Regulation of tyrosine aminotransferase in foetal rat liver.
ABSTRACT: A specific tyrosine aminotransferase, separate from the aspartate aminotransferases, is present in low concentration in foetal rat liver at the 21st day of gestation. Intraperitoneal injections of tyrosine methyl ester into the foetuses in utero increase the activity 2-fold, whereas glucose injections decrease it. Tyrosine, dexamethasone and dibutyryl cyclic AMP induce the enzyme activity in organ culture to the same extent as in adult rat liver in vivo.
Project description:Liver of rat foetuses from 14 to 19 days of gestation and cultured hepatocytes derived from foetuses of 14 or 15 days gestation show a limited capacity to transaminate tyrosine. This low tyrosine transamination activity can be ascribed to aspartate aminotransferase. Definitive tyrosine aminotransferase can be demonstrated in 1-day-old cultures of hepatocytes taken from 19-day foetuses, but not from 15-day foetuses. However, after 3 days of culture hepatocytes from 15-day foetuses are able to synthesize tyrosine aminotransferase. Induction studies reveal that dexamethasone is capable of increasing tyrosine aminotransferase activity once it is detectable in culture.
Project description:Incubation of liver explants from second-trimester human foetuses with dexamethasone, glucagon or dibutyryl cyclic AMP (plus theophylline) increased the activity of liver cystathionase from unmeasurable or trace values to adult values. Simultaneous incubation with cycloheximide or actinomycin D inhibited this effect.
Project description:1. Premature delivery of foetal rats by uterine section results in the rapid appearance of tyrosine aminotransferase activity in foetal liver, after an initial lag period of 3-6hr. 2. The premature induction of activity is completely repressible by actinomycin D given soon after delivery and partially repressible by puromycin and amino acid analogues. 3. Glucagon injections into foetal rats in utero lead to production of tyrosine aminotransferase in the foetal liver, but adrenalin and nor-adrenalin are without effect. 4. Injections of glucose, galactose, fructose and mannose into prematurely delivered rats repress the development of tyrosine aminotransferase activity about 50% when they are given 2hr. after delivery, but glucose has no significant effect when injected at delivery. 5. The results are discussed in relation to current hypotheses on the role of hormones in enzyme induction in foetal development.
Project description: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.
Project description:1. A precocious development of UDP-glucuronosyltransferase activity (EC 126.96.36.199) towards o-aminophenol is demonstrated in 15-17 day foetal rat liver in utero after dexamethasone administration to the mother. 2. This stimulation of liver transferase activity in utero is directly proportional to the dose of dexamethasone infected. 3. Precocious development of transferase activity in utero can also be effected with the natural glucocorticoid cortisol by multiple injections of large amounts of this hormone into the mother. 4. Transferase activity towards o-aminophenolin foetal lung, kidney and upper alimentary tract can also be precociously stimulated by dexamethasone in 17-day foetuses in utero. 5. Natural development of hepatic transferase activity between days 18 and 20 of gestation is retarded after foetal hypophysectomy by decapitation in utero. 6. Overall glucuronidation of o-aminophenol, as observed in foetal rat liver, is also precociously stimulated by dexamethasone. 7. From this and from evidence previously presented we suggest that glucocorticoids, which are known to increase in rat foetuses between days 17 and 20 of gestation, trigger the normal development in utero of hepatic transferase activity towards o-aminophenol which occurs at that time. We also suggest that these hormones are responsible for the rise in activity of the enzyme in foetal lung, kidney and upper alimentary tract which occurs during the same gestational period.
Project description:Tyrosine aminotransferase was purified to homogeneity from epimastigotes of Trypanosoma cruzi by a method involving chromatography on DEAE-cellulose, gel filtration on Sephacryl S-200 and chromatography on Mono Q in an f.p.l.c. system. The purified enzyme showed a single band in SDS/PAGE, with an apparent molecular mass of 45 kDa. Since the apparent molecular mass of the native enzyme, determined by gel filtration, is 91 kDa, the native enzyme is a dimer of similar subunits. The amino-acid composition was determined, as well as the sequences of three internal peptides obtained by CNBr cleavage at Met residues. Both criteria suggest considerable similarity with the tyrosine aminotransferases from rat and from human liver. The enzyme contains nine 1/2 Cys residues, three free and the others forming three disulphide bridges. The enzyme is not N-glycosylated. The isoelectric point is 4.6-4.8. The optimal pH for the reaction of the enzyme with tyrosine as a substrate is 7.0. The apparent Km values for tyrosine, phenylalanine and tryptophan, with pyruvate as a co-substrate, were 6.8, 17.9 and 21.4 mM, respectively, whereas those for pyruvate, alpha-oxoglutarate and oxaloacetate, with tyrosine as a substrate, were 0.5, 38 and 16 mM respectively. The purified tyrosine aminotransferase acts as an alanine aminotransferase as well and the activity seems to reside in the same enzyme molecule. The results suggest that the enzyme is a general aromatic-amino-acid transaminase, with high sequence similarity to tyrosine aminotransferases from rat and human liver.
Project description:1. Adrenaline increased hepatic tyrosine aminotransferase activity when injected into foetal rats or 2-day-old rats. 2. The inhibition of the postnatal increase in tyrosine aminotransferase activity which occurred in adrenalectomized newborn rats rapidly overcome by injection of adrenaline or dibutyryl cyclic AMP. 3. The effects of adrenaline or dibutyryl cyclic AMP on the tyrosine aminotransferase activity in foetal, adrenalectomized newborn and 2-day-old rats could be partially or completely blocked by prior treatment with actinomycin D. 4. Dibutyryl cyclic AMP induced tyrosine aminotransferase activity in hepatocytes cultured from 15-day foetal rats in glucocorticoid-free medium. 5. Actinomycin D at 0.2 microgram/ml in the culture medium completely prevented the induction of tyrosine aminotransferase activity by dibutyryl cyclic AMP in cultured cells. 6. The results suggest that adrenaline and cyclic AMP stimulate a transcriptional event during induction of tyrosine aminotransferase in perinatal liver.
Project description:1. The glycogen present in the liver of rat foetuses was labelled by injecting a trace amount of [6-(3)H]glucose into the mother at 19.5 days of gestation. The radioactivity incorporated in the glycogen 4h after the administration of the label was still present 38h later. A large proportion of this radioactivity was on the outer chains of the polysaccharide. These results indicate that there is normally almost no glycogen degradation in the foetal liver. In contrast, glycogen breakdown occurs very rapidly in the livers of foetuses whose mother is anaesthetized. 2. Glycogen synthetase is present in the liver at day 16 of gestation at a concentration as high as 30% of that in the adult, but essentially as an inactive (b) enzyme. The appearance of synthetase phosphatase between days 18 and 19 corresponds to that of synthetase a and to the beginning of glycogen synthesis. From day 19 to 21.5 the amount of synthetase a present in the foetal liver is just sufficient to account for the actual rate of glycogen deposition. 3. The content of total phosphorylase in the foetal liver increases continuously from day 16 to birth. However, a precise measurement of the a and b forms of the enzyme in the liver of non-anaesthetized foetuses is not possible. Taking the rate of glycogenolysis as an appropriate index of phosphorylase activity, we conclude that this enzyme is almost entirely in the inactive form in the foetal liver under normal conditions. 4. The accumulation of glycogen in the liver during late pregnancy may therefore be explained by a relatively slow rate of synthesis and a nearly total absence of degradation.
Project description:1. Glucose, formed from [1-(14)C]fructose or [6-(14)C]fructose in rat-liver slices, has been isolated as gluconate and degraded to give the radioactivity in C-1, C-2-5 and C-6. 2. By using this method it has been shown that, in liver from foetal rats younger than 20 days, glucose is formed from fructose without splitting of the molecule by the aldolase reaction. The rate of glucose formation from fructose in liver from these foetuses is approximately half of the rate in adult liver. 3. The direct conversion of fructose into glucose in foetal rat liver is not via sorbitol as in seminal vesicles, as this pathway cannot be detected. 4. When liver slices are incubated with [U-(14)C]fructose of high specific activity, the labelled intermediates are similar whether from liver from 18-day foetal, newborn or adult rats. 5. These findings are discussed with reference to the changing pathways of fructose metabolism during perinatal development of the liver in the rat.
Project description:1. Salicylate, in concentrations of 0.25mm and above, enhances the basal activity of tyrosine-2-oxoglutarate aminotransferase in homogenates of rat liver incubated in the absence of added pyridoxal 5'-phosphate (endogenous activity). The effect is decreased by increasing the concentration of the cofactor. 2. The intraperitoneal administration of sodium salicylate enhances the activity of rat liver tyrosine aminotransferase; the major effect during the first hour being on the enzyme in the absence of added pyridoxal phosphate. Actinomycin D prevents the induction of the enzyme by cortisol and tryptophan. Induction by pyridoxine or salicylate is 50% inhibited by actinomycin D. The effects of the injections of various combinations of cortisol, pyridoxine and salicylate were also studied in the absence or presence of actinomycin D. 3. It is suggested that salicylate induces rat liver tyrosine aminotransferase by displacing its protein-bound cofactor and that a cofactor-type induction of the hepatic enzyme occurs in pyridoxine-treated rats.