Tyrosine aminotransferase, inducers

GH has been shown to induce a number of enzymes concerned with amino acid metabolism in the liver of the hypophysectomized rat (in vivo or in perfused liver [78]). Induced enzymes include tyrosine aminotransferase, tryptophan oxygenase and ornithine decarboxylase. The effects are complex, particularly in relation to interaction with glucocorticoids, and in some experiments GH lowered enzyme levels induced by glucocorticoids, although given alone it led to induction of these enzymes. 

The major route of degradation of tyrosine in animals begins with transamination (Fig. 25-5, reaction c) to p-hydroxyphenylpyruvate. The enzyme tyrosine aminotransferase is induced in the liver in response to the action of glucorticoid hormones (Chapter 22). The S5mthesis of the enzyme is also controlled at the translational level, release of the newly formed protein from liver ribosomes being stimulated by cyclic AMP. The enzyme is subject to posttranscriptional... 

Alexandrova M (1994) Stress Induced Tyrosine Aminotransferase Activity via Glucocorticoid Receptor. Hormone Metabolic Res 26 97... 

The almost universal presence of adenosine 3 , 5 cyclic phosphate (cAMP) as a biologic regulator in microbial and hormone-sensitive animal tissues raises the question of whether this nucleotide might mediate the steroid hormone effect on specific enzyme synthesis. This possibility is made especially attractive because certain enzymes known to be induced by the steroids can also be induced by the cyclic nucleotide. Thus, several enzymes which are induced in the prostate or seminal vesicles by (dihydro) testosterone can also be induced by cAMP (Singhal et al., 1970). Likewise, tyrosine aminotransferase is inducible in embryonic and adult liver by the cyclic nucleotide as well as by the glucocorticoids (Wicks, 1968). 

Auricchio, F., D. W. Martin, Jr., and G. M. Tomkins. 1969. Control of degradation and synthesis of induced tyrosine aminotransferase studied in hepatoma cells in culture. Nature (London), 244 806. 

Granner, D. K., E. B. Thompson, and G. M. Tomkins. 1970. Dexamethasone phosphate induced synthesis of tyrosine aminotransferase in HTC cells. J. Biol. Chem., 245 1472. 

When adrenal glucocortocoid steroid hormones are administered to a rat, several different enzymic activities can be observed to acumulate in the liver [47]. Among these is tyrosine aminotransferase (TAT) activity, which is responsible for the first and rate-limiting step in the catabolism of the a-amino acid, L-tyrosine. The enzyme catalyzes the transfer of the a-amino group of L-tyrosine to the a-carbon of a-keto-glutarate and has been purified to homogeneity from induced rat livers... 

In liver glucocorticoids induce the synthesis of tyrosine aminotransferase. This activity of the enzyme is further increased if insulin is added to the incubation medium, but while actinomycin blocks the enzyme induction by glucocorticoids it is without effect on the stimulation of activity elicited by insulin and therefore, insulin is also believed to act at the level of translation. 

In rats fed a diet adequate in vitamin Bg, the fraction of total pyridoxal phosphate found in the nuclei of liver cells was 21%, and this increased to 39% in rats fed a vitamin Bg-deflcient diet, indicating a conservation of the vitamin in the nuclear compartment during deficiency. Pyridoxal phosphate in the cell nucleus is protein bound, and this protein has an apparent molecular mass of 50 to 55 kDa. Cells grown in the presence of 5 mM pyridoxine have a decreased glucocorticoid-dependent induction of enzymes such as tyrosine aminotransferase. Vitamin Bg regulates transcriptional activation of human glucocorticoid receptors in the HeLa cells. The modulatory role in transcription is not restricted to the glucocorticoid receptor but extends to other members of the steroid hormone super family. The intracellular concentration of PLP could have a profound influence on steroid hormone-induced... 

It is necessary to mention that the final effect of hormones on the morphogenetic processes is determined not only by the genetically controlled conditions of the endocrinal system, but also by the genetically controlled ability (competency) of the target-tissue which reacts to the hormone (Shire, 1974). For instance, liver tyrosine aminotransferase (TAT) is an enzyme which is induced by steroids both in vivo and in vitro in cultured cells. Sublines of hepatomas were described in which the level of activity of this enzyme was normal, but which were unable to react to steroid treatment by increasing TAT activity (Levisohn and Thompson,... 

It is known, also, that tyrosine aminotransferase synthesis in the hepatoma cell culture is induced by steroid hormones only in certain stages of the cell cycle (late Gj and S). It was suggested that gene expression in this case is controlled on the transcriptional and post-transcriptional level (Sellers and Granner, 1974). Post-... 

An analysis of the activation-inactivation of the gene for tyrosine aminotransferase has been made in rat hepatoma cell cultures (Martin and Tomkins, 1970). It is known that the synthesis of tyrosine aminotransferase can be induced by glucocorticoids during the latter half of the Gi and S periods of the cell cycle but not during the G2 or early part of the Gi periods. Although the mechanism that is postulated is complex, it involves, in part, the activation and inactivation of genes regulating the synthesis of tyrosine aminotransferase. 

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