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...     

Diurnal variations in response of rat liver tyrosine aminotransferase activity to food intake

Effects of fasting and refeeding on the hepatic tyrosine aminotransferase activity were examined in rats that had been fed during the night. The tyrosine aminotransferase activity showed clear diurnal variations, with a maximal activity after the feeding time. The tyrosine aminotransferase rhythm persisted even under starvation, though the amplitude decreased remarkably. When the starved rats were refed at night, the tyrosine aminotransferase activity increased rapidly to a high level, but it increased slowly to a rather lower level when they were refed in daytime.     

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.     

Purification and characterization of ascamycin-hydrolysing aminopeptidase from Xanthomonas citri.

Intact obese rats were hyperinsulinaemic, had higher rates of whole-body fatty acid synthesis, higher activities of hepatic acetyl-CoA carboxylase and tyrosine aminotransferase and a higher hepatic glycogen concentration than intact lean animals. Adrenalectomy abolished all these factors of the obese phenotype. Treatment of adrenalectomized rats with corticosterone for 24 h increased the rate of whole-body fatty acid synthesis to the same extent in both phenotypes, but caused a larger increase in glycogen concentration, tyrosine aminotransferase activity and plasma insulin concentration in obese rats.     

Induction of hepatic tyrosine aminotransferase mRNA by protein synthesis inhibitors.

Several protein synthesis inhibitors were as effective as the inducers hydrocortisone or cyclic AMP in elevating rat liver tyrosine aminotransferase mRNA levels when assayed in the wheat germ cell-free translational system. Cycloheximide, emetine, or puromycin increased this mRNA activity 6- to 7-fold within 4 h after in vivo administration. No increase in total hepatic mRNA levels or tryptophan oxygenase mRNA was found after treatment with these protein synthesis inhibitors. Furthermesults suggest that a short lived protein may specifically regulate the level of functional hepatic tyrosine aminotransferase mRNA or that ongoing translation of this mRNA is required for its degradation.     

The effect of stress or glucose feeding on hepatic tyrosine aminotransferase activity and liver and plasma tyrosine level of intact and adrenalectomized rats.

The increase of hepatic tyrosine aminotransferase and the fall of plasma tyrosine in rats subjected to immobilization is reconfirmed. Moreover, the same effects three hrs after exposuing the animals to 400 revolutions in Noble-Collip drums are described. However, in bilaterally adrenalectomized rats both hepatic tyrosine aminotransferase and plasma tyrosine remain unchanged after injury and the liver tyrosine level increase. Finally, in animals fed overnight exclusively with 15% glucose solution the well-known decrease of hepatic tyrosine aminotransferase was found paralleled by increased plasma tyrosine levels. A regulatory role of tyrosine aminotransferase in establishing the level of tyrosine in plasma is suggested.     

The influence of starvation and tryptophan administration on the metabolism of phenylalanine, tyrosine and tryptophan in isolated rat liver cells.

Liver cells from fed Sprague-Dawley rats metabolized phenylalanine, tyrosine and tryptophan at rates consistent with the known kinetic properties of the first enzymes of each pathway. Starvation of rats for 48 h did not increase the maximal activities of phenylalanine hydroxylase, tryptophan 2,3-dioxygenase and tyrosine aminotransferase in liver cell extracts, when results were expressed in terms of cellular DNA. Catabolic flux through the first two enzymes was unchanged; that through the aminotransferase was elevated relatively to enzyme activity. This is interpreted in terms of changes in the concentrations of 2-oxoglutarate and glutamate. Cells from tryptophan-treated animals exhibited significant increases in the catabolism of tyrosine and tryptophan, but not of phenylalanine. The activities of tyrosine aminotransferase and tryptophan 2,3-dioxygenase were also increased, although the changes in flux and enzyme activity did not correspond exactly. These results are discussed with reference to the control of aromatic amino acid catabolism in liver; the role of substrate concentration is emphasized.     

Tertiary structure of tRNAPhe. A possible correlation between the structural functional unit of this tRNA and its exonic sequence.

By using an antiserum raised against rat liver tyrosine aminotransferase, it was shown that about 60% of tryptophan aminotransferase activity in rat liver extracts is catalysed by this enzyme. Induction of tryptophan aminotransferase activity by intraperitoneal injections of tryptophan or triamcinolone can be entirely attributed to the effects of these agents on tyrosine aminotransferase. The origin of the other 40% of tryptophan aminotransferase activity remains to be established. This activity increases after starvation for 48 h. It is unlikely that tryptophan transamination plays a quantitatively important role in the metabolism of tryptophan by the liver.     

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.     

Effects of Dietary Amino Acids on Brain Amino Acids and Transmitter Amines in Rats with a Portacaval Shunt

The etiologic relationship between disturbances in metabolism of amino acids and amines and hepatic coma was investigated by examining the effects of diets containing various mixtures of amino acids on brain amine metabolism in rats with a portacaval shunt, using a method for simultaneous analysis of amino acids and amines. Rats with a portacaval shunt were fed on four different amino acid compositions with increased amounts of various amino acids suspected to be etiologically related to hepatic coma, such as methionine, phenylalanine, tyrosine, and tryptophan. The animals were killed 4 weeks after operation. During the experimental period, these animals did not become comatose, but exhibited various behavioral abnormalities. Marked increase in the plasma and brain levels of the augmented amino acids, especially methionine and tyrosine, were observed in rats with a portacaval shunt. Brain noradrenaline, dopamine, and serotonin levels were significantly decreased when the brain tyrosine level was increased. These results indicate that in rats with a portacaval shunt the dietary levels of amino acids greatly influence the brain levels of both amino acids and transmitter amines.     

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.     

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.     

The effects of corticosterone, cold exposure and overfeeding with sucrose on brown adipose tissue of obese Zucker rats (fa/fa).

GDP binding to brown-adipose-tissue mitochondria was decreased in obese Zucker rats. Adrenalectomy restored both GDP binding and serum tri-iodothyronine of obese rats to values observed in lean rats. The effects of adrenalectomy on GDP binding and serum tri-iodothyronine were reversed by corticosterone. Decreasing food intake had no effect on brown-adipose-tissue GDP binding in obese rats. Young (5-week-old) obese rats showed a normal increase in brown-adipose-tissue mitochondrial GDP binding after housing at 4 degrees C for 7 days, but this response was attenuated in 10-week-old obese rats. Overfeeding with sucrose increased brown-adipose-tissue thermogenesis in lean, but not in obese, rats. After adrenalectomy, overfeeding with sucrose enhanced brown-adipose-tissue mitochondrial GDP binding in obese rats.     

Involvement of the liver in the regulation of tryptophan availability. Possible role in the responses of liver and brain to starvation

The concentrations of free and total (free plus albumin bound) tryptophan were measured in plasma of blood taken from the portal vein, hepatic vein and abdominal aorta of male rats, fed, and starved for one and three days. Liver and brain tryptophan concentrations were measured in similar groups of rats.On starvation, there was an increase in arterial plasma free tryptophan concentration which took place peripherally and was paralleled by an increase in brain tryptophan. In both the fed and starved rats, the portal vein concentrations of free tryptophan were high and as the blood flowed through the liver they were reduced to relatively low levels not directly related to the arterial values. All these changes were due to alterations in degree of binding of tryptophan to plasma albumin.The measurements of plasma total tryptophan concentrations showed that postabsorptively and during starvation there was a net uptake of tryptophan by the peripheral tissues (which included brain), but no overall fall in plasma concentration. At the same time, there was a net release from the liver, and to a lesser extent from the portal-drained tissues. The released tryptophan largely entered the albumin bound plasma pool. Accompanying the hepatic output was a fall in tryptophan concentration in the liver which was apparently caused by altered cell membrane transport.The results suggest (1) that the liver protects the brain from the high free tryptophan level in portal blood, (2) that the availability of tryptophan to the brain is maintained postabsorptively and during starvation by hepatic output into the albumin bound pool and (3) that this release of tryptophan from the liver and the fall in intracellular tryptophan concentration are initiated by altered membrane transport. The pattern of changes is consistent with a role for tryptophan in the mediation of changes in liver protein synthesis and gluconeogenesis and cerebral serotonin turnover on starvation.     

Stimulation of tyrosine aminotransferase activity by dl-alpha-methyltryptophan.

DL-alpha-Methyltryptophan (alphaMeTrp), a synthetic analogue of tryptophan, has been found to be a potent inducer of hepatic tyrosine aminotransferase activity in the adrenalectomized rat. alphaMeTrp is inactive in vitro. Unlike the action of other known inducers (tryptophan, hydrocortisone, adenosine cyclic 3:5-monophosphate, and glucagon), maximal stimulation of enzyme activity occurs only 16 to 30 hours after alphaMeTrp administration and the activity is still elevated at 96 hours. Only the L isomer of alphaMeTrp is active, and addition of a hydroxyl group to position 5 of the indole ring renders an inactive compound. The induction can be prevented by actinomycin D or cycloheximide but not galactosamine. Administration of alphaMeTrp together with hydrocortisone produced an additive stimulation of enzyme activity. alphaMeTrp given along with glucagon or adenosine cyclic 3:5-monophosphate caused a further but not additive increase in enzyme activity. Tryptophan given along with alphaMeTrp promoted no extra stimulation whatsoever. These data indicate that alphaMeTrp and tryptophan may act via a common pathway which in part requires RNA synthesis. Other enzymes, namely alanine and aspartate aminotransferase, ornithine aminotransferase, ornithine carbamoyltransferase, serine dehydratase, and histidine ammonialyase, were not affected by treatment of rats with alphaMeTrp.     

The effect of hormones on glucocorticoid binding capacity of rat liver cytosol.

The relationship between the glucocorticoid binding capacity of rat liver cytosol and the activity of tyrosine aminotransferase has been studied in adrenalectomized male rats. Bilateral adrenalectomy of male rats caused an increase within 3 days in the level of specific dexamethasone binding of liver cytosol accompanied by a rapid decrease in tyrosine aminotransferase activity. Known inducers of tyrosine aminotransferase were administered in vivo to test their effect on dexamethasone binding capacity, in order to determine whether the induction was by an indirect mechanism involving an increase in glucocorticoid binding capacity. Insulin, adrenalin, glucagon, dibutyryl cyclic AMP and oestradiol caused a significant increase in the activity of the enzyme, with no change in the specific dexamethasone binding. Tetracosactrin, a synthetic analogue of ACTH, had no effect on either parameter. It was concluded that the induction of tyrosine aminotransferase by the compounds tested was not mediated by an increase in glucocorticoid receptor activity.     

Increase in hepatic tyrosine aminotransferase mRNA during enzyme induction by N6,O2'-dibutyryl cyclic AMP.

Tyrosine aminotransferase mRNA was quantitated by translation in a cell-free system derived from wheat germ followed by specific immunoprecipitation of the newly synthesized enzyme subunit. Hepatic poly(A)-containg RNA prepared from rats treated for 4 h with N6, O2'-dibutyryl cyclic AMP and theophylline was approximately 5.6 times more active in directing the synthesis of the tyrosine aminotransferase subunit relative to untreated controls. The overall template activity of the RNA prepared from control and cyclic AMP-treated animals was virtually identical, demonstrating that the cyclic nucleotide effect was specific for the tyrosine aminotransferase mRNA. At all times, after a single injection of dibutyryl cyclic AMP and theophylline, the increase in hepatic enzyme activity was accompanied by corresponding induction in the level of functional tyrosine aminotransferase mRNA. Other inducers of tyrosine aminotransferase, such as glucagon and hydrocortisone, also increased the level of tyrosine aminotransferase mRNA in proportion to their effect on enzyme activity. The RNA polymerase II inhibitor, alpha-amanitin, completely blocked the dibutyryl cyclic AMP-mediated increase in tyrosine aminotransferase mRNA activity. These studies demonstrate that, in intact animals, the induction of tyrosine aminotransferase activity by dibutyryl cyclic AMP can be completely accounted for by a corresponding increase in the level of functional mRNA coding for the enzyme.