The effect of pyridoxine deficiency on the metabolism of the aromatic amino acids by isolated rat liver cells

The total activity of three key enzymes and the flux through eight steps of aromatic amino acid metabolism have been determined in liver cells isolated from rats fed either control or pyridoxine-free diet for 5-6 weeks. The pyridoxine-free diet caused a decrease in the catabolism of tyrosine and phenylalanine because of a drop in the flux through tyrosine aminotransferase. This decrease of expressed cellular tyrosine aminotransferase activity can be fully explained in terms of loss of cofactor. Larger decreases in the catabolism of tryptophan were seen after pyridoxine deprivation. The decreased extent of tryptophan catabolism can be solely attributed to loss of cofactor or increased degradation of kynureninase. Inhibition of tryptophan 2,3-dioxygenase was seen in pyridoxine deficiency, probably because of the buildup of the kynurenine metabolites. The control strength of kynureninase, for flux through kynureninase, was calculated to be less than or equal to 0.004, but 0.41 after pyridoxine deprivation. The sensitivity of the three pathways to pyridoxine deprivation is interpreted and discussed in terms of the different affinities for pyridoxal phosphate and the control strengths of the pyridoxal phosphate-dependent enzymes, tyrosine aminotransferase and kynureninase.     

Quantification of the importance of individual steps in the control of aromatic amino acid metabolism.

The quantitative importance of the individual steps of aromatic amino acid metabolism in rat liver was determined by calculation of the respective Control Coefficients (Strengths). The Control Coefficient of tryptophan 2,3-dioxygenase for tryptophan degradation was determined in a variety of physiological conditions and with a range of activities of tryptophan 2,3-dioxygenase. The Control Coefficient varied from 0.75 with basal enzyme activity to 0.25 after maximal induction of the enzyme by dexamethasone. The remainder of the control for tryptophan degradation was associated with the transport of the amino acid across the plasma membrane, with only very small contributions from kynureninase and kynurenine hydroxylase. The Control Coefficients of tyrosine aminotransferase for tyrosine degradation were approx. 0.70 and 0.20 with basal and dexamethasone-induced tyrosine aminotransferase activities respectively; the Control Coefficients of the transport of the amino acid into the cell were 0.22 and 0.58 respectively. Phenylalanine hydroxylase was found to have a Control Coefficient for the degradation of phenylalanine of approx. 0.50 under conditions of basal enzyme activity; after maximal activation by glucagon, the Control Coefficient decreased to 0.12. The transport of phenylalanine was responsible for the remaining control in the pathway. These results have important implications, directly for the regulation of aromatic amino acid metabolism in the liver, and indirectly for the regulation of neuroamine synthesis in the brain.     

The role of tryptophan 2,3-dioxygenase in the hormonal control of tryptophan metabolism in isolated rat liver cells. Effects of glucocorticoids and experimental diabetes.

The metabolism of L-tryptophan by isolated liver cells prepared from control, adrenalectomized, glucocorticoid-treated, acute-diabetic, chronic-diabetic and insulin-treated chronic-diabetic rats was studied. Liver cells from adrenalectomized rats metabolized tryptophan at rates comparable with the minimum diurnal rates of controls, but different from rates determined for cells from control rats 4h later. Administration of dexamethasone phosphate increased the activity of tryptophan 2,3-dioxygenase (EC 1.13.11.11) 7-8-fold, and the flux through the kynurenine pathway 3-4-fold, in cells from both control and adrenalectomized rats. Increases in flux through kynureninase (EC 3.7.1.3) and to acetyl-CoA can be explained in terms of increased substrate supply from tryptophan 2,3-dioxygenase. The metabolism of tryptophan was increased 3-fold in liver cells isolated from acutely (3 days) diabetic rats, with a 7-8-fold increase in the maximal activity of tryptophan 2,3-dioxygenase. The oxidation of tryptophan to CO2 and metabolites of the glutarate pathway increased 4-5-fold, consistent with an increase in picolinate carboxylase (EC 4.1.1.45) activity. Liver cells isolated from chronic (10 days) diabetic rats metabolized tryptophan at rates comparable with those of cells from acutely diabetic rats, but with a 50% decrease in the activity of tryptophan 2,3-dioxygenase. The proportion of flux from tryptophan 2,3-dioxygenase to acetyl-CoA, however, was increased by 50%; this was indicative of further increases in the activity of picolinate carboxylase. Administration of insulin partially reversed the effects of chronic diabetes on the activity of tryptophan 2,3-dioxygenase and flux through the kynurenine pathway, but had no effect on the increased activity of picolinate carboxylase. The role of tryptophan 2,3-dioxygenase in regulating the blood tryptophan concentration is discussed with reference to its sensitivity to the above conditions.     

The metabolism of L-phenylalanine and L-tyrosine by liver cells isolated from adrenalectomized rats and from streptozotocin-diabetic rats.

Flux through, and maximal activities of, key enzymes of phenylalanine and tyrosine degradation were measured in liver cells prepared from adrenalectomized rats and from streptozotocin-diabetic rats. Adrenalectomy decreased the phenylalanine hydroxylase flux/activity ratio; this was restored by steroid treatment in vivo. Changes in the phosphorylation state of the hydroxylase may mediate these effects; there was no significant change in the maximal activity of the hydroxylase. Tyrosine metabolism was enhanced by adrenalectomy; this was not related to any change in maximal activity of the aminotransferase. Steroid treatment increased the maximal activity of the aminotransferase. Both acute (3 days) and chronic (10 days) diabetes were associated with increased metabolism of phenylalanine; insulin treatment in vivo did not reverse these changes. Although elevated hydroxylase protein concentration was a major factor, changes in the enzyme phosphorylation state may contribute to differences in phenylalanine degradation in the acute and chronic diabetic states. Tyrosine metabolism, increased by diabetes, was partially restored to normal by insulin treatment in vivo. These changes can, to a large extent, be interpreted in terms of changes in the maximal activity of the aminotransferase.     

Enzyme activities involved in tryptophan metabolism along the kynurenine pathway in rabbits

The following enzyme activities of the tryptophan-nicotinic acid pathway were studied in male New Zealand rabbits: liver tryptophan 2,3-dioxygenase, intestine indole 2,3-dioxygenase, liver and kidney kynurenine 3-monooxygenase, kynureninase, kynurenine-oxoglutarate transaminase, 3-hydroxyanthranilate 3,4-dioxygenase, and aminocarboxymuconate-semialdehyde decarboxylase. Intestine superoxide dismutase and serum tryptophan were also determined. Liver tryptophan 2,3-dioxygenase exists only as holoenzyme, but intestine indole 2,3-dioxygenase is very active and can be considered the key enzyme which determines how much tryptophan enters the kynurenine pathway also under physiological conditions. The elevated activity of indole 2,3-dioxygenase in the rabbit intestine could be related to the low activity of superoxide dismutase found in intestine. Kynurenine 3-monooxygenase appeared more active than kynurenine-oxoglutarate transaminase and kynureninase, suggesting that perhaps a major portion of kynurenine available from tryptophan may be metabolized to give 3-hydroxyanthranilic acid, the precursor of nicotinic acid. In fact, 3-hydroxyanthranilate 3,4-dioxygenase is much more active than the other previous enzymes of the kynurenine pathway. In the rabbit liver 3-hydroxyanthranilate 3,4-dioxygenase and aminocarboxymuconate-semialdehyde decarboxylase show similar activities, but in the kidney 3-hydroxyanthranilate 3,4-dioxygenase activity is almost double. These data suggest that in rabbit tryptophan is mainly metabolized along the kynurenine pathway. Therefore, the rabbit can also be a suitable model for studying tryptophan metabolism in pathological conditions.     

Non-steroidal anti-inflammatory agents, tolmetin and sulindac, inhibit liver tryptophan 2,3-dioxygenase activity and alter brain neurotransmitter levels

Hepatic tryptophan 2,3-dioxygenase (TDO) is one of the rate-limiting enzymes in tryptophan catabolism and plays an important role in regulating the physiological flux of tryptophan into relevant metabolic pathways. In this study, we determined the effect of the non-steroidal anti-inflammatory agents, tolmetin and sulindac, on rat liver TDO activity and the subsequent changes in the hippocampal and striatal neurotransmitter levels. The amount of melatonin produced by the pineal gland was also measured using high performance liquid chromatography (HPLC). Treatment of rats with tolmetin or sulindac (5 mg/kg/bd for 5 days) significantly inhibited liver TDO activity. The results show that whilst tolmetin and sulindac increase serotonin levels in the hippocampus, these agents also significantly reduce dopamine levels in the striatum. Tolmetin, but not sulindac, increased the amount of melatonin produced by the pineal gland. The results of this study suggest that whilst tolmetin and sulindac may be beneficial for patients suffering from depression, these agents also have the potential to induce adverse effects in patients suffering with neurological disorders such as Parkinson's disease.     

Effect of hypobaric stress on enzymes of tryptophan metabolism

1. On exposure of rats to hypobaric stress the tryptophan pyrrolase and tyrosine aminotransferase activities of the liver increased about threefold in 4h. 2. The tryptophan hydroxylase activity increased about 50% on exposure for 24h or more. 3. The increased activities reverted to the basal value on removal of the stress. 4. Treatment with cycloheximide inhibited the increase in the enzyme activities when the time of exposure was short (4h). However, the inhibitor-treated animals showed paradoxically high tyrosine aminotransferase activity on prolonged exposure (24h). 5. The pattern of haematin saturation indicated that the increase in pyrrolase activity under low pressure resembled that obtained with cortisol and not with tryptophan. 6. Repeated administration of cortisol or tryptophan did not have any effect on the activity of tryptophan hydroxylase. 7. The stress-induced increase in hydroxylase activity was not eliminated by the prior administration of 5-hydroxytryptophan to the animals.     

Effect of high- and low-protein diets on the regulation of rat liver tyrosine aminotransferase and tryptophan oxygenase activities by l-tyrosine and l-tryptophan

Induction of rat liver tyrosine aminotransferase by l-tyrosine and tryptophan oxygenase by l-tryptophan was studied in groups of rats fed on diets containing 18 or 5% protein. The basal activity of hepatic tyrosine aminotransferase of rats receiving 5% protein gradually increased with the age of the animals but that of rats receiving 18% protein did not. l-Tyrosine induced hepatic tyrosine aminotransferase in rats receiving 18% protein when tested at ages from 4 to 20 weeks. When induction by l-tyrosine was carried out in rats receiving the 5% protein diet, significant induction of tyrosine aminotransferase occurred only in 4- or 6-week-old rats. Induction by l-tryptophan of tryptophan oxygenase in liver or the basal activity of this enzyme in liver did not differ between the groups fed on 5 and 18% protein. On changing the diet from 0 to 18% protein, the above-mentioned effects on the induction of hepatic tyrosine aminotransferase were reversed.     

Cycloheximide causes increased accumulation of translatable mRNA for tyrosine aminotransferase and tryptophan oxygenase in livers of cortisol-treated rats.

Messenger RNA activities for two cortisol-inducible enzymes, tyrosine aminotransferase and tryptophan oxygenase, have been determined by translation in a wheat germ system. The effects of cycloheximide on the two mRNA activities have been evaluated. Cortisol leads to an increase of the translatable mRNAs for tyrosine aminotransferase and tryptophan oxygenase with a maximum at approximately 6 h. Cycloheximide was administered 4 h after treatment with cortisol; 2 h later, the activities of tyrosine aminotransferase and tryptophan oxygenase mRNA had increased five-fold and two-fold, respectively, compared to the activities reached with cortisol alone. Thereafter the amount of the two translatable mRNAs declined, though 14 h after cortisol administration the mRNA activities were still several fold higher than in control animals. Application of alpha-amanitin together with cycloheximide did not prevent an increased accumulation of specific translatable mRNAs. The increase in tyrosine aminotransferase and tryptophan oxygenase activity by cortisol was immediately blocked by cycloheximide. Whereas tryptophan oxygenase activity rapidly declined after cycloheximide application, tyrosine aminotransferase activity remained at the same level. Approximately 4 h thereafter, both enzyme activities increased again.     

Alterations of tryptophan metabolism in a rat strain (Osborne-Mendel) predisposed to obesity

1. Osborne-Mendel (O-M) rats displayed differences in brain and systemic tryptophan metabolism. O-M rats had decreased brainstem tryptophan-5-hydroxylase activity and decreased serotonin (5-HT) levels as compared to Sprague-Dawley rats. However, brain tryptophan levels were actually increased in O-M rats. Norepinephrine, dopamine and 5-hydroxyindole-3-acetic acid levels were not different between strains. 2. Pineal serotonin levels were increased in O-M rats. 3. Liver tryptophan 2,3-dioxygenase activity was increased in O-M rats while tyrosine aminotransferase activity was not different between strains. 4. Total blood cholesterol was decreased in O-M rats while triglycerides, free fatty acids and albumin was not different between strains. Total serum tryptophan was not different between strains while O-M rats had an increased level of free (unbound) tryptophan.     

Hormonal control of the development of tryptophan oxygenase in primary cultures of young rat hepatocytes

Developmental increase of tryptophan oxygenase (L--tryptophan: oxygen 2,3-oxidoreductase (decyclizing), EC 1.13.11.11) was studied using hepatocytes of neonatal rats in primary culture. Hepatocytes from rats of 2–30-days-old were isolated and cultured for 2 days. In cultured hepatocytes of 2-day-old rats, tryptophan (2.5 mM), dexamethasone (1.10^?5 M) and glucagon (1.10^?7 M) did not cause the appearance of tryptophan oxygenase. But the enzyme activity became detectable, when heptocytes from 5-day-old rats were incubated wiht tryptophan, the oxygenase could be induced precociously by dexamethasone, but not by glucagon. The effect of glucagon was first seen 2 weeks after birth. However, in hepatocytes of 9-day-old rats glucagon stimulated formation of cyclic AMP and protein kinase activity (EC 2.7.1.37) and also induced tyrosine aminotransferase (EC 2.6.1.5). When heptocytes of 9-day-old rats were cultured for 4 days, their tryptophan oxygenase became inducible by glucagon. Insulin almost completely inhibited precocious appearance of the enzyme activity evoked by tryptophan plus dexamethasone in hepatocytes of 9-day-old rats. These results suggest that the appearance of tryptophan oxygenase in rat liver during development is due to first the onset of gene coding for tryptophan oxygenase and then stimulation by the sequential of glucocorticoid and glucagon.     

Changes in kynurenine pathway metabolism in the brain, liver and kidney of aged female Wistar rats

The kynurenine pathway of tryptophan catabolism plays an important role in several biological systems affected by aging. We quantified tryptophan and its metabolites kynurenine (KYN), kynurenine acid (KYNA), picolinic acid (PIC) and quinolinic acid (QUIN), and activity of the kynurenine pathway enzymes indoleamine 2,3-dioxygenase (IDO), tryptophan 2,3-dioxygenase (TDO) and quinolinic acid phosphoribosyltransferase (QPRTase), in the brain, liver and kidney of young, middle-aged and old female Wistar rats. Tryptophan levels and TDO activity decreased in all tissues with age. In contrast, brain IDO activity increased with age, while liver and kidney IDO activity decreased with age. The levels of KYN, KYNA, QUIN and PIC in brain all increased with age, while the levels of KYN in the liver and kidney showed a tendency to decrease. The levels of KYNA in the liver did not change, but the levels of KYNA in the kidney increased. The levels of PIC and QUIN increased significantly in the liver but showed a tendency to decrease in the kidney. QPRTase activity in both brain and liver decreased with age but was elevated in the kidney in middle-aged (12-month-old) rats. These age-associated changes in tryptophan metabolism have the potential to impact upon major biological processes, including lymphocyte function, pyridine (NAD(P)(H)) synthesis and N-methyl-d-aspartate (NMDA)-mediated synaptic transmission, and may therefore contribute to several degenerative changes of the elderly.     

Altered tyrosine and tryptophan metabolism during hypothermic hibernation in the 13-lined ground squirrel (Spermophilus tridecemlineatus)

(1) Tyrosine and tryptophan metabolism in brain and peripheral tissues were studied in hypothermic hibernating and normothermic nonhibernating 13-lined ground squirrels (Spermophilus tridecemlineatus). (2) In the hypothermic hibernating state, there were significant elevations of brain stem tyrosine, norepinephrine, and dopamine levels; forebrain norepinephrine and dopamine levels; and cerebellum norepinephrine and tyrosine levels. (3) On the other hand, plasma norepinephrine levels were significantly decreased in hypothermic hibernating squirrels while plasma tyrosine levels were increased. Kidney norepinephrine levels were significantly increased in hypothermic hibernating squirrels, while kidney tyrosine levels were decreased. Total plasma tryptophan and free plasma tryptophan were significantly reduced in hypothermic hibernating squirrels. Hepatic tyrosine aminotransferase Km and Vmax were decreased in hypothermic hibernating squirrels, while tryptophan 2,3-dioxygenase activity was not altered. Plasma and liver albumin were increased in hypothermic hibernating squirrels, while plasma and liver total protein were not altered. (4) These results demonstrate that significant changes in tyrosine and tryptophan metabolism occur in both central and peripheral tissues with concomitant alterations in metabolites during hypothermic hibernation in 13-lined ground squirrels.     

Comparative effects of oxygen on indoleamine 2,3-dioxygenase and tryptophan 2,3-dioxygenase of the kynurenine pathway

Indoleamine 2,3-dioxygenase (IDO) reacts with either oxygen or superoxide and tryptophan (trp) or other indoleamines while tryptophan 2,3-dioxygenase (TDO) reacts with oxygen and is specific for trp. These enzymes catalyze the rate-limiting step in the kynurenine (KYN) pathway from trp to quinolinic acid (QA) with TDO in kidney and liver and IDO in many tissues, including brain where it is low but inducible. QA, which does not cross the blood-brain barrier, is an excitotoxin found in the CNS during various pathologies and is associated with convulsions. We proposed that HBO-induced convulsions result from increased flux through the KYN pathway via oxygen stimulation of IDO. To test this, TDO and IDO of liver and brain, respectively, of Sprague Dawley rats were assayed with oxygen from 0 to 6.2 atm HBO. TDO activity was appreciable at even 30 microM oxygen and rose steeply to a maximum at 40 microM. Conversely, IDO had almost no detectable activity at or below 100 microM oxygen and maximum activity was not reached until about 1150 microM. (Plasma contains about 215 microM oxygen and capillaries about 20 microM oxygen when rats breathe air.) KYN was 60% higher in brains of HBO-convulsed rats compared to rats breathing air. While the oxygen concentration inside cells of rats breathing air or HBO is not known precisely, it is clear that the rate-limiting, IDO-catalyzed step in the brain KYN pathway (but not liver TDO) can be greatly accelerated in rats breathing HBO.     

Purification, characterization and identification of rat liver histidine-pyruvate aminotransferase isoenzymes.

1. Histidine-pyruvate aminotransferase (isoenzyme 1) was purified to homogeneity from the mitochondrial and supernatant fractions of rat liver, as judged by polyacrylamide-gel electrophoresis and isolectric focusing. Both enzyme preparations were remarkably similar in physical and enzymic properties. Isoenzyme 1 had pI8.0 and a pH optimum of 9.0. The enzyme was active with pyruvate as amino acceptor but not with 2-oxoglutarate, and utilized various aromatic amino acids as amino donors in the following order of activity: phenylalanine greater than tyrosine greater than histidine. Very little activity was found with tryptophan and 5-hydroxytryptophan. The apparent Km values were about 2.6mM for histidine and 2.7 mM for phenylalanine. Km values for pyruvate were about 5.2mM with phenylalanine as amino donor and 1.1mM with histidine. The aminotransferase activity of the enzyme towards phenylalanine was inhibited by the addition of histidine. The mol.wt. determined by gel filtration and sucrose-density-gradient centrifugation was approx. 70000. The mitochondrial and supernatant isoenzyme 1 activities increased approximately 25-fold and 3.2-fold respectively in rats repeatedly injected with glucagon for 2 days. 2. An additional histidine-pyruvate aminotransferase (isoenzyme 2) was partially purified from both the mitochondrial and supernatant fractions of rat liver. Nearly identical properties were observed with both preparations. Isoenzyme 2 had pI5.2 and a pH optimum of 9.3. The enzyme was specific for pyruvate and did not function with 2-oxoglutarate. The order of effectiveness of amino donors was tyrosine = phenylalanine greater than histidine greater than tryptophan greater than 5-hydroxytryptophan. The apparent Km values for histidine and phenylalanine were about 0.51 and 1.8 mM respectively. Km values for pyruvate were about 3.5mM with phenylalanine and 4.7mM with histidine as amino donors. Histidine inhibited phenylalanine aminotransferase activity of the enzyme. Gel filtration and sucrose-density-gradient centrifugation yielded a mol.wt. of approx. 90000. Neither the mitochondrial nor the supernatant isoenzyme 2 activity was elevated by glucagon injection.     

A pathway for the interconversion of hexose and pentose in the parasitic amoeba Entamoeba histolytica.

The following three potent inhibitors of hepatocytic proteolysis were investigated to see if they would inhibit the intracellular inactivation of enzymes: chymostatin and leupeptin (proteinase inhibitors) and methylamine (a lysosomotropic weak base). Chymostatin inhibited the inactivation of two of the three enzymes tested: tyrosine aminotransferase (EC 2.6.1.5) and tryptophan oxygenase (tryptophan 2,3-dioxygenase, EC 1.13.11.11). Leupeptin had no effect on any of the enzymes, whereas methylamine had only a weak inhibitory effect on tyrosine aminotransferase inactivation. Apparently proteolytic cleavage (probably by a non-lysosomal proteinase, since only chymostatin is effective) is involved in the inactivation of tyrosine aminotransferase and tryptophan oxygenase. The third enzyme, benzopyrene hydroxylase (flavoprotein-linked mono-oxygenase, EC 1.14.14.1), is probably inactivated by a non-proteolytic mechanism.     

The effect of stress on the activity of hepatic tryptophan pyrrolase, of tyrosine aminotransferase in various organs and on the level of tryptophan in the liver and plasma of rats.

In rats subjected to 400 revolutions in Noble-Collip drums, hepatic tryptophan pyrrolase activity increases and plasma tryptophan level decreases. After bilateral adrenalectomy, the alterations of plasma tryptophan are even more pronounced and liver tryptophan increases in contrast to tryptophan pyrrolase activity which remains unchanged after injury. The possible significance of the posttraumatic increase of tryptophan pyrrolase in intact animals for brain serotonin metabolism and hepatic gluconeogenesis is underlined. The activity of tyrosine aminotransferase in liver, brain, adrenal, kidney and muscle tissue of rats was determined with special reference to the possible effect of the before-mentioned stress procedure. Organ homogenates were centrifuged at 15000 x g and both supernatants and pellets were investigated for enzyme activity with the exception of the liver, where only the supernatant fraction was used. Tyrosine aminotransferase activity in the liver supernatant considerably exceeded the corresponding values in both supernatant and pellet of the remaining organs, in which a prevalence of the mitochondrial enzyme was obvious. In contrast to the clear-cut increase of the hepatic enzyme during stress, essentially no changes were noted in the brain, the adrenals, kidney or muscle under similar conditions...     

Enzyme activities along the tryptophan-nicotinic acid pathway in alloxan diabetic rabbits

Recent data from our laboratory have indicated that the rabbit is a suitable animal model for the study of enzyme activities of the tryptophan-nicotinic acid pathway. We report here the pattern of tryptophan metabolism in rabbits made diabetic with alloxan treatment, and hypercholesterolemic with a high-cholesterol diet. A group of rabbits with only hypercholesterolemia was also considered. The enzymes assayed were: liver tryptophan 2,3-dioxygenase (TDO), intestine indoleamine 2,3-dioxygenase (IDO), liver and kidney kynurenine 3-monooxygenase, kynurenine-oxoglutarate transaminase, kynureninase, 3-hydroxyanthranilate 3,4-dioxygenase and aminocarboxymuconate-semialdehyde decarboxylase.TDO showed a reduction of specific activity in liver of diabetic-hyperlipidemic and hyperlipidemic rabbits compared to controls. Intestine IDO activities and liver and kidney kynurenine monooxygenase were unchanged with respect to controls.Kynurenine-oxoglutarate transaminase and kynureninase activities were reduced in the kidneys, but not in the liver, of diabetic-hyperlipidemic rabbits.The main finding was the reduction of 3-hydroxyanthranilate 3,4-dioxygenase activity (expressed as activity per g of fresh tissue) in the liver and kidneys of diabetic-hypercholesterolemic and hyperlipidemic rabbits compared to controls. Conversely, aminocarboxymuconate-semialdehyde decarboxylase activity was significantly higher in diabetic hypercholesterolemic rabbits in comparison with control and hypercholesterolemic rabbits.These data demonstrate that also in diabetic rabbits there is an alteration of tryptophan metabolism at the level of 3-hydroxyanthranilic acid-->nicotinic acid step. Also dyslipidemia seems to be involved in enzyme activity variations of the tryptophan metabolism along the kynurenine pathway.     

Purification and characterization of aromatic-amino-acid-glyoxylate aminotransferase from monkey and rat liver.

Aromatic-amino-acid-glyoxylate aminotransferase was highly purified from the mitochondrial fraction of livers from monkey and glucagon-injected rats. The two enzyme preparations showed physical and enzymic properties different from a kynurenine aminotransferase previously described. The two enzymes had nearly identical molecular weights (approximate 80 000), isoelectric points (pH 8.0) and pH optima (pH 8.0 - 8.5). However, a difference in substrate specificity was observed between the two enzymes. Both enzymes utilized glyoxylate, pyruvate, hydroxypyruvate and 2-oxo-4-methyl-thiobutyrate as effective amino acceptors. 2-Oxoglutarate was active for rat enzyme but not for monkey enzyme. With glyoxylate, amino donors were effective in the following order of activity; phenylalanine greater than histidine greater than tyrosine greater than tryptophan greater than 5-hydroxytrypotphan greater than kynurenine for the rat enzyme, and phenylalanine greater than kynurenine greater than histidine greater than tryptophan greater than 5-hydroxy-tryptophan for the monkey enzyme.     

Regulation of hepatic tyrosine aminotransferase in genetically obese rats

The activities of hepatic tyrosine aminotransferase, tryptophan oxygenase and serine dehydratase were increased in obese rats shortly after weaning. Immunotitration experiments showed that the increase in tyrosine aminotransferase activity resulted from an increase in enzyme protein in obese rats. No increase in hepatic tyrosine aminotransferase was observed in suckling pre-obese rats. The post-weaning increase in hepatic tyrosine aminotransferase of obese rats was only observed during the light phase of the diurnal cycle, but was prevented by pair-feeding and by starvation. Tryptophan increased hepatic tyrosine aminotransferase of lean rats to obese levels but had no effect in obese rats until tyrosine aminotransferase levels were reduced by starvation or adrenalectomy. Adrenalectomy abolished the increase in hepatic tyrosine aminotransferase activity in obese rats although serum corticosterone was normal in these animals. Hepatic and brain tyrosine concentrations were decreased in obese rats but normalized after adrenalectomy. The results suggest that the corticosteroid-dependent increase in food and tryptophan intake may be the primary cause of the increased hepatic amino acid catabolism of obese rats.