

Acute effects of L-tryptophan on tryptophan hydroxylation rate in brain regions (hypothalamus and medulla) of rainbow trout (Oncorhynchus mykiss).

Levels of 5-hydroxytryptophan (5-HTP) in brain regions (hypopthalamus and medulla) of rainbow trout were analysed by HPLC-EC 0, 10, 30, and 40 min after intraperitoneal administration of different doses of L-tryptophan (Trp) (0, 12.5, and 25 mg. kg(-1) body weight) in fish treated with 3-hydroxybenzylhydrazine (NSD1015; 75 mg. kg(-1)). The results show that, in control fish, 5-HTP levels in hypothalamus (58.03 +/- 6.36 pg. mg(-1) brain tissue) were significantly higher than those observed in medulla (28.04 +/- 4.32 pg. mg(-1) brain tissue). Basal tryptophan hydroxylation rates (after 0 mg. kg(-1) Trp administration) were 0.42 +/- 0.07 pg 5-HTP. mg(-1). min(-1), and 0.63 +/- 0.24 pg 5HTP. mg(-1). min(-1), for hypothalamus and medulla respectively. On the other hand, the results demonstrate that L-tryptophan administration induced significant increases in the rate of tryptophan hydroxylation, both in hypothalamus and medulla. These findings indicate that, in a way similar to that observed in mammals, brain tryptophan hydroxylase is unsaturated by its substrate (tryptophan) under normal physiological conditions. J. Exp. Zool. 286:131-135, 2000.

Eosinophilia-Myalgia Syndrome in Germany: An Epidemiologic Review

To review the epidemiologic features of eosinophilia-myalgia syndrome (EMS) in Germany. We determined the incidence of EMS in Germany through May 1992 and analyzed the dose of L-tryptophan used, the duration of intake, and the concurrent medications. All patients receiving L-tryptophan preparations in Germany were already under medical supervision before the onset of the disease; thus, information on patient history and other potential risk factors was readily available. Because of the drug status of L-tryptophan preparations, brands could be accurately traced back to suppliers of raw materials. Statistical differences in age and gender of patients, dose of L-tryptophan, eosinophil count, and skin involvement were sought between patients with and those without concurrent medications. On the basis of guidelines established by the Centers for Disease Control and Prevention, 105 patients fulfilled the criteria for EMS. No apparent correlation was found between the incidence of EMS and the dose of L-tryptophan or the duration of intake before onset of EMS. Assessment of the study group showed that 45% were taking various other medications, whereas 55% were taking L-tryptophan only. Analysis by type of pharmaceutical agent showed no preponderance of a specific concurrent drug (in particular, psychotropic drugs). Thus, concurrently used medications did not seem to be an important variable. All cases of EMS were associated with L-tryptophan from formulators that had used raw materials from the previously implicated source. This study supports the pathophysiologic role of a contaminant in L-tryptophan in the occurrence of cases of EMS in Germany.

Allelopathy of oats. II. Allelochemical effect ofl-Tryptophan and its concentration in oat root exudates

L-Tryptophan caused growth inhibition of roots and hypocotyls (or coleoptiles) of cockscomb (Amaranthus caudatus L.), lettuce (Lactuca sativa L.), cress (Lepidium sativum L.), timothy (Phleum pratense L.), rice (Oryza sativa L.), wheat (Triticum aestivum L.), and oat (Avena sativa L.), increasing the dose ofL-tryptophan increased the inhibition. The concentrations for 50% inhibition of the root growth were 0.14, 0.15, 0.21, 0.79, 0.95, 1.7, and 2.4 mM for cockscomb, cress, lettuce, timothy, rice, wheat, and oat, respectively; the concentrations for 40% inhibition of the hypocotyl (or coleoptile) growth were 0.28, 0.33, 0.43, 2.7, 4.5, 7.2, and 15 mM for cockscomb, cress, lettuce, timothy, rice, wheat and oat, respectively. The levels ofL-tryptophan in oat seedlings and in its root exudates were 29.3 mg/kg fresh wt and 0.25 mM under light conditions, and 21.1 mg/kg fresh wt and 0.18 mM under dark conditions, respectively. The presence ofL-tryptophan in the root exudates coupled with its effect on growth suggested thatL-tryptophan may play an important role in the growth inhibition of other plants in nature.

Changes in serotonin metabolism may elicit obstructive apnoea in the newborn rat.

1. Experiments were performed on anaesthetized newborn rats (aged 3-10 days) to know whether an increase in central serotonin levels might favour the occurrence of obstructive apnoeas as previously suggested by in vitro results from our group. 2. The levels of serotonin (5-HT), its precursor 5-hydroxytryptophan (5-HTP) and its metabolite, 5-hydroxyindole-acetic acid (5-HIAA), were analysed in cerebrospinal fluid samples collected at the level of the obex prior to and after intraperitoneal injection of L-tryptophan (50 mg kg-1) in sixty-eight anaesthetized newborn rats (control rats prior to injection and injected rats 15, 30 and 45 min after the injection). A significant increase in 5-HT and 5-HTP levels occurred 30 min after the injection, attesting to the activation of 5-HT biosynthesis. 3. The EMG activity of both the genioglossus and the diaphragm was recorded before and after L-tryptophan load (50 mg kg-1) in twenty-two newborn rats. After the injection of L-tryptophan, the amplitude of the integrated genioglossus activity decreased, or was even abolished, either during a few respiratory cycles or for long periods in twenty-one out of twenty-two animals. Recovery of the genioglossus activity occurred within 45-60 min. 4. The thoracic respiratory movements and the resulting upper airway pressure changes were recorded before and after L-tryptophan injection (50 mg kg-1) in sixty-two animals. In some litters (n = 7), most of the animals (21/25) displayed, within 20-40 min of the injection, recurrent episodes of obstructive apnoea often followed by central ones. These respiratory difficulties became severe and drastic, and led in two instances to respiratory distress and death. Lower doses of L-tryptophan (10 mg kg-1) did not induce any respiratory disorders unless these were potentiated by pargyline treatment (50 mg kg-1, n = 7). The obstructive apnoeas liable to occur after an L-tryptophan load (50 mg kg-1) were prevented by blocking the 5-HT receptor with methysergide (50 mg kg-1, n = 5) or by blocking the 5-HT biosynthesis by applying p-chlorophenylalanine (PCPA) pretreatment at birth (300 mg kg-1, n = 7). In other litters (n = 6), none of the eighteen newborn rats tested were affected by L-tryptophan, however, In five young adult rats, L-tryptophan again had no effect.4+ ĕ

Effect of Chronic Dietary Treatment with Z-Tryptophan on the Development of Cold-Induced Hypertension in Rats

This study was designed to assess the effect of chronic dietary administration (2.5 and 5.0% by weight) of the neutral amino acid, L-tryptophan, on the development of hypertension during chronic exposure to cold. In addition, a warm-adapted and cold-treated control group receiving unsupplemented food were used. Chronic administration of the lower dose of L-tryptophan (850 mg/day) prevented the elevation of blood pressure attenuated cardiac hypertrophy, and had no effect on body weight during exposure to cold. The higher dose of L-tryptophan (1,690 mg/day) attenuated the rate of blood pressure increase, did not affect cardiac hypertrophy, attenuated the gain in body weight, and increased the urinary output of epinephrine. Thus, this dose may be associated with some toxicity. Both doses of tryptophan failed to prevent certain other responses characteristically occurring during exposure to cold: i.e. increased weight of the kidneys, adrenal glands and brown adipose tissue; increased food and water consumption; increased dipsogenic responsiveness to angiotensin II, and increased plasma aldosterone concentration. The results indicate that chronic dietary administration of L-tryptophan (850 mg/day) can prevent the development of cold-induced hypertension, as it can in all other models of hypertension tested thus far in rats.

Hormonal and behavioral effects associated with intravenousl-tryptophan administration

Doses of 5.0, 7.5 and 10.0 g L-tryptophan, the amino acid precursor of serotonin, or saline alone were administered by IV infusion to a group of 11 healthy male subjects, and both hormonal and behavioral responses were monitored. Significant increases were observed in plasma concentrations of growth hormone and prolactin after all three doses of L-tryptophan, but not after saline infusion. No alterations in cortisol or thryotropin were noted at any level. Examination of behavioral effects of L-tryptophan revealed a dose-dependent impairment in performance on the symbol copying test. In addition, L-tryptophan produced significant effects on mental and physical sedation, but did not alter subjective ratings of tranquilization. In agreement with some prior reports, these observations support the ability of L-tryptophan, when administered IV in high doses, to produce pronounced effects on the central nervous system in humans, and suggest the potential utility of this paradigm as a neuroendocrine challenge test.

Audiogenic seizures and brain serotonin after l-tryptophan and p-chlorophenylalanine

Despite intense investigation, the role of serotonergic neurons in audiogenic seizures in mice remains uncertain. In the work reported here, audiogenic seizure susceptibility and brain tryptophan and serotonin concentrations were measured in DBA 2J mice after administration of three doses of l-tryptophan or p-chlorophenylalanine. p-Chlorophenylalanine reduced brain serotonin and significantly prolonged the latency to appearance of all seizure phases. l-tryptophan was largely ineffective in protecting against seizures and in elevating brain serotonin content, despite the fact that it caused a marked increase in brain tryptophan content. Thus, it appears that DBA 2J mice have an impaired ability to synthesize serotonin from tryptophan.

Tryptophan availability, central serotonergic function and methionine sulphoximine-induced convulsions

A convulsant dose (100 mg/kg) of l-methionine-RS-sulphoximine (MSO) produced temporally correlated decreases in central 5-hydroxytryptamine (5-HT) and plasma and brain tryptophan (TRY) concentrations in C57BL/6J mice. In contrast, a subconvulsant dose of MSO (50 mg/kg) had no effect on central 5-HT levels and only decreased brain tryptophan at one time-point after its administration. The pattern of findings suggests that the decrease in central 5-HT levels resulting from a convulsant dose of MSO is related to an impairment in 5-HT synthesis resulting from a restriction of tryptophan availability. Administration of 25 mg/kg and 100 mg/kg doses of l-tryptophan with convulsant doses of MSO significantly increased seizure latency. It is suggested that the central 5-HT system modulates the expression of MSO-induced seizures.

Tryptophan tolerance and metabolism in endogenous depression.

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.

Studies on the tryptophan load test in man.

Studies on the tryptophan load test were carried out by measuring the urinary excretion of 10 tryptophan metabolites via the kynurenine pathway in 25 healthy men and 20 women after oral L-tryptophan loading with dosages of 2 g. 50 mg/kg or 75 mg/kg body weight. With increasing doses of the amino acid, a difference was found between the male and female groups in the pattern of excretion of the tryptophan metabolites. In both groups, kynurenine and kynurenic acid were the metabolites excreted in the greatest amounts. The results indicate that the most useful loading dose of L-tryptophan for both men and women is 50 mg/kg body weight, as with 2 g L-tryptophan the excretion of the metabolites is too low and varies too much between the two groups of male and female subjects. On the other hand, the load with 75 mg/kg body weight can cause an overloading, particularly in women. No significant variations in the levels of the metabolites were found in the urine of two female subjects at different stages of the menstrual cycle after loading doses of 2 g and 50 mg/kg body weight. On the basis of the determination of the serum tryptophan levels, the period of urine collection was established as 10 h.

The effect of l-tryptophan administration on the concentration of probenecid in plasma and cerebrospinal fluid in patients

The administration of large doses of probenecid has been used to study the central nervous system metabolism of catecholamines and indoleamines in patients with affective disease. It has been reported that alterations of the binding of L-tryptophan to plasma albumin binding sites occur during probenecid administration. The present study sought to determine if the administration of large doses of L-tryptophan affected probenecid concentrations in cerebrospinal fluid and/or plasma. The data indicate that during L-tryptophan treatment, plasma probenecid concentrations are reduced but that no significant alterations in cerebrospinal fluid probenecid concentrations occur. This would suggest that the kinetics of the probenecid blockade of transport of acidic biogenic amine metabolites out of cerebrospinal fluid are not altered by L-tryptophan loading.

Studies on tryptophan-niacin metabolism in streptozotocin diabetic rats.

Streptozotocin diabetic male albino rats were observed to exhibit faulty conversion of tryptophan into niacin. Streptozotocin-injected rats, kept on niacin-deficient diet and given a dose of L-tryptophan (intraperitoneal or oral) excreted comparatively less quantities of niacin and N1-methylnicotinamide in urine than the control nondiabetic rats. Rats continued to show this effect for at least three months after streptozotocin injection. Dietary intake was found to have no influence on the excretory pattern. Streptozotocin diabetic rats were also observed to excrete greater amounts of xanthurenic acid and other tryptophan metabolites on tryptophan administration as compared to the nondiabetic rats.

Long-term effects of 4-chloroamphetamine on brain 5-hydroxyindole metabolism in rats

Four weeks after a single intraperitoneal injection of 4-chloroamphetamine hydrochloride (0·1 mmol/kg) into rats, brain serotonin and 5-hydroxyindoleacetic acid levels were still depressed to approximately half the control values, though tryptophan levels in brain were normal. Serotonin turnover in brain was likewise depressed to about half of the control rate as measured by (a) the rate of serotonin accumulation after monoamine oxidase inhibition and (b) the rate of 5-hydroxyindoleacetic acid accumulation after probenecid administration to block its efflux. Despite this decreased turnover rate, the capacity of the brain to synthesize 5-hydroxyindoles, as indicated by the increases in serotonin and 5-hydroxyindoleacetic acid after the injection of various doses of l-tryptophan, was not altered.

Cerebrospinal fluid 5-hydroxyindoles and behavior after l-tryptophan and pyridoxine administration to psychiatric patients

L-Tryptophan(2–8 g/day) and vitamin B 6 (100 mg/day) were administered for 1–3 weeks to 15 newly-admitted psychiatric inpatients who were diagnosed as depressive or schizophrenic and who had taken no other psychotropic medications for at least a week. Measurements for cerebrospinal fluid 5-hydroxyindoles, the Brief Psychiatric Rating Scale (BPRS), and sleep were made before and at the termination of the l-tryptophan regimen. For the patient group as a whole, increases in cerebrospinal fluid 5-hydroxyindoles correlated significantly with decreases in certain individual BPRS items, with the total sickness score, and with the total items score. In depressives, neither depressive symptoms nor insomnia were remarkably ameliorated by these doses of l-tryptophan and B 6. Schizophrenic patients as a group seemed more responsive to the l-tryptophan regimen than depressives.

THE TRYPTOPHAN LOAD AS A TEST FOR PYRIDOXINE DEFICIENCY IN HOSPITALIZED PATIENTS

Measurement of urinary excretion of xanthurenic acid after administration of an oral dose of tryptophan has been the technique most widely used for detection of subclinical pyridoxine deficiency. Although alterations in urinary excretion of this and other metabolites of tryptophan have been described in a wide variety of conditions and diseases,’ comparison of results obtained by different investigators has been difficult because of variability in dose and kind of tryptophan, urine collections, method of analysis and definitions of “abnormality.” Recently, as a result of the development of more accurate methods by Price and associates2 and the plea for standardization made by C o ~ r s i n , ~ greater agreement has been reached concerning proper methods of study. For the past ten years our laboratory has been concerned with the investigation of the prevalence of biochemical evidence of deficiency of several of the watersoluble vitamins in il l patients in our hospital. Several years ago we began to assess the relative “sensitivity” of several methods proposed for the detection of pyridoxine deficiency in normal adults placed upon a pyridoxine-deficient diet supplemented with vitamin-free casein. Studies included serial determinations of absolute lymphocyte count, serum and erythrocyte glutamic-oxalacetic transnminase and glutamic-pyruvic transaminase, taurine excretion after a cysteine load, estimation of phosphorylated and nonphosphorylated derivatives of pyridoxine in plasma, urinary excretion of 4-pyridoxic acid and 24-hour urinary excretion of xanthurenic acid and hydroxykynurenine after a 5 g dose of Ltryptophan. Afte’r the study of three patients, we arrived at the same conclusions as those in the then just published papers of Yess, Swan and co l leagi ie~~-~ that the earliest and most consistent abnormalities observed were increases in excretion of hydroxykynurenine and xanthurenic acid after a tryptophan load. At this time, Coursin made his plea for standardization of methods and for utilization of a 2 g dose of ~t ryptophan .~ For this reason and also because we had observed considerable interindividual variation in excretion of tryptophan metabolites in our normal subjects tested during control periods before and after the period of deficiency, a comparison of the 2 g and 5 g dose of L-tryptophan was made in sixteen healthy adults both prior to and following administration of pyridoxine.’ Urinary hydroxyanthranilic acid was also determined, since the ratio of excretion of hydroxyknurenine to that of hydroxyanthranilic acid has been proposed as a helpful adjunct in the assessment of pyridoxine deficiency.8 In order to clarify the rationale for this approach, FIGURE 1 presents a diagram of the metabolic pathway of tryptophan metabolism of importance in the tryptophan load test. Tryptophan pyrrolase, which catalyzes the first reaction in this pathway, is inducible by substrate (tryptophan) and by adrenocortical steroids. The vitamin B6-containing enzymes are not subject to adaptive change (enzyme induction),9 and therefore a relative or absolute decrease in activity of these enzymes could bring about alterations in tryptophan metabolism. Since the