Regulation of the tyrosine oxidizing system in fetal rat liver

Inhibition of p-hydroxyphenylpyruvate dioxygenase by the diketonitrile of isoxaflutole: a case of half-site reactivity

p-Hydroxyphenylpyruvate dioxygenase (HPPD) catalyzes the formation of homogentisate from p-hydroxyphenylpyruvate and molecular oxygen. In plants, this enzyme is the molecular target of new families of very active bleaching herbicides. In the study presented here, we report for the first time on the purification to homogeneity of a plant enzyme, as obtained from recombinant Escherichia coli cells expressing a cDNA encoding carrot HPPD. The purified enzyme allowed us to carry out a detailed characterization of the inhibitory properties of a diketonitile (DKN), the active inhibitor formed from the benzoylisoxazole herbicide isoxaflutole. Inhibition kinetic analyses confirmed that DKN exerts a slow and tight-binding inhibition of HPPD, competitive with respect to the p-hydroxyphenylpyruvate substrate. The stoichiometry of DKN binding to HPPD determined by kinetic analyses or by direct binding of [(14)C]DKN revealed a half-site reactivity of DKN.

Levels of Cu-Zn and Mn superoxide dismutases in rat liver during development

Superoxide dismutase is the main scavenger of superoxide radicals in the mammalian body. The liver has high levels of two types of superoxide dismutase enzymes, cytosolic Cu-Zn and mitochondrial Mn enzymes. The aim of the present study was to investigate the content of two distinct superoxide dismutases in liver during the perinatal transition from a hypoxic to a hyperoxic environment. Both isozymes were purified to homogeneity and used as immunogens in rabbits. Antisera raised were found to recognize only polypeptides of molecular weight 16,900 or 23,400, which correspond to Cu-Zn and Mn superoxide dismutases, respectively. It was found that the level of Cu-Zn superoxide dismutase enzymatic activity and protein as assessed by immunoquantitation increased 10-fold during the postnatal period, reaching adult levels by 3 weeks. In contrast, the amount of Mn superoxide dismutase content increased only twofold to adult levels during the first week of life. Neither of the superoxide dismutases showed an alteration in specific activity or apparent molecular weight in rat livers during ontogeny. These results show that the levels of two intracellular superoxide dismutases are differentially elevated during the perinatal period. It is suggested that each dismutase plays a different physiological role for superoxide scavenging in liver as a function of the hypoxic/hyperoxic environment at birth.

The effect of dexamethasone and glucagon on the expression of hepatocyte plasma membrane proteins during development

The regulation of different maturational processes in the liver is believed to be influenced by the hormonal system. The aim of this study was to investigate the effect of two hormones, glucagon and dexamethasone, on levels of plasma membrane proteins in rat liver cells during late fetal and early postnatal stages of development. For this purpose, 18-day-old rat fetuses and 1-day-old newborns were treated with glucagon or dexamethasone and killed at 22 days of gestation and 3, 5 and 7 days of age, respectively. Postnuclei liver membranes were isolated using a sucrose gradient method and assessed for levels of specific membrane proteins. Asialoglycoprotein receptor and 110,000 Mr glycoprotein, denoted GP 110, representing the sinusoidal and bile canalicular domains, respectively, were quantitated using the immunoblot method. Membrane enzymes alkaline phosphatase, leucine aminopeptidase and gamma-glutamyl transferase were evaluated using enzymatic methods. The data showed that glucagon and dexamethasone have a differential effect on membrane constituents according to the stage of development. Glucagon increased the levels of membrane enzymes during the late fetal stage but had no effect on liver membrane proteins in the newborn animal. In contrast, although dexamethasone elevated GP 110 in fetal rat livers, none of the other marker proteins was significantly affected. On the other hand, in newborns dexamethasone reduced the amount of asialoglycoprotein receptor and alkaline phosphatase and leucine aminopeptidase enzyme activities but greatly augmented the level of gamma-glutamyl transferase. Thus, glucagon primarily affects plasma membrane proteins in late gestation while dexamethasone does so during the early postnatal period. The roles that these two hormones may play during ontogeny is discussed with respect to liver development.

Some aspects of amino acid metabolism in the rat fetus

1. In spite of an eventual catabolic phase during the last third of pregnancy, nitrogen retention seems to increase in pregnant rats. Furthermore, the high uterine blood flow and the high placental transfer of amino acids maintains an adequate nutrient supply to the fetuses. 2. The terminal rat fetus has a high circulating plasma amino acid level, as well as an increased free amino acid tissue pool when compared to its mother's. 3. In the rat fetus the development of enzymatic capabilities shows a sudden emergence (also denomined clustering) in late fetal life. In a general trend, the activities of enzymes related with amino acid metabolism are not well developed during rat fetal life. 4. The rate of amino nitrogen excretion in rat fetus is low, mainly due to the low development of urea cycle enzyme activities. 5. The rates of protein synthesis in many tissues are high in the rat fetus and they show a progressive decrease until delivery. On the other hand, the rates of protein breakdown are also higher during fetal life than in the adult.

Quantification of the importance of individual steps in the control of aromatic amino acid metabolism

The quantitative importance of the individual steps of aromatic amino acid metabolism in rat liver was determined by calculation of the respective Control Coefficients (Strengths). The Control Coefficient of tryptophan 2,3-dioxygenase for tryptophan degradation was determined in a variety of physiological conditions and with a range of activities of tryptophan 2,3-dioxygenase. The Control Coefficient varied from 0.75 with basal enzyme activity to 0.25 after maximal induction of the enzyme by dexamethasone. The remainder of the control for tryptophan degradation was associated with the transport of the amino acid across the plasma membrane, with only very small contributions from kynureninase and kynurenine hydroxylase. The Control Coefficients of tyrosine aminotransferase for tyrosine degradation were approx. 0.70 and 0.20 with basal and dexamethasone-induced tyrosine aminotransferase activities respectively; the Control Coefficients of the transport of the amino acid into the cell were 0.22 and 0.58 respectively. Phenylalanine hydroxylase was found to have a Control Coefficient for the degradation of phenylalanine of approx. 0.50 under conditions of basal enzyme activity; after maximal activation by glucagon, the Control Coefficient decreased to 0.12. The transport of phenylalanine was responsible for the remaining control in the pathway. These results have important implications, directly for the regulation of aromatic amino acid metabolism in the liver, and indirectly for the regulation of neuroamine synthesis in the brain.

Transport of the aromatic amino acids into isolated rat liver cells. Properties of uptake by two distinct systems

The transport of the aromatic amino acids into isolated rat liver cells was studied. There was a rapid and substantial binding of the aromatic amino acids, L-alanine and L-leucine to the plasma membrane. This has important consequences for the determination of rates of transport and intracellular concentrations of the amino acids. Inhibition studies with a variety of substrates of various transport systems gave results consistent with aromatic amino acid transport being catalysed by two systems: a 2-aminobicyclo(2,2,1)heptane-2-carboxylic acid (BCH)-insensitive aromatic D- and L-amino acid-specific system, and the L-type system (BCH-sensitive). The BCH-insensitive component of transport was Na+-independent and facilitated non-concentrative transport of the aromatic amino acids; it was unaffected by culture of liver cells for 24 h, by 48 h starvation, dexamethasone phosphate or glucagon. Kinetic properties of the BCH-inhibitable component were similar to those previously reported for the L2-system in liver cells. The BCH-insensitive component was a comparatively low-Km low-Vmax. transport system that we suggest is similar to the T-transport system previously seen only in human red blood cells. The results are discussed with reference to the importance of the T- and L-systems in the control of aromatic L-amino acid degradation in the liver.

Biochemistry of liver development in the perinatal period

Just before birth, changes occur in the metabolic capacities of rat liver so that the animal can adapt to changes in the substrate supply. In utero, glucose is the main energy-generating fuel and the liver metabolism is directed towards glucose degradation. The activities of the rate-limiting enzymes of glycolysis, hexokinase and phosphofructokinase, are high. In preparation for post-natal life, when the continuous glucose supply from the mother is interrupted, very large amounts of glycogen are stored in the late fetal liver. With the intake of the fat-rich and carbohydrate-poor milk diet, the animal develops the ability to synthesize glucose de novo from non-carbohydrate precursors. During suckling, metabolic energy is derived mainly from the beta-oxidation of fatty acids, which in turn is an essential prerequisite for the high rate of gluconeogenesis, by yielding acetyl-CoA for the activation of pyruvate carboxylase and by generating a high NADH/NAD ratio for the shift of the glyceraldehyde 3-phosphate dehydrogenase reaction in the direction of glucose formation.--The developmental adaptation of metabolism and the process of enzymatic differentiation are closely connected with the maturation of the endocrine system and the changes in the concentration of circulating hormones. The neonatal regulation of phosphoenolpyruvate carboxykinase and of tyrosine aminotransferase by variations in the hormonal milieu around birth, and also the interaction of hormonal and nutritional factors in the induction of serine dehydratase and glucokinase at the end of the suckling period, will be discussed in detail.

Development of tyrosine aminotransferase and para-hydroxyphenylpyruvate dioxygenase activities in fetal and neonatal human liver

In livers of fetuses of 220--340 g body wt, total cytosolic tyrosine aminotransferase activity was 1.0 nmol of product/mg of protein per min, and the corresponding values for autopsy livers of newborns of 740--1,475 g and 2,600--3,650 g were 1.5 and 5.7, respectively, as compared with the adult value of 12.7. On the other hand, para-hydroxyphenylpyruvate dioxygenase activity is at adult level already in fetuses less than 340 g body wt. The Km value for tyrosine of tyrosine aminotransferase (1 mM) was considerably higher than the corresponding value for para-hydroxyphenylpyruvate of para-hydroxyphenylpyruvate dioxygenase (50 micro M). These results suggest that tyrosine aminotransferase is the rate limiting enzyme in the catabolism of tyrosine in premature infants.

Induction of cytosolic tyrosine aminotransferase by dexamethasone in organ culture of fetal human liver

Total tyrosine aminotransferase activity in organ culture of fetal human liver is increased by 347% after 15 h incubation with 2 micro M of dexamethasone. Isoelectric focusing reveals that the increased activity is due to induction of cytosolic tyrosine aminotransferase (EC 2.6.1.5) while the activity of the nonspecific mitochondrial aspartate aminotransferase (EC 2.6.1.1) remains unchanged.

Tyrosine aminotransferase deficiency in mink (Mustela vision): a model for human tyrosinemia II

Mink pseudodistemper, a recessive disease associated with high blood tyrosine levels, is an animal analogue of the human inborn error of metabolism, tyrosinemia II. Affected mink and man have eye and skin lesions. Affected mink have no hepatic tyrosine aminotransferase (TAT) activity, as measured immunologically and biochemically. Hepatic mitochondrial aspartate aminotransferase is increased to 188% of control. This new genetic animal model of TAT deficiency should allow new studies of tyrosine metabolism.