Regulation of tyrosine aminotransferase activity by glucagon and cAMP analogues in chick embryos in ovo.

Glucagon, dibutyryl cAMP (Bt2cAMP) and 8-(4-chlorophenylthio) cAMP (CptcAMP), singly or when combined, stimulated tyrosine aminotransferase (TAT) activity in 17-day-old chick embryos in ovo. Maximal induction was produced within 4 hr of injection of the inducers. The effects of glucagon and the cAMP analogues were not additive. Glucagon administration was accompanied by a rapid increase in hepatic cAMP concentration which remained elevated for at least 4 hr. The stimulated increase in TAT activity elicited by the hormone or cyclic nucleotide was prevented by injection of cycloheximide or cordycepin. These results are discussed vis-à-vis the possible regulation of TAT in ovo by physiological concentrations of glucagon and the likely role of cAMP as a second messenger in this process during chick embryogenesis.

A comparative study of effect of oral chronic cyanide administration on some rabbit tissue ATPases.

The effect of oral KCN administration on Na(+)-K(+)-ATPase, Ca(2+)-ATPase and Mg(2+)-ATPase activities was studied in some rabbit tissues. Generally, the order of activity of the enzymes was kidney > liver > ileum > colon. There were significant decreases (P < 0.05) in the activities of all the ATPases in the tissues of KCN-treated rabbits relative to controls. The decreases in Na(+)-K(+)-ATPase and Ca(2+)-ATPase activities were most pronounced in liver and kidney. The decreases in ATPases suggest impairment of membrane function as a result of the toxic effect of cyanide.

Effects of glucocorticosteroids and insulin on tyrosine aminotransferase activity in isolated chick embryo hepatocytes and in intact embryos in ovo.

Hepatocytes were isolated from 17-day-old chick embryos. Steroid hormones or insulin, alone or when combined, did not alter tyrosine aminotransferase (TAT) activity in the isolated hepatocytes. However, TAT was stimulated by glucocorticoids in mixed hepatocyte and fibroblast cocultures; hormonal effects were not observed in pure hepatocyte cultures. Administration of hydrocortisone, dexamethasone, or triamcinolone (singly) to chick embryos in ovo resulted in an increase in hepatic TAT activity; insulin injection was without effect on the enzyme. The stimulation of TAT activity evoked by glucocorticosteroids in ovo was abolished by injection of cycloheximide or cordycepin. These observations contrast with reported glucocorticosteroid actions on TAT activity in fetal rat liver. It would appear that the differential regulatory effects on hepatic TAT by glucocorticosteroids are imposed by the distinct nutrient environments of chick embryos and fetal rats.

Hormonal control of glycogenolysis in isolated chick embryo hepatocytes.

Hepatocytes were isolated from 15-, 16-, 17-, and 18-day-old chick embryos. Glucagon, adrenaline, and dibutyryl cAMP (Bt2cAMP), individually or in combination, activated glycogenolysis in the isolated hepatocytes. The alpha-adrenergic agonist phenylephrine did not increase glycogen breakdown. The action of adrenaline was abolished upon treatment of hepatocytes with a combination of the hormone and propranolol, a beta-adrenergic blocker. The effects of glucagon, adrenaline, and dibutyryl cAMP on glycogenolysis were not additive. Either hormone induced an increase in the concentration of cAMP. The activities of dephosphophosphorylase kinase and phosphorylase a were stimulated by each hormone or Bt2cAMP. It appears likely that glucagon and adrenaline serve as physiological regulators of hepatic glycogen breakdown during embryogenesis in chickens.

Effects of glucagon and an analogue of cAMP on tyrosine aminotransferase in isolated chick embryo hepatocytes.

Hepatocytes were isolated from 17-day-old chick embryos by the use of collagenase. Glucagon and dibutyryl cAMP (bt2cAMP), individually or in combination, stimulated tyrosine aminotransferase (TAT) activity and synthesis in the isolated hepatocytes; maximal stimulation occurred 4 h after exposure of hepatocytes to the inducers. The stimulatory effects produced by glucagon and bt2cAMP were abolished by treatment of hepatocytes with cordycepin or cycloheximide. The effects of the hormone and the cyclic nucleotide were not additive. The induction of the enzyme by glucagon suggests a physiological role for the hormone in the expression of TAT activity during chick embryonic development.

Tyrosine aminotransferase of chicken liver: purification and properties.

Tyrosine aminotransferase has been purified from chicken liver to homogeneity by a 5-step procedure. The resultant enzyme preparation has a specific activity (256 units activity/mg protein) comparable to results published for the enzyme purified from rat liver and represented an overall recovery of 35-40%. In terms of structure (native and subunit molecular weights, immunological reactivity, and kinetic parameters) (apparent Michaelis constants for L-tyrosine and 2-oxoglutarate, oxoacid specificity, pH optimum) the purified enzyme from chicken liver exhibits remarkable similarities to tyrosine amino-transferase from rat liver.