Multiple forms of tyrosine aminotransferase in rat liver and their hormonal induction in the neonate

Purification and properties of human liver tyrosine aminotransferase

Tyrosine aminotransferase (EC 2.6.1.5) of human liver was purified 2200-fold by successive chromatography on DEAE-cellulose DE-52, Ultrogel AcA-34, CM-Sephadex C-50 and hydroxyapatite to a specific activity of 64 units/mg of protein. The purified enzyme had a molecular mass of 95 500. The Km-values were 1.04 X 10(-3) mol/l, 0.17 X 10(-3) mol/l and 0.69 X 10(-6) mol/l for tyrosine, 2-oxoglutarate and pyridoxal 5'-phosphate, respectively. In the final purification step the enzyme activity was divided into two major fractions and a minor third one. On isoelectric focusing, three distinct fractions of specific tyrosine aminotransferase activity were obtained. The isoelectric points of these fractions were 4.9, 5.1 and 5.3, respectively. These findings imply that the human tyrosine aminotransferase consists of three subforms. No differences in properties studied could be found between the subforms. The coenzyme, pyridoxal 5'-phosphate, could be removed by dialysis.

Characterization of the metallothioneins induced in HeLa cells by dexamethasone and zinc

The metallothioneins induced in HeLa cells by dexamethasone and Zn2+ were labeled with [3H]cysteine and [35S]cysteine respectively. These labeled metallothioneins were compared to one another and to metallothioneins from human liver by (a) gel permeation chromatography on Sephadex G-75 columsn, (b) ion-exchange chromatograhy on DEAE-Sephadex A-25 columns, (c) electrophoresis on non-denaturing polyacrylamide gels, and (d) electrophoresis of reduced and alkylated proteins on dodecylsulfate/polyacrylamide gels. The dexamethasone-induced metallothioneins of HeLa cells, the Zn2+-induced metallothioneins of HeLa cells, and the metallothioneins of human liver were indistinguishable by these four criteria.

Studies on the different molecular forms of tyrosine aminotransferase from rat liver and hepatoma cells

In an attempt to clarify the significance of the separable forms of tyrosine aminotransferase, the enzyme from rat liver and from cultured hepatoma cells was studied by carboxymethyl-Sephadex chromatography. Our studies of the form conversion during the purification procedure of the enzyme, where all cellular components were quickly discarded, do not allow us to invoke a specific "converting factor", the existence of which in the particulate fraction has been suggested. Moreover the addition of serine protease inhibitors is not sufficient to prevent the classical conversion. More probably, several factors depending on the environmental conditions might influence different reactions which lead to a preferential conformation of the enzyme in vitro. The difference in the PO4- content of the various enzyme forms and the consecutive differences in negative charge may be the determining factor in the elution pattern of the three forms of the isolated soluble enzyme. This observation raises the possibility that phosphorylation might play a specific role in the regulation of tyrosine aminotransferase synthesis.

Subcellular distribution of a factor inactivating tyrosine aminotransferase. Study of its mechanism and relationship to different forms of the enzyme

The subcellular distribution of a tyrosine aminotransferase inactivating factor in rat liver has been investigated. Most of its activity is associated with plasma membranes, with minor amounts in mitochondria and endoplasmatic reticulum. The factor is also found in kidney and inactivates the enzyme reversibly in presence of cysteine, most likely by modification of -SH groups. ATP counteracts this inactivation only, when crude enzyme extracts are inactivated by purified subcellular fractions or when the purified enzyme is inactivated in presence of liver or kidney cortex homogenates. The relationship of this inactivation to reported different forms of the enzyme has been investigated. Form I of three different forms, that can be obtained by hydroxyl-apatite chromatography, is readily inactivated, form III can be partly converted to form I by incubation in presence of purified plasma membranes. The relationship of these findings to a possible multistep mechanism in the turnover of the enzyme discussed.