Biochemistry of tryptophan in health and disease

Effects of alpha-lactalbumin on emotional processing in healthy women

The synthesis of serotonin (5-HT) in the central nervous system is dependent on the availability to the brain of its precursor amino acid, tryptophan (TRP). Recent studies suggest that alpha-lactalbumin, a whey-derived protein with a relatively high TRP content, increases plasma TRP and produces endocrine and cognitive changes consistent with facilitation of brain 5-HT function. In the present study we assessed the biochemical and cognitive effects of alpha-lactalbumin (40 g) in 28 healthy female subjects in a parallel group, placebo-controlled design. Relative to a casein-derived control protein, alpha-lactalbumin increased plasma TRP and the ratio of TRP to neutral amino acids. However, there was no effect on salivary cortisol secretion or tasks of emotional processing shown previously to be sensitive to pharmacological manipulation of 5-HT in healthy volunteers. The results suggest that alpha-lactalbumin produces a relatively modest increase in TRP availability which may not be sufficient to produce the changes in emotional processing seen with administration of pure TRP in healthy subjects. Further studies in subjects more vulnerable to stress are needed to assess the potential therapeutic effects of alpha-lactalbumin in clinical populations.

Tissue expression and biochemical characterization of human 2-amino 3-carboxymuconate 6-semialdehyde decarboxylase, a key enzyme in tryptophan catabolism

2-amino 3-carboxymuconate 6-semialdehyde decarboxylase (ACMSD, EC 4.1.1.45) plays a key role in tryptophan catabolism. By diverting 2-amino 3-carboxymuconate semialdehyde from quinolinate production, the enzyme regulates NAD biosynthesis from the amino acid, directly affecting quinolinate and picolinate formation. ACMSD is therefore an attractive therapeutic target for treating disorders associated with increased levels of tryptophan metabolites. Through an isoform-specific real-time PCR assay, the constitutive expression of two alternatively spliced ACMSD transcripts (ACMSD I and II) has been examined in human brain, liver and kidney. Both transcripts are present in kidney and liver, with highest expression occurring in kidney. In brain, no ACMSD II expression is detected, and ACMSD I is present at very low levels. Cloning of the two cDNAs in yeast expression vectors and production of the recombinant proteins, revealed that only ACMSD I is endowed with enzymatic activity. After purification to homogeneity, this enzyme was found to be a monomer, with a broad pH optimum ranging from 6.5 to 8.0, a K(m) of 6.5 microM, and a k(cat) of 1.0 s(-1). ACMSD I is inhibited by quinolinic acid, picolinic acid and kynurenic acid, and it is activated slightly by Fe(2+) and Co(2+). Site-directed mutagenesis experiments confirmed the catalytic role of residues, conserved in all ACMSDs so far characterized, which in the bacterial enzyme participate directly in the metallocofactor binding. Even so, the properties of the human enzyme differ significantly from those reported for the bacterial counterpart, suggesting that the metallocofactor is buried deep within the protein and not as accessible as it is in bacterial ACMSD.

Xanthurenic aciduria due to a mutation in KYNU encoding kynureninase

Massive urinary excretion of xanthurenic acid, 3-hydroxykynurenine and kynurenine, known as xanthurenic aciduria or hydroxykynureninuria, in a young Somali boy suggested kynureninase deficiency. Mutation analysis of KYNU encoding kynureninase of the index case revealed homozygosity for a c.593 A > G substitution leading to a threonine-to-alanine (T198A) shift. A younger brother was found to have a similar excretion pattern and the same genotype. At present, neither of the two boys has symptoms of niacin deficiency. This is the first report linking xanthurenic aciduria to a mutation in the gene encoding kynureninase.

Regulation of mouse hepatic alpha-amino-beta-carboxymuconate-epsilon-semialdehyde decarboxylase, a key enzyme in the tryptophan-nicotinamide adenine dinucleotide pathway, by hepatocyte nuclear factor 4alpha and peroxisome proliferator-activated receptor alpha

Nicotinamide adenine dinucleotide (NAD) plays a critical role in the maintenance of cellular energy homeostasis. alpha-Amino-beta-carboxymuconate-epsilon-semialdehyde decarboxylase (ACMSD) is the key enzyme regulating de novo synthesis of NAD from l-tryptophan (Trp), designated the Trp-NAD pathway. Acmsd gene expression was found to be under the control of both hepatocyte nuclear factor 4alpha (HNF4alpha) and peroxisome proliferator-activated receptor alpha (PPARalpha). Constitutive expression of ACMSD mRNA levels were governed by HNF4alpha and downregulated by activation of PPARalpha by the ligand Wy-14,643 ([4-chloro-6-(2,3-xylidino)-2-pyrimidinylthio]acetic acid]), as revealed by studies with hepatic HNF4alpha-null mice and PPARalpha-null mice, respectively. Transient transfection and electrophoretic mobility shift analyses showed an HNF4alpha binding site in the Acmsd gene promoter that directed transactivation of reporter gene constructs by HNF4alpha. The Acmsd promoter was not responsive to PPARalpha in transactivation assays. Wy-14,643 treatment decreased HNF4alpha protein levels in wild-type, but not PPARalpha-null, mouse livers, with no changes in HNF4alpha mRNA. These results show that Wy-14,643, through PPARalpha, post-transcriptionally down-regulates HNF4alpha protein levels, leading to reduced expression of the HNF4alpha target gene Acmsd.

Regulation of sexual behaviour in male macaques by sex steroid modulation of the serotonergic system

The view that androgen action is the primary impetus underlying male-typical behaviour has been irrevocably altered by the profound perturbations in social and sexual behaviour observed in recent models of oestrogen insufficiency in male mice. Evidence is also accumulating for an involvement of oestrogens in the modulation of neural systems that are thought to play important roles in male reproductive functioning. Specifically, the serotonergic system is implicated in diverse autonomic functions, most or all of which are sensitive to oestradiol as well. Although their interaction domains have yet to be examined in male primates, roles have been established for both oestrogen and serotonin in the regulation of male sexual behaviour. We used a blinded, sham-treated and self-controlled, randomized, multitreatment cross-over design to test the hypothesis that male sexual behaviour is regulated by oestrogen modulation of the serotonergic system in intact male Japanese macaques. Regression analysis revealed that oestradiol and whole blood tryptophan, but not testosterone or 5alpha-dihydrotestosterone, had additive, independent effects on male potentia over a range of hormone concentrations, whereas androgens were confirmed to be the primary determinants of sexual motivation. We suggest that modulation of the serotonergic system by 'female hormones' may be fundamental to the regulation of male mating success in higher primates. This might also explain, at least in part, why significant correlations between steroid hormones and male copulatory behaviour have traditionally proven so elusive in this order, thereby warranting a re-evaluation of the current notion that male sexual behaviour has been emancipated from activational hormonal control in higher primates.

Differential effects of dietary fatty acids on rat liver alpha-amino-beta-carboxymuconate-epsilon-semialdehyde decarboxylase activity and gene expression

Hepatic alpha-amino-beta-carboxymuconate-epsilon-semialdehyde decarboxylase (ACMSD; formerly termed picolinic carboxylase) [EC4.1.1.45] plays a key role in regulating NAD biosynthesis and the generation of quinolinate (quinolinic acid) from tryptophan. Quinolinate is a potent endogenous excitotoxin of neuronal cells. We previously reported that ingestion of fatty acids by rats leads to a decrease in their hepatic ACMSD activity. However, the mechanism of this phenomenon is not clarified. We previously purified ACMSD and cloned cDNA encoding rat ACMSD. Therefore, in this study, we examined the differential effect of fatty acids on ACMSD mRNA expression by Northern blot. Moreover, we measured quinolinic acid concentration in rats fed on fatty acid. When diets containing 2% level of fatty acid were given to male Sprague-Dawley rats (4 weeks old) for 8 days, long-chain saturated fatty acids and oleic acid did not affect ACMSD mRNA expression in the liver. Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) strongly suppressed the liver ACMSD mRNA expression. In rats fed with high linoleic acid diet for 8 days, serum quinolinic acid was significantly increased as compared with the rats fed on a fatty acid-free diet under the condition of the approximately same calorie ingestion. These results suggest that the transcription level of ACMSD is modulated by polyunsaturated fatty acids, and suppressive potency of ACMSD mRNA is n-3 fatty acid family>linoleic acid (n-6 fatty acid)>saturated fatty acid. Moreover, this study provides the information that a high polyunsaturated fatty acid diet affects the production of quinolinic acid in serum by suppressing the ACMSD activity.

Carcinoid--a comprehensive review

Carcinoid tumors originate from the neuroendocrine cells throughout the body and are capable of producing various peptides. Their clinical course is often indolent but can also be aggressive and resistant to therapy. We examined all aspects of carcinoid tumors including the molecular biology oncogenesis, role of angiogenesis, recent advances in imaging, and therapy. The Medline and Cancerlit databases were searched using carcinoid as the keyword. English language manuscripts were reviewed and relevant references from a total of 7741 were found. All titles were screened and all the relevant manuscripts were analyzed; we found 307 references pertinent to the history, epidemiology, clinical behavior, pathology, pathophysiology, molecular biology, radiologic imaging, supportive care of carcinoid syndrome, and results of therapeutic clinical trials. Management of patients with carcinoid tumors requires an understanding of the disease process and a multimodality approach. Introduction of long-acting somatostatin analogues has resulted in significant advances in the palliative care of patients with carcinoid syndrome. However, advanced carcinoid tumor remains incurable. Existing therapies for advanced disease have low biologic activity, high toxicity, or both. Clearly, more research is necessary in the areas of molecular biology, targeted therapy, and development of new drugs Future advances in this field need to focus on clinical and biological predictors of outcome. Early works in the area of tumor biology such as the role of p53, bcl-2, bax, MEN1, FGF TGF PDGF and VEGF expression are of interest and need to be explored further.

Glutaric aciduria type I and kynurenine pathway metabolites: a modified hypothesis

Glutaric aciduria type I is an inborn error of organic acid metabolism that demonstrates a particular temporal vulnerability (acute encephalopathic episodes in infancy) and a spatial vulnerability (acute striatal necrosis, focused on the putamen). Excitotoxic mechanisms involving 3-hydroxyglutaric acid as the major neurotoxin have been suggested. This paper proposes a role for metabolites of the kynurenine pathway in the pathogenic process and modifies the hypothesis of Heyes. Deficiency of glutaryl-CoA dehydrogenase blocking the glutarate pathway and activation of indoleamine 2,3-dioxygenase in macrophages/monocytes by intercurrent inflammation may increase flux down the kynurenine pathway towards the production of quinolinic acid. Quinolinic acid is neurotoxic and is an endogenous agonist at N-methyl-D-aspartate receptors. Synergistic excitation of these receptors by quinolinic acid and 3-hydroxyglutaric acid, which alone does not have sufficient potency, may be involved in the pathogenesis of striatal necrosis.

Neuropsychological investigation into the carcinoid syndrome

RATIONALE

In patients suffering from metastatic carcinoid tumors, chronic disturbances of serotonergic metabolism are frequently present. Serotonin is supposed to influence a range of cognitive functions.

OBJECTIVES

The present study evaluated the cognitive performance of carcinoid patients.

METHODS

In 14 patients with proven carcinoid syndrome, neuropsychological functioning was studied. Visual search, sustained attention, set shifting ability and spatial working memory were assessed using tests from the CANTAB neuropsychological battery. This was compared with the performance of matched healthy controls.

RESULTS

Plasma tryptophan levels were lower than controls. Patients showed an enhanced ability to learn new stimulus-response associations. Sustained visual attention, however, was impaired.

CONCLUSION

Cognitive patterns were different from those found in depressive patients and partly mimicked those found in tryptophan depletion experiments. Further investigation has to point out the role of serotonergic changes in the accomplishment of affective states.

Tryptophan as a link between psychopathology and somatic states

OBJECTIVE

Several somatic illnesses are associated with psychiatric comorbidity. Evidence is provided that availability of the essential amino acid tryptophan, which is the precursor of serotonin, may cause this phenomenon.

METHODS

We performed a database search to find relevant articles published between 1966 and 2002. For our search strategy, we combined several diseases from the categories hormonal, gastrointestinal, and inflammatory with the search terms "tryptophan" and "serotonin."

RESULTS

The catabolism of tryptophan is stimulated under the influence of stress, hormones and inflammation by the induction of the enzymes tryptophan pyrrolase (in the liver) and IDO (ubiquitous). Because of the reduction in blood levels of tryptophan under these circumstances the formation of cerebral serotonin is decreased.

CONCLUSIONS

It is argued that the coupling of peripheral tryptophan levels and cerebral serotonin levels has physiological significance. The clinical implications and therapeutic consequences of changes in tryptophan and consequently serotonin metabolism are discussed.

Tryptophan metabolism was accelerated by exercise in rat

We have already reported that exercise activates kynurenine pathway. But, the mechanism for this activation by exercise is still unclear. Kynurenine is metabolized to NAD, which is an essential factor for energy metabolism. In this study, exercise on treadmill was loaded to rats until all-out and tryptophan metabolites and tryptophan 2,3-dioxygenase (TDO) and indoleamine 2,3-dioxygenase (IDO) activities were determined in the blood, livers, and macrophages, respectively, in the exercise-loaded rats. The mean values of serum tryptophan concentration decreased from 92.6 +/- 6.0 nmol/ml to 52.4 +/- 10.2 nmol/ml (p<0.05) just after treadmill load. The serum kynurenine concentration had increased from 2.06 +/- 0.25 nmol/ml to 3.08 +/- 0.62 nmol/ml (p<0.005). And whole blood NAD concentration increased from 68.8 +/- 14.6 nmol/ml to 77.9 +/- 19.1 nmol/ml (p<0.005). These results showed that exercise activated the kynurenine pathway of tryptophan metabolism and made NAD which will be concerned with energy metabolism in mitochondria. Tryptophan-NAD pathway was initiated by cleavage of indole ring of tryptophan by TDO in the liver and IDO in many organs. We had also found that the exercise increase IDO activity of macrophages, but not TDO activity.

L-tryptophan administration promotes the reversion of pre-established chronic liver injury in rats treated with carbon tetrachloride

We examined the effect of L-tryptophan (Trp) administration on the reversion of CCl(4)-induced chronic liver injury after hepatotoxicant withdrawal in rats. When rats treated with CCl(4) twice a week for 6 weeks were released from CCl(4) treatment for 2 weeks, there was an incomplete reversion of liver injury. The reversion was enhanced by 2 weeks of daily intraperitoneal administration of Trp (50 mg/kg body weight), starting just after CCl(4) withdrawal. There were increases in the levels of thiobarbituric acid reactive substances, an index of lipid peroxidation, Ca(2+), triglycerides, and Trp, and decreases in tryptophan 2,3-dioxygenase activity and serum triglyceride concentrations in the liver of rats treated with CCl(4) for 6 weeks. Serum albumin concentrations and in vitro hepatic protein synthesis activity did not change in the CCl(4)-treated rats. The changes in the CCl(4)-treated rats were partially attenuated 2 weeks after CCl(4) withdrawal. The attenuation was enhanced by 2 weeks of daily Trp administration. The increases in hepatic thiobarbituric acid reactive substances and triglycerides and the decreases in hepatic tryptophan 2,3-dioxygenase activity and serum triglyceride concentrations observed 2 weeks after CCl(4) withdrawal were almost completely attenuated by Trp administration. In vitro hepatic protein synthesis in CCl(4)-treated and untreated rats was increased by 2 weeks of daily Trp administration. These results indicate that Trp administration promotes the reversion of pre-established chronic liver injury in rats treated with CCl(4,) and suggest that Trp exerts this effect by enhancing the improvement of several parameters of liver dysfunction associated with chronic liver injury and by stimulating hepatic protein synthesis.

Increased levels of 3-hydroxykynurenine in different brain regions of rats with chronic renal insufficiency

Tryptophan (TRP) metabolism via the kynurenine pathway leads to formations of neuroactive substances like kynurenine (KYN) and 3-hydroxykynurenine (3-HK), which may be involved in the pathogenesis of several human brain diseases. 3-Hydroxykynurenine especially is known to have strong neurotoxic properties. The generation of reactive oxygen species (ROS) leads to neuronal cell death with apoptotic features. Because the chronic renal insufficiency (CRI) results in disturbances in the functioning of the central nervous system (CNS), it is conceivable that the metabolism of some kynurenines may be altered and could play an important role in uremic encephalopathy. The levels of TRP, KYN and 3-HK were measured in the plasma and in different brain regions of uremic rats. The total plasma concentration of TRP as well as in all the studied brain samples was significantly diminished during uremia. Surprisingly, the level of KYN and 3-HK were elevated both in the plasma and different brain regions of CRI animals. KYN concentrations were approximately two times higher in the cerebellum, midbrain and cortex compared to the control group. The changes of 3-HK levels were more pronounced in the striatum and medulla than in other structures. This data suggests that CRI results in deep disturbances on the kynurenine pathway in CNS, which could be responsible for neurological abnormalities seen in uremia.

17Beta-estradiol protects against quinolinic acid-induced lipid peroxidation in the rat brain

The neurotoxin quinolinic acid has been identified as a causative agent in Huntington's disease and is a metabolite of the tryptophan pathway in the brain. In the present study, the in vivo and in vitro effect of 17beta-estradiol on lipid peroxidation induced by quinolinic acid was investigated. For the in vivo experiments ovariectomized female rats were administered with 100 microg 17beta-estradiol daily for seven days prior to and seven days following the intrahippocampal injection of 1 micromol quinolinic acid. The level of lipid peroxidation in brain homogenate was investigated using the thiobarbituric acid test. The in vitro experiments were performed in brain homogenates of ovariectomized female rats. The homogenate was treated with quinolinic acid alone or in combination with 17beta-estradiol. Quinolinic acid increased lipid peroxidation in a dose dependent manner in vitro, while homogenate co-treated with 17beta-estradiol showed a significant reduction in lipid peroxidation. 17Beta-estradiol was also shown to be protective against quinolinic acid in vivo. These results could explain the neuroprotective effect of 17beta-estradiol.

Mechanism of increases in L-kynurenine and quinolinic acid in renal insufficiency

Marked increases in metabolites of the L-tryptophan-kynurenine pathway, L-kynurenine and quinolinic acid (Quin), were observed in serum and cerebrospinal fluid (CSF) of both the rat and human with renal insufficiency. The mechanisms responsible for their accumulation after renal insufficiency were investigated. In patients with chronic renal insufficiency, elevated levels of serum L-kynurenine and Quin were reduced by hemodialysis. In renal-insufficient rats, Quin and L-kynurenine levels in serum, brain, and CSF were also increased parallel to the severity of renal insufficiency. Urinary excretion of Quin (3.5-fold) and L-kynurenine (2.8-fold) was also increased. Liver L-tryptophan 2,3-dioxygenase activity (TDO), a rate-limiting enzyme of the kynurenine pathway, was increased in proportion to blood urea nitrogen and creatinine levels. Kynurenine 3-hydroxylase and quinolinic acid phosphoribosyltransferase were unchanged, but the activities of kynureninase, 3-hydroxyanthranilate dioxygenase, and aminocarboxymuconate-semialdehyde decarboxylase (ACMSDase) were significantly decreased. Systemic administrations of pyrazinamide (ACMSDase inhibitor) increased serum Quin concentrations in control rats, demonstrating that changes in body ACMSDase activities in response to renal insufficiency are important factors for the determination of serum Quin concentrations. We hypothesize the following ideas: that increased serum L-kynurenine concentrations are mainly due to the increased TDO and decreased kynureninase activities in the liver and increased serum Quin concentrations are due to the decreased ACMSDase activities in the body after renal insufficiency. The accumulation of CSF L-kynurenine is caused by the entry of increased serum L-kynurenine, and the accumulation of CSF Quin is secondary to Quin from plasma and/or Quin precursor into the brain.

Changes in membrane fluidity induced by tryptophan and its metabolites

Incorporation of fatty acids into phospholipids as well as cholesterol and phospholipid concentration has been investigated using samples of rat liver homogenate, Krebs-Ringer-phosphate buffer (pH = 7.4), containing 0.3% albumin, fatty acid mixture and glycerol. The addition of kynurenine, kynurenic, xanthurenic, picolinic, and 5-hydroxyindoleacetic acids to incubation medium for phospholipid biosynthesis in vitro induced the elevation of cholesterol/phospholipid ratio, cholesterol concentration in samples, an increase of saturated and a decrease of polyunsaturated fatty acids, especially arachidonic acid incorporation into phospholipids. It allowed us to suggest that these metabolites of tryptophan can decrease the membrane fluidity, depress cell cycle, cell transformation and may stimulate cholesterol precipitation. The addition of tryptophan, 3-hydroxykynurenine, 3-hydroxyanthranilic, quinolinic, nicotinic acids, serotonin together with iproniasid, acetylserotonin, and melatonin to incubation medium for phospholipid biosynthesis in vitro induced an inverse relationship. Tryptophan and above mentioned metabolites decreased cholesterol/phospholipid ratio, cholesterol concentration in samples and incorporation of saturated fatty acids into phospholipids. The incorporation of polyunsaturated fatty acids, especially arachidonic acid increased. It allowed us to suggest that tryptophan and these metabolites, may increase membrane fluidity, stimulate cell cycle, cell transformation and can protect against cholesterol precipitation.

Kynurenine and neopterin in chronic glomerulonephritis

The results of our clinical observations of 102 patients with chronic glomerulonephritis with normal renal function have shown that hyperkynureninemia in 22.5% of patients develops in cases of pyridoxal-5-phosphate deficiency (hyperkynureninemia after peroral L-tryptophan load), but in 14.8% of patients through the stimulation of the cellular immune system (hyperkynureninemia at fasting state, increase of serum neopterin concentration). In all 20 patients with chronic renal failure hyperkynureninemia develops due to decreased renal function (increased serum kynurenine, neopterin and creatinine concentrations). Therefore, L-tryptophan peroral loading test with the determination of serum concentration of kynurenine before and at 3rd hour after the load, as well as the detection of serum concentration of neopterin and creatinine are helpful for the differentiation of following pathologies in patients with chronic glomerulonephritis: pyridoxal-5-phosphate deficiency, cellular immune stimulation and chronic renal failure.

Depletion of systemic macrophages by liposome-encapsulated clodronate attenuates increases in brain quinolinic acid during CNS-localized and systemic immune activation

Quinolinic acid is a neurotoxic tryptophan metabolite produced locally during immune activation. The present study tested the hypothesis that macrophages are an important source. In normal gerbils, the macrophage toxin liposome-encapsulated clodronate depleted blood monocytes and decreased quinolinic acid levels in liver (85%), duodenum (33%), and spleen (51%) but not serum or brain. In a model of CNS inflammation (an intrastriatal injection of 5 microg of lipopolysaccharide), striatal quinolinic acid levels were markedly elevated on day 4 after lipopolysaccharide in conjunction with infiltration with macrophages (lectin stain). Liposome-encapsulated clodronate given 1 day before intrastriatal lipopolysaccharide markedly reduced parenchymal macrophage invasion in response to lipopolysaccharide infusion and attenuated the increases in brain quinolinic acid (by 60%). A systemic injection of lipopolysaccharide (450 microg/kg) increased blood (by 38-fold), lung (34-fold), liver (23-fold), spleen (8-fold), and striatum (25-fold) quinolinic acid concentrations after 1 day. Liposome-encapsulated clodronate given 4 days before systemic lipopolysaccharide significantly attenuated the increases in quinolinic acid levels in blood (by 80%), liver (87%), spleen (80%), and striatum (68%) but had no effect on the increases in quinolinic acid levels in lung. These results are consistent with the hypothesis that macrophages are an important local source of quinolinic acid in brain and systemic tissues during immune activation.

Alleviation of carbon tetrachloride-induced chronic liver injury and related dysfunction by L-tryptophan in rats

We examined whether L-tryptophan (Trp) alleviates carbon tetrachloride (CCl4)-induced chronic liver injury and related dysfunction. Fifty rats were classified into four groups: the first (15 rats) served as the control; the second (10 rats) received subcutaneous injections of CCl4 (1.0 mL/kg) twice weekly for 8 weeks; the third (15 rats) received daily intraperitoneal injections of Trp (50 mg/kg) for 2 weeks after 6 weeks of CCl4 treatment; the fourth (10 rats) received both treatments. The activities of serum transaminases and the content of liver total hydroxyproline increased after 6 and 8 weeks of CCl4 treatment. The concentrations of serum albumin and liver protein and the in vitro activity of liver protein synthesis fell after 8 weeks of the treatment. Trp administration alleviated all these changes. At 6 and 8 weeks of CCl4 treatment the serum triglyceride concentration fell, whereas the liver triglyceride and lipid peroxide concentrations were elevated. Trp administration hardly affected these changes. These results indicate that Trp alleviates CCl4-induced chronic liver injury possibly by maintaining the activity of protein synthesis.

Isoniazid induced pellagra despite pyridoxine supplementation

Although pellagra is a recognized complication of isoniazid therapy, the diagnosis may be overlooked or delayed--sometimes with life-threatening consequences. We report a case of isoniazid-induced pellagra which occurred despite pyridoxine supplementation. Drug withdrawal and supplementation with niacin led to a rapid and sustained clinical improvement. The possible mechanisms of isoniazid induced pellagra are discussed.

Sources of the neurotoxin quinolinic acid in the brain of HIV-1-infected patients and retrovirus-infected macaques

This study investigated the sources of quinolinic acid, a neurotoxic tryptophan-kynurenine pathway metabolite, in the brain and blood of HIV-infected patients and retrovirus-infected macaques. In brain, quinolinic acid concentrations in HIV-infected patients were elevated by > 300-fold to concentrations that exceeded cerebrospinal fluid (CSF) by 8.9-fold. There were no significant correlations between elevated serum quinolinic acid levels with those in CSF and brain parenchyma. Because nonretrovirus-induced encephalitis confounds the interpretation of human postmortem data, rhesus macaques infected with retrovirus were used to examine the mechanisms of increased quinolinic acid accumulations and determine the relationships of quinolinic acid to encephalitits and systemic responses. The largest kynurenine pathway responses in brain were associated with encephalitis and were independent of systemic responses. CSF quinolinic acid levels were also elevated in all infected macaques, but particularly those with retrovirus-induced encephalitis. In contrast to the brain changes, there was no difference in any systemic measure between macaques with encephalitis vs. those without. Direct measures of the amount of quinolinic acid in brain derived from blood in a macaque with encephalitis showed that almost all quinolinic acid (>98%) was synthesized locally within the brain. These results demonstrate a role for induction of indoleamine-2,3dioxygenase in accelerating the local formation of quinolinic acid within the brain tissue, particularly in areas of encephalitis, rather than entry of quinolinic acid into the brain from the meninges or blood. Strategies to reduce QUIN production, targeted at intracerebral sites, are potential approaches to therapy.

Use of the somatostatin analogue octreotide acetate in the treatment of encephalopathy associated with carcinoid tumor. Case report

Carcinoid tumors may present in a variety of ways dependent on sites of disease and ectopic hormone secretion. This case report describes a patient having metastatic carcinoid tumor who developed encephalopathy of uncertain etiology. Treatment with the somatostatin analogue octreotide acetate resulted in dramatic improvement in his mental status. Several plausible mechanisms are discussed.

Different kynurenine pathway enzymes limit quinolinic acid formation by various human cell types

Substantial increases in the tryptophan-kynurenine pathway metabolites, l-kynurenine and the neurotoxin quinolinic acid, occur in human brain, blood and systemic tissues during immune activation. Studies in vitro have shown that not all human cells are capable of synthesizing quinolinate. To investigate further the mechanisms that limit l-kynurenine and quinolinate production, the activities of kynurenine pathway enzymes and the ability of different human cells to convert pathway intermediates into quinolinate were compared. Stimulation with interferon gamma substantially increased indoleamine 2,3-dioxygenase activity and L-kynurenine production in primary peripheral blood macrophages and fetal brains (astrocytes and neurons), as well as cell lines derived from macrophage/monocytes (THP-1), U373MG astrocytoma, SKHEP1 liver and lung (MRC-9). High activities of kynurenine 3-hydroxylase, kynureninase or 3-hydroxyanthranilate 3,4-dioxygenase were found in interferon-gamma-stimulated macrophages, THP-1 cells and SKHEP1 cells, and these cells made large amounts of quinolinate when supplied with L-tryptophan, L-kynurenine, 3-hydroxykynurenine or 3-hydroxyanthranilate. Quinolinate production by human fetal brain cultures and U373MG cells was restricted by the low activities of kynurenine 3-hydroxylase, kynureninase and 3-hydroxyanthranilate 3,4-dioxygenase, and only small amounts of quinolinate were synthesized when cultures were supplied with L-tryptophan or 3-hydroxyanthranilate. In MRC-9 cells, quinolinate was produced only from 3-hydroxykynurenine and 3-hydroxyanthranilate, consistent with their low kynurenine 3-hydroxylase activity. The results are consistent with the notion that indoleamine 2,3-dioxygenase is an important regulatory enzyme in the production of L-kynurenine and quinolinate. Kynurenine 3-hydroxylase and, in some cells, kynureninase and 3-hydroxyanthranilate 3,4-dioxygenase are important determinants of whether a cell can make quinolinate.

The kynurenine pathway and neurologic disease. Therapeutic strategies

The neurotoxic effects of QUIN have been well established. Clinical conditions have been identified where substantial elevations in CNS QUIN levels occur. There is a relationship between the severity of neurologic impairments and macrophage activation, with the magnitude of the increases in QUIN. The magnitude of QUIN increases in experimental immune activation, and macrophages in vitro, are highest in non-human primates, intermediate in gerbils and guinea pigs, and lowest in mice and rats. Macrophages in vitro are a useful screening system to evaluate potential inhibitors of the kynurenine pathway. Several models of CNS inflammation are available, including brain injury in post-ischemic gerbils and spinal cord injury in guinea pigs. 4-Chloro-3-hydroxyanthranilate is a potent inhibitor of QUIN production by macrophages and reduces QUIN accumulations in spinal cord injury. Such reductions are associated with significant neurologic improvements in the early post-injury period. The results support further investigation of QUIN as a mediator of neurologic dysfunction and damage in neurologic diseases.

Cerebrospinal fluid levels of kynurenine pathway metabolites in patients with eating disorders: relation to clinical and biochemical variable

In brain, most L-tryptophan is metabolized to indoleamines, whereas in systemic tissues L-tryptophan is catabolized to kynurenine pathway metabolites. Among these latter compounds are: quinolinic acid, an N-methyl-D-aspartate receptor agonist; kynurenic acid, an antagonist of excitatory amino acid receptors that also reduces quinolinic acid-mediated neurotoxicity; and L-kynurenine, a possible convulsant. Because the metabolism of L-tryptophan through the kynurenine pathway is dependent upon adequate nutrition, we sought to determine whether the impaired nutrition characteristic of eating-disordered patients might be associated with specific disturbances in this metabolic pathway. Cerebrospinal fluid levels of L-tryptophan, quinolinic acid, kynurenic acid, L-kynurenine, and 5-hydroxyindoleacetic acid were measured in medication-free female patients meeting DSM-III-R criteria for either anorexia nervosa (n = 10) or normal-weight bulimia nervosa (n = 22), studied at varying stages of nutritional recovery. Eight healthy, normal-weight females served as a comparison group. Cerebrospinal fluid levels of kynurenic acid were significantly reduced in underweight anorectics, compared to normal females, but returned to normal values with restoration of normal body weight. Although cerebrospinal fluid quinolinic acid levels were not different from controls, the ratio of quinolinic acid to kynurenic acid was significantly increased during the underweight phase of anorexia nervosa. Furthermore, in the eating-disordered patients, kynurenic acid levels in cerebrospinal fluid correlated positively with percent-of-population average body weight.(ABSTRACT TRUNCATED AT 250 WORDS)

6-Chloro-D,L-tryptophan, 4-chloro-3-hydroxyanthranilate and dexamethasone attenuate quinolinic acid accumulation in brain and blood following systemic immune activation

Accumulations of the neurotoxin quinolinic acid (QUIN) occur in the brain and blood following immune activation and are attributed to increased metabolism of L-tryptophan through the kynurenine pathway. Systemic administration of 4-chloro-3-hydroxyanthranilate (an inhibitor of 3-hydroxyanthranilate-3,4-dioxygenase), 6-chloro-D,L-tryptophan (a substrate of the kynurenine pathway) and dexamethasone (an anti-inflammatory agent) attenuated the accumulation of QUIN in the brain and blood following systemic pokeweed mitogen administration to mice. 6-Chloro-D,L-tryptophan and dexamethasone also attenuated the increases in brain and lung indoleamine-2,3-dioxygenase activity and elevations in plasma L-kynurenine levels. We conclude that QUIN formation can be modified by drugs which act at different levels of the cascade of events that link immune stimulation to increased kynurenine pathway metabolism.

Inflammation-associated changes in the cellular availability of tryptophan and kynurenine in renal transplant recipients

In the course of evaluating the hypothesis that tryptophan or tryptophan metabolites mediate some of the physiological or pathological aspects of the inflammatory response, we assessed the bioavailability of tryptophan and kynurenine in renal allograft recipients during periods of stable graft function, acute rejection and OKT3 therapy. In normal controls and patients with stable function, approximately 8% of the tryptophan and less than 5% of the kynurenine in serum were present in the freely diffusable form. The free tryptophan concentration was significantly increased during acute rejection, while free tryptophan as well as total and free kynurenine concentrations were significantly increased during OKT3 therapy. In each case the ratio of free indole to the sum of the plasma concentrations of large neutral amino acids was also increased. In vitro studies of indole binding to human serum proteins demonstrated the parallel displacement of bound tryptophan and kynurenine by physiological changes in pH, serum albumin concentration and free fatty acid concentration. The results suggest that inflammation associated increases in the oxidative metabolism of tryptophan are accompanied by the increased availability of serum indoles for intracellular metabolism in the tissues.

Effects of inhibition of serotonin synthesis on 5-hydroxyindoleacetic acid excretion, in healthy subjects

The urinary excretion of 5-hydroxyindoleacetic acid (5-HIAA), the main metabolite of serotonin, reflects the content and turnover of gastrointestinal (GI) serotonin. Employing longitudinal measurements of 5-HIAA, the authors investigated in healthy subjects (n = 43) how manipulations of serotonin synthesis affect GI serotonin. Under conditions of serotonin-free diets, the intersubject and intrasubject variability (coefficient of variation) for 5-HIAA excretion averaged 33% and 14%, respectively. Dietary tryptophan restrictions to 50% of minimal daily requirements (which is equivalent to a 10-fold reduction in baseline tryptophan intake) decreased by half the urinary excretion of 5-HIAA, irrespective of the caloric content of diet. Restoration to the regular tryptophan intake produced a rapid normalization of the 5-HIAA excretion. Neutral amino acids are known to compete with the intestinal transport absorption mechanisms of tryptophan. Administration of neutral amino acids (1.8 g, by mouth, three times a day, before each meal) or of carbidopa (50 mg, by mouth, three times a day for 3 days) to a normal tryptophan diet failed to alter significantly the 5-HIAA excretion. Further, neutral amino acids failed to enhance the reduction in 5-HIAA produced by the low-tryptophan diet. The failure of these treatments to reduce 5-HIAA excretion could be due to large capacity transport and decarboxylation systems for tryptophan. Other possibilities are discussed. In summary, dietary tryptophan is essential for the maintenance of GI serotonin. Reductions or increases in dietary tryptophan are the easiest and most effective method to alter GI serotonin.(ABSTRACT TRUNCATED AT 250 WORDS)

4-Chloro-3-hydroxyanthranilate, 6-chlorotryptophan and norharmane attenuate quinolinic acid formation by interferon-gamma-stimulated monocytes (THP-1 cells)

Accumulation of quinolinic acid and L-kynurenine occurs in the brain and/or blood following immune activation, and may derive from L-tryptophan following induction of indoleamine 2,3-dioxygenase and other kynurenine-pathway enzymes. In the present study a survey of various cell lines derived from either brain or systemic tissues showed that, while all cells examined responded to interferon-gamma by increased conversion of L-[13C6]tryptophan into L-kynurenine (human: B-lymphocytes, neuroblastoma, glioblastoma, lung, liver, kidney; rat brain: microglia, astrocytes and oligodendrocytes), only macrophage-derived cells (peripheral-blood mononuclear cells; THP-1, U-937) and certain liver cells (SKHep1) synthesized [13C6]quinolinic acid. Tumour necrosis factor-alpha enhanced the effects of interferon-gamma in THP-1 cells. Norharmane, 6-chloro-DL-tryptophan and 4-chloro-3-hydroxyanthranilate attenuated quinolinic acid formation by THP-1 cells with IC50 values of 51 microM, 58 microM and 0.11 microM respectively. Norharmane and 6-chloro-DL-tryptophan attenuated L-kynurenine formation with IC50 values of 43 microM and 51 microM respectively, whereas 4-chloro-3-hydroxyanthranilate had no effect on L-kynurenine accumulation. The reductions in L-kynurenine and quinolinic acid formation are consistent with the reports that norharmane is an inhibitor of indoleamine 2,3-dioxygenase, 6-chloro-DL-tryptophan is metabolized through the kynurenine pathway, and 4-chloro-3-hydroxyanthranilate is an inhibitor of 3-hydroxyanthranilate 3,4-dioxygenase. These results suggest that many tissues may contribute to the production of L-kynurenine following indoleamine 2,3-dioxygenase induction and immune activation. Quinolinic acid may be directly synthesized from L-tryptophan in both macrophages and certain types of liver cells, although uptake of quinolinic acid precursors from blood may contribute to quinolinic acid synthesis in cells that cannot convert L-kynurenine into quinolinic acid.

Mechanism of delayed increases in kynurenine pathway metabolism in damaged brain regions following transient cerebral ischemia

Delayed increases in the levels of an endogenous N-methyl-D-aspartate receptor agonist, quinolinic acid (QUIN), have been demonstrated following transient ischemia in the gerbil and were postulated to be secondary to induction of indoleamine-2,3-dioxygenase (IDO) and other enzymes of the L-tryptophan-kynurenine pathway. In the present study, proportional increases in IDO activity and QUIN concentrations were found 4 days after 10 min of cerebral ischemia, with both responses in hippocampus > striatum > cerebral cortex > thalamus. These increases paralleled the severity of local brain injury and inflammation. IDO activity and QUIN concentrations were unchanged in the cerebellum of postischemic gerbils, which is consistent with the preservation of blood flow and resultant absence of pathology in this region. Blood QUIN and L-kynurenine concentrations were not affected by ischemia. Brain tissue QUIN levels at 4 days postischemia exceeded blood concentrations, minimizing a role for breakdown of the blood-brain barrier. Marked increases in the activity of kynureninase, kynurenine 3-hydroxylase, and 3-hydroxyanthranilate-3,4-dioxygenase were also detected in hippocampus but not in cerebellum on day 4 of recirculation. In vivo synthesis of [13C6]QUIN was demonstrated, using mass spectrometry, in hippocampus but not in cerebellum of 4-day postischemic animals 1 h after intracisternal administration of L-[13C6]tryptophan. However, accumulation of QUIN was demonstrated in both cerebellum and hippocampus of control gerbils following an intracisternal injection of 3-hydroxyanthranilic acid, which verifies the availability of precursor to both regions when administered intracisternally. Notably, although IDO activity and QUIN concentrations were unchanged in the cerebellum of ischemic gerbils, both IDO activity and QUIN content were increased in cerebellum to approximately the same degree as in hippocampus, striatum, cerebral cortex, and thalamus 24 h after immune stimulation by systemic pokeweed mitogen administration, demonstrating that the cerebellum can increase IDO activity and QUIN content in response to immune activation. No changes in kynurenic acid concentrations in either hippocampus, cerebellum, or cerebrospinal fluid were observed in the postischemic gerbils compared with controls, in accordance with the unaffected activity of kynurenine aminotransferase activity. Collectively, these results support roles for IDO, kynureninase, kynurenine 3-hydroxylase, and 3-hydroxyanthranilate-3,4-dioxygenase in accelerating the conversion of L-tryptophan and other substrates to QUIN in damaged brain regions following transient cerebral ischemia. Immunocytochemical results demonstrated the presence of macrophage infiltrates in hippocampus and other brain regions that parallel the extent of these biochemical changes.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of immune activation on quinolinic acid and neuroactive kynurenines in the mouse

Accumulation of quinolinic acid and neuroactive kynurenines derived from tryptophan are of potential significance in human neuropathologic diseases because of their neurotoxic and convulsant properties. Clinical studies have established that sustained elevations of quinolinic acid, L-kynurenine and kynurenic acid within the cerebrospinal fluid occur in patients with a broad spectrum of inflammatory diseases and correlate with markers of immune activation and interferon-gamma activity. The present study describes an animal model that replicates these clinical observations and investigates the role of interferon-gamma as a mediator between immune activation and increased kynurenine pathway metabolism. Marked elevations in quinolinic acid, L-kynurenine and 3-hydroxykynurenine as well as an increased ratio of quinolinic acid: kynurenic acid in brain occurred 24 h after systemic pokeweed mitogen administration to C57BL6 mice. In plasma, L-tryptophan and kynurenic acid levels were reduced by pokeweed mitogen, while the concentrations of L-kynurenine, 3-hydroxykynurenine and quinolinic acid were increased. Interferon-gamma, pokeweed mitogen and lipopolysaccharide induced indoleamine-2,3-dioxygenase, the first enzyme of the kynurenine pathway, and increased both L-kynurenine and quinolinic acid concentrations of brain and systemic tissues, particularly in the lung, gastrointestinal tract and spleen. In contrast, hepatic tryptophan-2,3-dioxygenase activity was either reduced or unaffected. Increases in kynurenine pathway metabolism were sustained in mice given daily injections of interferon-gamma for seven days and subsequent responses to interferon-gamma were further enhanced. In contrast, daily administration of lipopolysaccharide was associated with subsequent attenuated responsiveness (tolerance) to lipopolysaccharide, pokeweed mitogen and interferon-gamma. Systemic administration of a monoclonal antibody to mouse interferon-gamma either attenuated or abolished the responses of kynurenine pathway metabolism to pokeweed mitogen and interferon-gamma. We conclude that acute and chronic increases in quinolinic acid and neuroactive kynurenines follow immune stimulation in mice, and result from indoleamine-2,3-dioxygenase induction. The results demonstrate that interferon-gamma is an important mediator between immune stimulation and indoleamine-2,3-dioxygenase induction. These increases in kynurenine pathway metabolism closely parallel the responses documented in patients with a broad spectrum of inflammatory diseases. Mice treated with immune stimuli are a useful model to investigate the relationships between immune activation and kynurenine pathway metabolism.

Oxidative tryptophan metabolism in renal allograft recipients: increased kynurenine synthesis is associated with inflammation and OKT3 therapy

Serum concentrations of tryptophan (TRP) and kynurenine (KYN) were determined in renal allograft recipients (RAR) as an index of interferon-gamma-induced, indoleamine-dioxygenase-catalysed TRP degradation. Serum TRP and KYN in RAR during periods of stable graft function were typically within the normal range, however, the median values for serum KYN demonstrated significant increases 5-7 days prior to biopsy-confirmed acute rejection (1.6-fold, P less than 0.01) and on the day of biopsy (1.7-fold, P less than 0.001). Serum KYN was also markedly elevated in patients who contracted viral or Gram-negative bacterial infections in the absence of graft rejection. Serum KYN was not correlated with serum creatinine in RAR nor were serum TRP or KYN affected by antirejection therapy with high dose steroids. Retrospective analysis of intra-patient changes in serum KYN demonstrated that KYN monitoring was a useful adjunct to serum creatinine in the early detection of first acute rejection episodes. The first course of OKT3 therapy was associated with low serum TRP and significant increases in serum KYN (two- to three-fold) following the first three doses. The time course of these abnormalities corresponded to that over which many of the side effects of the OKT3 'first dose reaction' have been reported to occur. Significant changes in serum KYN were not observed in patients receiving repeat courses of OKT3 therapy. Significant decreases in serum TRP and significant increases in serum KYN were both prevalent and frequent in RAR during the first two postoperative months.(ABSTRACT TRUNCATED AT 250 WORDS)

Postoperative pain treated by intravenous L-tryptophan: a double-blind study versus placebo in cholecystectomized patients

The effectiveness of intravenous administration (i.v.) of L-tryptophan, which is the precursor of cerebral serotonin, was verified in the treatment of postoperative pain. The study was carried out on 45 female patients, aged between 34 and 61 years, undergoing cholecystectomy who were randomly divided into three groups. Group 1 (age: 50.33 +/- 8.64 years) received 100 ml of 5% mannitol solution i.v.; group 2 (age: 49.80 +/- 11.11 years) 100 ml of a mannitol solution containing 7.5 mg/kg L-tryptophan; and group 3 (age: 53.46 +/- 9.60 years) 100 ml of a mannitol solution containing 15 mg/kg L-tryptophan. Vital capacity (preoperative VC) was measured before surgery. Anesthesia used was isoflurane. Narcotics or neuroleptics were not used. Pain was assessed before treatment (T-0 min), at the end of administration (T-30) and at T-60, 120, 180, 240, 300 and 360 min by the following variables: respiratory rate (RR), heart rate (HR), mean arterial pressure (MAP), Scott-Huskisson test (VAS), pain vital capacity (PVC), analgesic vital capacity (AVC), and respiratory restoration factor (RRF) calculated from Bromage's formula (RRF = (AVC - PVC/preoperative VC - PVC) X 100). As regards variables RR, HR, MAP and VAS, differences between the values from T-30 to T-360 and the value at T-0 were calculated. Means and S.E.M. were calculated on the obtained values and on RRF values for each group. The significance of the differences between groups was calculated using Student's t test and Bonferroni's test. Results show a significant decrease of pain in groups 2 and 3 treated with L-tryptophan, in comparison with group 1 (controls). No significant difference was observed between the treated groups, although more lasting pain relief was observed in group 3 in comparison with group 2. Intravenous L-tryptophan showed its effectiveness in the treatment of postoperative pain even when used alone. Its use may be considered for patients with renal failure, in order to strengthen pharmacological analgesia or to prevent postoperative pain by its intraoperative administration.

Free amino acids during chronic cyclosporine A toxicity in intact and partially nephrectomized rats

The effects of 40 days of treatment with Cyclosporine A (CSA) on plasma and urine free amino acids were investigated in sham-operated (C) and partially nephrectomized (Pnx) female Fischer 344 rats. High Dose CSA (30 mg/kg/day ip) was associated with reduced weight gain, increased plasma urea nitrogen, and hypoproteinemia in C and Pnx animals. These animals also demonstrated increased plasma levels of alanine, markedly reduced levels of tryptophan, and an increase in urinary excretion of methylhistidines. C but not Pnx animals also showed a significant increase in plasma serine and a decrease in plasma taurine. CSA treatment of group C resulted in a progressive aminoaciduria involving substrates of the neutral and acidic renal amino acid transport systems; however, the renal excretion of taurine and beta-alanine by these animals was markedly reduced as compared to vehicle treated controls. High dose CSA exacerbated aminoaciduria in Pnx animals, but in this group, the excretion of beta amino acids was also increased. Our findings demonstrate that chronic CSA toxicity in rodents with normal renal function is characterized by increased muscle protein catabolism, significant reductions in plasma tryptophan, and an apparent decrease in whole body taurine pools. With the exception of the taurine abnormalities. CSA treatment had similar effects on Pnx animals; however, in this group, CSA-induced pathological changes were superimposed on the changes due to renal insufficiency per se. CSA toxicity as identified by the parameters investigated in this study was no more severe in Pnx animals with moderate chronic renal insufficiency than in controls with intact renal function.

Chronic effects of gamma-interferon on quinolinic acid and indoleamine-2,3-dioxygenase in brain of C57BL6 mice

Chronic infections are associated with increased concentrations of the neuroactive kynurenine pathway metabolite, quinolinic acid (QUIN), in blood and cerebrospinal fluid. In the present study, repeated injections of gamma-interferon (5000 IU, every 3 days for 39 days) to C57BL6 mice were associated with persistent activation of indoleamine-2,3-dioxygenase (IDO), the first enzyme of the kynurenine pathway, in lung and brain, sustained increases in brain QUIN concentration and increases in plasma L-kynurenine and QUIN levels. Mice chronically treated with gamma-interferon offer an animal model to investigate the effects of sustained immune stimulation on kynurenine pathway metabolism.

Neurobiology of simian and feline immunodeficiency virus infections

Experimental and clinical evidence indicates that all lentiviruses of animals and humans are neurotropic and potentially neurovirulent. The prototypic animal lentiviruses, visna virus in sheep and caprine arthritis encephalitis virus in goats have been known for decades to induce neurologic disease. More recently, infection of the brain with the human immunodeficiency virus (HIV) has been linked to an associated encephalopathy and cognitive/motor complex. While the visna virus and caprine arthritis encephalitis virus are important models of neurologic disease they are not optimal for the study of HIV encephalitis because immune deficiency is only a minor component of the disease they induce. By contrast, the recently isolated lentiviruses from monkeys and cats, the simian and feline immunodeficiency viruses (SIV and FIV respectively), are profoundly immunosuppressive as well as neurotropic. SIV infection of the central nervous system of macaques now provides the best animal model for HIV infection of the human brain due to the close evolutionary relationship between monkeys and man, the genetic relatedness of their respective lentiviruses, and the similarities in the neuropathology. This chapter will compare and contrast the neurobiology of SIV and FIV with HIV.

Cerebral cortex and lung indoleamine-2,3-dioxygenase activity is increased in type-D retrovirus infected macaques

Increased concentrations of the neuroactive kynurenine pathway metabolites, quinolinic acid (QUIN) and kynurenic acid (KYNA), occur in the CSF of humans infected with the human immunodeficiency virus and macaques infected with retroviruses, including the D/1/California serotype. In the present study, increased activity of indoleamine-2,3-dioxygenase (IDO), the first enzyme of the kynurenine pathway, occurred in cerebral cortex and lung of macaques with clinical SAIDS. Such increases provide a mechanism to accelerate the formation of kynurenine pathway metabolites in both systemic tissues and the central nervous system.