The activity of gamma-glutamyltranspeptidase in regenerating rat liver

gamma-Glutamyltranspeptidase is expressed at low levels in the liver of the male Fischer 344 rat where it exhibits 15-fold purification and 33% recovery in isolated plasma membranes. While the activity of the enzyme is unaltered in regenerating liver 24 h after partial hepatectomy, it increases steadily thereafter over a period of one week. Seven days after partial hepatectomy the enzyme is maximally activated: 5.6-fold in liver homogenates and 5.3-fold in isolated liver plasma membranes. The enzyme declines in activity over the next fourteen days and is expressed at normal levels three weeks after partial hepatectomy. These results demonstrate that the activity of gamma-glutamyltranspeptidase increases in regenerating liver but that the increase is out of phase with the proliferative response.     

Purification and properties of beta-alanine aminotransferase from rabbit liver

beta-Alanine aminotransferase from rabbit liver has been purified 1,700-fold over the initial liver extract. The purified enzyme was shown to be homogeneous by disc electrophoresis and SDS polyacrylamide electrophoresis. The molecular weight of the purified enzyme determined by gel filtration was 95,000 +/- 5,700 and the subunit molecular weight was 48,000 +/- 2,100. The enzyme showed absorption maxima at 282, 330, and 414 nm and contained only 1 mol of pyridoxal 5'-phosphate/mol of dimer. The pH optimum for enzyme activity was 8.8 and the Km values for beta-alanine and 2-oxoglutaric acid were calculated to be 3.9 and 1.4 mM, respectively. The enzyme catalyzed transamination of various omega-amino acids with 2-oxoglutaric acid, which was a favourable amino acceptor. beta-Alanine, gamma-aminobutyric acid, and beta-aminoisobutyric acid, which are naturally occurring substrates, were preferred amino donors, but taurine, alanine, ornithine, spermine, and spermidine were not. 6-Azauracil inhibited the enzyme activity with a Ki of approximately 1.5 mM. From the above properties, beta-alanine aminotransferase from rabbit liver was seen to closely resemble with 4-aminobutyrate aminotransferase from liver and brain.     

Sialyltransferase activity in regenerating rat liver

Liver microsomal fractions catalyse the transfer of sialic acid from CMP-N-acetyl-neuraminic acid to various exogenous acceptors such as desialylated fetuin, desialylated human Tamm–Horsfall glycoprotein and desialylated bovine submaxillary-gland mucin. An increase in the rate of incorporation of sialic acid into desialylated glycoproteins was found after a lag period (7h) in regenerating liver. The increase was maximum 24h after partial hepatectomy for all acceptors tested. At later times after operation the sialyltransferase activity remained high only for desialylated fetuin. No soluble factors from liver or serum of partially hepatectomized animals influenced the activity of the sialyltransferases bound to the microsomal fraction. The sensitivity of sialyltransferases to activation by Triton X-100, added to the incubation medium, was unchanged in the microsomal preparation from animals 24h after sham operation or partial hepatectomy. The full activity of sialyltransferases towards the various desialylated acceptors showed some differences. Human Tamm–Horsfall glycoprotein was a good acceptor of sialic acid only when desialylated by mild acid hydrolysis. After this treatment, but not after enzymic hydrolysis, a decrease in molecular weight of human Tamm–Horsfall glycoprotein was observed. Further, the sialyltransferase activity as a function of incubation temperature gave different curves according to the acceptor used. The relationship between the biosynthesis of glycoproteins by regenerating liver and the sialyltransferase activity of microsomal fraction after partial hepatectomy is discussed.     

Thymidine triphosphate dephosphorylating activity in regenerating rat liver

The enzyme activity of dephosphorylation of thymidine triphosphate was found in microsomal fraction of rat liver. The enzyme activity decreased at the time when [³H]thymidine incorporation into DNA of regenerating liver increased. When the [³H]thymidine incorporation was suppressed by 1,3-diaminopropane, the enzyme activity remained elevated. These results suggest that the enzyme activity appears to be closely linked to DNA synthesis.     

Diamine oxidase activity induction in regenerating rat liver

The synthesis and turnover of diamine oxidase (EC 1.4.3.6) activity was studied in regenerating rat liver after partial hepatectomy using inhibitors of protein and RNA syntheses. The administration to animals of cycloheximide or actinomycin D prevented the increase in diamine oxidase activity normally observed during the first hours after hepatectomy. The study of the turnover rate of diamine oxidase with cycloheximide demonstrated that the half-life of this enzyme was about 15 h in normal and regenerating liver. These results suggest that the rise in diamine oxidase activity in regenerating rat liver was due to the synthesis of new enzyme rather than to a lengthening of its turnover.     

Adenosine aminohydrolase activity in the regenerating rat liver

The specific activity of adenosine aminohydrolase in the regenerating rat liver is significantly increased 12 h after partial hepatectomy. There is a twofold increase in enzyme activity at 48 h, after which the activity begins to decline. However, increased values still persist 7 days postsurgery. The enzyme is located mainly in the soluble supernatant (90-95%) of the cell. The purified enzyme from 48-h regenerating liver and control liver has similar kinetic properties (Km 54-58 microM for adenosine), similar molecular weights (30,000-35,000), and are equally inhibited by an irreversible transition-state analog and a reversible competitive inhibitor. It is concluded that adenosine aminohydrolase in regenerating liver is an integral component of a salvage pathway designed for the reutilization of nucleotides, and thus helps maintain a "growth state" for the regenerating liver.     

Distribution pattern of alanine aminotransferase activity in rat liver

Summary Activities of the alanine aminotransferase were measured along the entire sinusoidal paths (1) between small portal tracts and central veins and (2) between regions of adjoining septal branches and central veins in the livers of male Wistar rats using a Lowry technique. The established profiles of enzyme activity give support to previous studies, suggesting functional heterogeneity of liver sinusoids and their abutting hepatocytes related to morphological differences of the sinusoidal bed.Dedicated to Professor Dr. T.H. Schiebler on occasion of his 65th birthdaySupported by grants of the Forschungsförderung des Landes Nordrhein-Westfalen, No. 40002585 and the Verein der Förderer und Freunded der Universität Köln     

Distribution pattern of alanine aminotransferase activity in rat liver

Activities of the alanine aminotransferase were measured along the entire sinusoidal paths (1) between small portal tracts and central veins and (2) between regions of adjoining septal branches and central veins in the livers of male Wistar rats using a Lowry technique. The established profiles of enzyme activity give support to previous studies, suggesting functional heterogeneity of liver sinusoids and their abutting hepatocytes related to morphological differences of the sinusoidal bed.     

The maximum of the histone acetyltransferase activity precedes DNA-synthesis in regenerating rat liver

A pronounced transitory increase of histone acetyltransferase (EC 2.3.1.48) activity before the onset of DNA replication is shown to occur in regenerating rat liver. This effect is followed by an increase in the H1/H1(0) ratio at the start of DNA synthesis; H1(0) de is replaced by H1. Histone acetylation and H1/H1(0) exchange are discussed as steps in the signal transduction for DNA replication.     

Identity of beta-alanine-oxo-glutarate aminotransferase and L-beta-aminoisobutyrate aminotransferase in rat liver

L-beta-Aminoisobutyrate served as an amino donor for purified beta-alanine-oxo-glutarate aminotransferase from rat liver when 2-oxoglutarate was employed as an amino acceptor, but the D-isomer did not. L-beta-Aminoisobutyrate acted as a competitive inhibitor with respect to beta-alanine and had a Ki of approximately 2.6 mM, which is the same value as the Km of 2.7 mM. When the crude extract was applied to a DEAE-Sepharose CL-6B column, L-beta-aminoisobutyrate aminotransferase and beta-alanine-oxo-glutarate aminotransferase activities were found in the same fractions with a single peak. Antiserum to rat liver beta-alanine-oxo-glutarate aminotransferase inhibited L-beta-aminoisobutyrate aminotransferase activity in rat liver in the same way as beta-alanine-oxo-glutarate aminotransferase activity.     

Diurnal variations in inducibility of rat liver tyrosine aminotransferase activity.

The activity of tyrosine aminotransferase (TAT) was measured in the livers of rats which were entrained to eat for the first 2 hours of a daily 12 hour dark period (‘2+22’ schedule) and were treated with the synthetic glucocorticoid dexamethasone and with glucagon at several times of day. TAT activity in untreated animals varies diurnally with a maximum 4 to 6 hours after the beginning of feeding. In both fed and fasted rats there was a small diurnal variation in inducibility by dexamethasone: in fed rats induction was greatest near the beginning of the dark period, shortly after feeding; in fasted rats induction increased towards the end of the dark period. Glucagon induction showed a marked diurnal variation in fed rats with a decrease coincident with the decline in control TAT activity after its food-induced peak. This variation did not appear to be depemdent on food intake, however, since the decline in inducibility occurred in fasted rats at the same time as in fed rats. Co-treatment with dexamethasone did not affect the decrease in glucagon inducibility. The diurnal variation in TAT induction may reflect a diurnal rhythm in the components of the enzyme synthesizing system (e.g. in the availability of mRNA or in enzyme degradation).