Liver Tryptophan Pyrrolase Activity Research Articles

Dual Effect of Exogenous Progesterone on the Activity of Tyrosine Aminotransferase in the Liver of Female Rats

An as yet undescribed biphasic response of liver tyrosine aminotransferase (TAT) activity to a single administration of a microcrystallic watery suspension of 25 mg of progesterone (Agolutin Depot, SPOFA Praha) is described in adult female rats subjected to immobilization stress for 150 min. Exaggeration of the stress induced increase of TAT activity 3 and 8 h after hormone administration and its suppression 20 h after it was observed. The stress induced serum corticosterone increase is not correlated with the described changes, however, in non-stressed animals an increased TAT activity at 3 and 8 h after progesterone injection tightly follows the increased plasma corticosterone values. No statistically significant changes were found with respect to liver tryptophan pyrrolase activity.

Activation of Liver Tryptophan Pyrrolase Mediates the Decrease in Tryptophan Availability to the Brain after Acute Alcohol Consumption by Normal Subjects

We have previously suggested that acute ethanol consumption by normal subjects decreases the availability of circulating tryptophan (Trp) to the brain by activating liver Trp pyrrolase, the first and rate-limiting enzyme of the (major) kynurenine pathway of Trp degradation. The aim of the present study was to examine this hypothesis further by measuring plasma levels of kynurenine metabolites following alcohol consumption. After an overnight fast and a light breakfast, each of 10 healthy subjects received one of five drinks (placebo and doses of ethanol of 0.2, 0.4, 0.6 and 0.8 g/kg body weight in tonic water) on five different occasions. Blood samples were withdrawn 2 h later and plasma was analysed for concentrations Trp, competing amino acids (CAA) and kynurenine metabolites. Along with the depletion of plasma Trp and the decrease in its availability to the brain, as expressed by the ratio of [Trp]/[CAA], plasma kynurenine was elevated by doses of ethanol of 0.2-0.8 g/kg body weight. The ratio% of [kynurenine]/[Trp], an index of the expression of Trp pyrrolase activity, was also increased by all doses of ethanol. We conclude that activation of liver Trp pyrrolase mediates the depletion of plasma Trp and the decrease in its availability to the brain induced by acute ethanol consumption.

Inhibition of tryptophan - pyrrolase activity and elevation of brain tryptophan concentration by fluoxetine in rats.

To investigate in-vitro as well as in-vivo effects of various doses of fluoxetine (SSRI) on tryptophan metabolism in rats. A pre-clinical study. Clinical Biochemistry Research Laboratory, Department of Biochemistry, University of Karachi. The investigation was carried out during 2000 to 2001. Male Wistar rats (150-200 g body wt) were selected and divided into control and test groups (n = 5) for comparison. In in-vitro (10 - 1000mM) as well in-vivo (0.5-30 mg/kg body wt.) studies, fluoxetine showed a statistically significant inhibition of rat liver tryptophan pyrrolase (tryptophan-2,3-dioxygenase; EC 1.13.11.11) activity. Significant increases were noted at 10 and 30 mg/kg doses in brain, serum (total and free) and liver L-tryptophan concentrations. Similarly, serum non-estrified free fatty acids showed a significant increase at both doses. There was no effect on serum glucose and albumin concentrations. It is suggested that major mechanism of action of fluoxetine is that of elevating brain tryptophan concentration and hence 5-HT synthesis by increasing the availability of circulating tryptophan to the brain secondarily to inhibition of major tryptophan degrading enzyme, hepatic tryptophan pyrrolase. It is assumed that fluoxetine inhibits the binding of apoenzyme form of tryptophan pyrrolase with its cofactor haem. The results are discussed in relation to possible involvement of disturbed hepatic tryptophan metabolism in depressive illness.

Rat liver tryptophan pyrrolase activity and gene expression during alcohol withdrawal.

Rat liver tryptophan (Trp) pyrrolase activity and gene expression were studied in relation to the alcohol-withdrawal syndrome (AWS). Both activity and gene expression were enhanced after withdrawal of ethanol-containing liquid diets and the time-course of these changes mirrored that of development and intensity of the behavioural disturbances of the AWS. By contrast, no correlation was observed between the AWS-induced behaviour and changes in activity of another hepatic glucocorticoid-inducible enzyme, tyrosine aminotransferase, and a negative correlation was noted between behaviour and the gene expression of this latter enzyme and also of that of the hepatic glucocorticoid receptor. We suggest that the metabolic consequences of activation of liver Trp pyrrolase during alcohol withdrawal may play a role in the behavioural features of the AWS.

Body weight loss and changes in tryptophan homeostasis by chlorinated dibenzo-p-dioxin congeners in the most TCDD-susceptible and the most TCDD-resistant rat strain.

We compared the effects of 2,3,7,8-tetra (TCDD), 1,2,3,7,8-penta (PeCDD), 1,2,3,4,7,8-hexa (HxCDD) and 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin (HpCDD) on brain serotonin metabolism, plasma tryptophan and liver tryptophan pyrrolase activity in two rat strains, TCDD-sensitive Long-Evans (Turku AB; L-E) and TCDD-resistant Han/Wistar (Kuopio; H/W). Previously it was shown that L-E rats exhibit the expected rank order of potency for CDDs in terms of acute toxicity with TCDD being the most potent, followed by PeCDD, HxCDD and HpCDD. In contrast, to H/W rats HxCDD was the most toxic and TCDD the least toxic of these congeners. In the present study, the CDDs decreased body weight in L-E rats in the following order of potency: TCDD > PeCDD > HxCDD > HpCDD. The same rank order was recorded for elevations in brain tryptophan and plasma free tryptophan concentrations as well as for inhibition of the main hepatic tryptophan metabolizing enzyme, tryptophan pyrrolase. By contrast, in H/W rats HxCDD was the most effective congener in producing loss of body weight, followed by HpCDD, PeCDD and TCDD. This was also true of changes in tryptophan homeostasis. These findings imply that in TCDD-susceptible L-E and TCDD-resistant H/W rats the potency of dioxin congeners in inducing acute toxicity highly correlates with their ability to disrupt tryptophan homeostasis. However, there may not be a direct causal relationship between body weight loss and altered tryptophan homeostasis, because the magnitudes of these two phenomena were not consistently parallel across the dioxin congeners tested.

PREVENTION BY CYCLOHEXIMIDE OF THE AUDIOGENIC SEIZURES AND TRYPTOPHAN METABOLIC DISTURBANCES OF ETHANOL WITHDRAWAL IN RATS

Cycloheximide (20 mg/kg body wt, given intraperitoneally at-1 and 3 h after withdrawal of an ethanol-containing liquid diet) prevents the activation of liver tryptophan pyrrolase, the consequent inhibition of synthesis of brain 5-hydroxytryptamine, and the audiogenic seizures observed at 7 h after alcohol withdrawal. We suggest that a rapidly-turning-over protein mediates the alcohol withdrawal syndrome and discuss the possible role of liver tryptophan pyrrolase.

Increased delta aminolevulinic acid and decreased pineal melatonin production. A common event in acute porphyria studies in the rat.

Tryptophan (TRP) is the precursor of melatonin, the primary secretory product of the pineal gland. Hepatic heme deficiency decreases the activity of liver tryptophan pyrrolase, leading to increased plasma TRP and serotonin. As a paradox, patients with attacks of acute intermittent porphyria (AIP), exhibit low nocturnal plasma melatonin levels. This study using a rat experimental model was designed to produce a pattern of TRP and melatonin production similar to that in AIP patients. Pineal melatonin production was measured in response to: (a) a heme synthesis inhibitor, succinylacetone, (b) a heme precursor, delta-aminolevulinic acid (Ala), (c) a structural analogue of Ala, gamma-aminobutyric acid. Studies were performed in intact rats, perifused pineal glands, and pinealocyte cultures. Ala, succinylacetone, and gamma-aminobutyric acid significantly decreased plasma melatonin levels independently of blood TRP concentration. In the pineal gland, the key enzyme activities of melatonin synthesis were unchanged for hydroxyindole-O-methyltransferase and decreased for N-acetyltransferase. Our results strongly suggest that Ala overproduced by the liver acts by mimicking the effect of gamma-aminobutyric acid on pineal melatonin in AIP. They also support the view that Ala acts as a toxic element in the pathophysiology of AIP.

The Relationship Between Tryptophan Pyrrolase Activity and mRNA Expression in Rats Following Chronic Ethanol Administration and Withdrawal

Ethanol exerts a variety of effects on tryptophan and 5-hydroxytryptamine (5-HT) metabolism in man and in experimental animals. In rat models. whose tryptophan metabolism is nearest to that of man, chronic ethanol administration enhances brain 5-HT synthesis whereas inhibition of synthesis occurs during the subsequent withdrawal period. Both effects are caused by corresponding changes in tryptophan availability to the brain and are thought to be due to inhibition and enhancement of liver tryptophan pyrrolase activity respectively. It has been suggested that ethanol acts chronically by inhibiting apo-tryptophan pyrrolase synthesis. We have studied the relationship between tryptophan pyrrolase activity and the expression of hepatic tryptophan pyrrolase mRNA in rats chronically administered ethanol by means of an ethanol-containing liquid diet. Wistar rats were fed ethanol diet at concentrations of 5% and 8% (v/v) and for duration's of two and four weeks. Rats were withdrawn in a starved state for seven hours by which time there were demonstrable signs of withdrawal. Matched controls were fed equal amounts of isocaloric ethanol-free liquid diet. The livers were removed from each of the animals and both tryptophan pyrrolase activity and synthesis were measured. Tryptophan pyrrolase activity was decreased before withdrawal and greatly increased over baseline following withdrawal.

Activity and Partial Purification of Liver Tryptophan Pyrrolase in Rainbow Trout.

Compounds that activate tryptophan pyrrolase in other animals like methylene blue, hematin, EDTA, and calciumion did not affect fish enzyme activity. Neutralized ascorbic acid showed an inhibitory effect. Time-activity and enzyme concentration-activity curves were determined in three methods of assay. Km was estimated via the Lineweaver-Burk plot to be 80mM. The enzyme was partially purified by a four-step process: solubilization, DEAE Sephadex A-50 chromatography (batch method), and by two consecutive DEAE Sepharose CL-6B chromatographs. The partially purified enzyme was highly unstable so that no characterization was at all possible.

The effects of lofepramine and desmethylimipramine on tryptophan metabolism and disposition in the rat

Acute and chronic administration of lofepramine and its major metabolite desmethylimipramine (DMI) to rats elevates brain tryptophan concentration, thereby enhancing cerebral 5-hydroxytryptamine (5-HT) synthesis, by increasing the availability of circulating tryptophan to the brain, secondarily to inhibition of liver tryptophan pyrrolase (tryptophan 2,3-dioxygenase, l-tryptophan:O 2 oxidoreductase, decyclizing; EC 1.13.11.11) activity. The pyrrolase inhibition by lofepramine occurs independently of metabolism to DMI, because it can be demonstrated directly in vitro. Lofepramine also differs from DMI in its action profile on the above and related aspects of tryptophan metabolism and disposition. These results demonstrate that lofepramine influences tryptophan and 5-HT metabolism and disposition independently of its major metabolite DMI, and are discussed briefly in relation to the mechanism of action of antidepressants.

Effects of acute paroxetine administration on tryptophan metabolism and disposition in the rat

1 The effects of acute oral administration of paroxetine on tryptophan metabolism and disposition were examined in the rat. 2 Basal liver tryptophan pyrrolase activity was inhibited by paroxetine in vitro and after oral administration. Maximum inhibition was caused by a 1 mg kg-1 dose. 3 Paroxetine administration also inhibited pyrrolase activity that had previously been enhanced by hormonal induction by cortisol or cofactor activation by haematin. The cortisol induction of the enzyme was, however, not inhibited by pretreatment of rats with paroxetine. 4 Paroxetine increased tryptophan availability to the brain, because of the above pyrrolase-inhibitory mechanism. Cerebral 5-hydroxytryptamine (5-HT) synthesis was accordingly enhanced, though this was apparent only with doses of the drug of up to 1 mg kg-1. With larger doses, decreased 5-HT turnover, probably as a result of 5-HT uptake inhibition, was the more dominant feature. 5 Paroxetine lowered circulating corticosterone concentration, but did not influence those of albumin, non-esterified fatty acids or glucose. 6 It is concluded that, in addition to inhibiting brain 5-HT turnover, paroxetine also, in common with 20 other antidepressants, enhances 5-HT synthesis by increasing brain tryptophan concentration secondarily to inhibition of liver tryptophan pyrrolase activity.

Kinetic properties of human liver tryptophan pyrrolase.

Human liver tryptophan pyrrolase (TPO) activity exhibited substrate level regulation. TPO showed biphasic activity to tryptophan when low ascorbate was used as an activator. The high affinity form (Km for tryptophan: 0.05 mM) was promoted by low ascorbate and low tryptophan. The low affinity form (Km for tryptophan: 0.4 mM) was induced by high concentrations of tryptophan or ascorbate. Both high and low affinity forms showed the same affinity to oxygen. The high affinity form was also induced by pyrroloquinoline quinone, but this effect was decreased by catalase, suggesting the participation of H2O2.

In vivo evidence that insulin does not inhibit hepatic tryptophan pyrrolase activity in rats

Previous reports have indicated that insulin administration triggers an early increase in plasma tryptophan (TRP) levels in fasted rats. Then, the present study was undertaken to investigate the putative role of liver tryptophan pyrrolase (TPO) in this short-term effect of insulin. In 24 hr fasted rats, doses of insulin that triggered an increase in plasma TRP levels (i.e., 2–3 I.U./kg, 1 hr) did not alter either holoenzyme or total enzyme activity. In another series of experiments, the administration of insulin (2 I.U./kg) to 24 hr fasted rats promoted biphasic time effects on plasma TRP levels and liver TPO activity. Thus, insulin initially triggered a rise in plasma TRP (without any change in liver TPO activity) and then increased liver TPO activity whilst plasma TRP returned toward control levels. In addition, hypercorticosteronemia was evidenced throughout the first phasis. Lastly, the influence of insulin administration (2 I.U./kg) on fasting-induced TPO induction was analysed. Whereas fasting increased liver TPO activity in a time-dependent manner, insulin administration (2 I.U./kg, 30 min) did not modify either plasma TRP or liver TPO activity. The data reported herein bring evidence that the effect of insulin administration on circulating TRP is not mediated by an inhibition of hepatic TPO.

Liver tryptophan pyrrolase. A major determinant of the lower brain 5-hydroxytryptamine concentration in alcohol-preferring C57BL mice

The lower brain 5-hydroxytryptamine concentration in alcohol-preferring C57BL, compared with -non-preferring CBA, mice is caused by a decrease in circulating tryptophan availability to the brain secondarily to a higher liver tryptophan pyrrolase activity associated with a higher circulating corticosterone concentration. Activity or expression of liver tryptophan pyrrolase and/or their induction by glucocorticoids may be important biological determinants of predisposition to alcohol consumption.

Depletion of liver regulatory heme in benzene exposed rats

The effect of a single dose of benzene (0.5 ml/kg body wt i.p.) on the heme saturation of tryptophan pyrrolase activity in liver was examined. There was a significant decrease in the heme saturation of hepatic tryptophan pyrrolase, suggesting depletion of “regulatory heme”. After benzene administration there was significant increase in σ-aminolevulinate (ALA) synthetase activity (approx. 2-fold) while σ-aminolevulinate dehydratase activity was significantly decreased, however, ferrochelatase and heme oxygenase activities were unaltered. Administration of tryptophan to benzene pretreated rats showed a reversal of benzene effects on heme synthesizing enzymes: there is an increase in the heme saturation of tryptophan pyrrolase and decrease in σ-aminolevulinate synthetase. However, there was no significant alteration in the activity of σ-aminolevulinate dehydratase.

Inhibition of rat Liver Tryptophan Pyrrolase Activity and Elevation of Brain Tryptophan Concentration by Administration of DL-α-Amino-β-Pyridinepropanoic Acid (Pyridylalanine) Analogs

Single doses of DL-alpha-amino-beta-(2-pyridine)propanoic acid (2-PA, 100 mg/kg) significantly decreased the holoenzyme and apoenzyme activities of rat liver tryptophan pyrrolase (TP) and increased brain tryptophan, serotonin (5-HT) and 5-hydroxyindole-3-ylacetic acid concentrations. 2-PA had no inhibitory effect on either of the enzyme activities in vitro, but its expected metabolites were effective. Single doses of DL-alpha-amino-beta-(3-pyridine)propanoic acid (3-PA, 100 mg/kg) decreased only the holoenzyme activity and elevated brain tryptophan and its metabolites levels in rats. 3-PA and its metabolite, 3-pyridylpyruvate, inhibited only the holoenzyme activity in vitro. DL-alpha-Amino-beta-(4-pyridine)propanoic acid (4-PA) caused significant changes in liver TP (holo- and apoenzyme forms) activity and brain tryptophan concentration only after repeated administration (100 mg/kg/day). 4-PA was a weak inhibitor of the holoenzyme, but its metabolites apparently inhibited the holo- and apoenzyme activities in vitro. These findings suggest that PA analogs (and/or their metabolites) increased brain tryptophan (and hence 5-HT synthesis) by directly inhibiting liver TP activity.

Effect of combined administration of tryptophan with putative tryptophan pyrrolase inhibitors, DL-3-pyridylalanine, allopurinol or nicotinamide, on brain serotonin concentration.

1) Single administration of tryptophan (25-200 mg / kg) to Wistar male rats resulted in a dose-related increase in liver tryptophan pyrrolase (TP) activity. The increase in brain serotonin (5-HT) concentration was not proportional to the dose given, and brain 5-HT concentration rapidly fell to the control (saline-treated) level or below. 2) The tryptophan-induced increase in liver TP activity was clearly prevented by DL-3-pyridylalanine (3-PA, 100 mg/kg). Serum free tryptophan and brain 5-HT concentrations increased more markedly and maintained higher levels after the combined administration of tryptophan with 3-PA than after a single administration of tryptophan. 3) Although allopurinol (20 mg/kg) reduced liver TP activity under control and tryptophan-treated conditions, it did not increase serum free tryptophan and brain 5-HT concentrations. 4) Nicotinamide also failed to alter the catabolism of tryptophan and the concentration of brain 5-HT in control or tryptophan-treated rats. 5) These data suggest that 3-PA would increase the therapeutic effect of tryptophan given to treat depressive illness, but that allopurinol and nicotinamide would not be suitable drugs for this purpose.

Does naloxone always act as an opiate antagonist?

Evidence for the ability of the opiate antagonist naloxone to block a variety of metabolic effects exerted by morphine and non-opiate drugs is reviewed. Naloxone prevents or reverses the following effects in the rat: (a) the chronic morphine-induced increase in liver [NADPH]; (b) the consequent chronic morphine-induced inhibition of liver tryptophan pyrrolase activity; (c) the resultant chronic morphine-induced enhancement of brain 5-hydroxytryptamine synthesis; (d) the similar effects on liver and brain tryptophan metabolism exerted chronically by other drugs of dependence (ethanol, nicotine and phenobarbitone); (e) the acute ethanol-induced increase in the hepatic [NADH]/[NAD] ratio. Naloxone also (f) inhibits basal and stimulated lipolysis in fed and 24hr-starved rats. This leads to prevention of (g) the consequent increase in the availability of circulating free tryptophan, and (h) the resultant tryptophan-mediated decrease in liver 5-aminolaevulinate synthase activity. The question of how many of these effects involve changes in endogenous opiates or at opiate receptors is not clearly understood at present and thus merits investigation. However, because most of the above effects are explained on biochemical grounds, and in view of evidence from behavioural and pharmacological studies [see (1)], the possibility must be considered that many of the actions of naloxone may be unrelated to its opiate-receptor-antagonistic properties.

Tryptophan pyrrolase in haem regulation. The mechanisms of enhancement of rat liver 5-aminolaevulinate synthase activity by starvation and of the glucose effect on induction of the enzyme by 2-allyl-2-isopropylacetamide.

1. Rat liver tryptophan pyrrolase activity is enhanced by a hormonal-type mechanism during the first 2 days of starvation and by a substrate-type mechanism during the subsequent 2 days. 5-Aminolaevulinate synthase activity is also enhanced during the first 2 days of starvation, but returns thereafter to values resembling those observed in the fed rat. Treatments that prevent or reversé the enhancement of tryptophan pyrrolase activity in 24-48h-starved rats also abolish that of 5-aminolaevulinate synthase activity. Starvation of guinea pigs, which does not enhance the pyrrolase activity, also fails to alter that of the synthase. It is suggested that the decrease in 5-aminolaevulinate synthase activity in 72-96h-starved rats represents negative-feedback repression of synthesis, possibly involving tryptophan participation, whereas the enhancement observed in 24-48h-starved animals is caused by positive-feedback induction secondarily to increased utilization of the regulatory-haem pool by the newly synthesized apo-(tryptophan pyrrolase). 2. Glucose, fructose and sucrose abolish the 24h-starvation-induced increases in rat liver tryptophan pyrrolase and 5-aminolaevulinate synthase activities. Cortisol reverses the glucose effect on 5-aminolaevulinate synthase activity, presumably by enabling pyrrolase to re-utilize the regulatory-haem pool after induction of synthesis of this latter enzyme. 3. The impaired ability of 2-allyl-2-isopropylacetamide to enhance markedly 5-aminolaevulinate synthase activity in 24h-starved rats treated with glucose is associated with a failure of the porphyrogen to cause loss of tryptophan pyrrolase haem. Cortisol restores the ability of the porphyrogen to destroy tryptophan pyrrolase haem and to enhance markedly 5-aminolaevulinate synthase activity, presumably by enhancing tryptophan pyrrolase synthesis and, thereby, its re-utilization of the regulatory-haem pool. It is tentatively suggested that 2-allyl-2-isopropylacetamide destroys the above pool only after it has become bound to (or utilized by) apo-(tryptophan pyrrolase).

Inhibition of rat liver tryptophan pyrrolase activity and elevation of brain tryptophan concentration by acute administration of small doses of antidepressants.

1 Administration to rats of a 0.5 mg/kg dose of any of 19 antidepressants, but not that of many other drugs, causes a significant inhibition of the total enzyme and apoenzyme activities of liver tryptophan pyrrolase (of 24-48% and 37-65% respectively) and elevates brain tryptophan concentration by 13-66%. 2 When liver tryptophan pyrrolase activity is enhanced by pretreatment with cortisol or haematin, subsequent administration of a 0.5 mg/kg dose of some, but not other, antidepressants causes inhibition, which is weak (up to 38%). 3 This weak inhibition of the enhanced pyrrolase activity together with other pharmacological and physiological factors could explain the time lag between the start of antidepressant medication and the occurrence of a therapeutic response. 4 The cortisol-induced and haematin-activated pyrrolases respond differentially to inhibition by imipramine and amitriptyline, and this may explain the differential response to these two drugs of depressed patients in relation to urinary excretion of the noradrenaline metabolite 3-methoxy-4-hydroxyphenylglycol. 5 The results are discussed in relation to the mechanism of action of antidepressants and the possible involvement of disturbed hepatic tryptophan metabolism in depressive illness.