Possible role of the membrane Na+/H+ antiport in ornithine decarboxylase induction by L-asparagine

Amino acids of transport systems A and N play certain important role in cell activation. For example, the presence of these amino acids is essential in the induction of ornithine decarboxylase by growth factors and hormones. At mM concentrations, each of these amino acids, particularly L-asparagine, can also induce the enzyme without being further metabolized or incorporated into proteins. We have reported that the addition of 10 mM L-asparagine to quiescent Reuber's H-35 rat hepatoma cells caused an immediate and transient increase in intracellular pH. Here we report that concomitant with the intracellular alkalinization was an increase in H+ extrusion which was amiloride-sensitive and Na+-dependent. The induction of ornithine decarboxylase by L-asparagine was also amiloride-sensitive.     

A role for ornithine in the regulation of putrescine accumulation and ornithine decarboxylase activity in Reuber H35 hepatoma cells

We investigated the ability of intracellular ornithine to alter both the biosynthesis of putrescine and the activity of ornithine decarboxylase in Reuber H35 hepatoma cells in culture incubated with 12-O-tetradecanoylphorbol 13-acetate (TPA). In confluent cultures of H35 cells, the addition of TPA (1.6 μM) caused the activity of ornithine decarboxylase to increase by more than 100-fold within 4 h. When exogenous ornithine (0.1–1.0 mM) was added to the culture medium with TPA, a marked dose-dependent increase in the production of putrescine was observed. The activity of ornithine decarboxylase in the same cultures incubated with ornithine decreased in a similar dose-dependent manner. The addition of arginine (0.1–1.0 mM) (but not lysine or histidine) to the H35 cells in culture concomitant with TPA also led to a relative increase in putrescine biosynthesis and a decrease in ornithine decarboxylase activity compared to cultures not receiving the amino acids. A similar response to exogenous ornithine and TPA was observed in a series of less confluent rapidly growing cultures which were in culture for a shorter period of time. The confluent cultures possessed a basal level of arginase (55 units/mg protein) which increased approx. 2-fold upon treatment with TPA. The intracellular concentration of ornithine in the unstimulated cells was in the order of 0.02–0.03 mM. Upon incubation of the cells with exogenous ornithine or arginine, the intracellular pools of these amino acids increased 4- to 8-fold.     

NADP-specific isocitrate dehydrogenase in regulation of urea synthesis in rat hepatocytes.

The effect of inhibition of NADP-specific isocitrate dehydrogenase (EC 1.1.1.42) by DL-threo-alpha-methylisocitrate (3-hydroxy-1,2,3-butanetricarboxylase) on urea synthesis was studied in isolated rat hepatocytes. alpha-Methylisocitrate substantially inhibited the rate of urea synthesis (35--84%) with substrates requiring net reductive amination of 2-oxoglutarate to glutamate for aspartate synthesis (i.e., L-serine, D-alanine, or NH4Cl + L-lactate). alpha-Methylisocitrate did not inhibit synthesis of urea from substrates not requiring reductive formation of glutamate (i.e. L-alanine, L-glutamine, L-asparagine, or NH4Cl + L-ornithine). The rate-limiting role of NADPH in urea synthesis was correlated with the decrease in NADPH content that occurred upon addition of NH4Cl or of alpha-methylisocitrate to hepatocytes incubated with lactate and pyruvate, indicating utilization of NADPH for reductive amination of 2-oxoglutarate and inhibition of NADPH generation via NADP-isocitrate dehydrogenase, respectively. Similar results were obtained with D-alanine and L-serine; however, alpha-methylisocitrate or NH4Cl did not substantially decrease NADPH content when L-alanine was the substrate. Inhibitors or ornithine--2-oxo acid transaminase (L-canaline or gabaculine) decreased the uptake of ornithine by hepatocytes and inhibited the alpha-methylisocitrate insensitive urea synthesis from ornithine and NH4Cl. Canaline did not inhibit urea synthesis from lactate, ornithine, and NH4Cl but the inhibition by alpha-methylisocitrate of urea formation from this combination was appreciably larger with canaline (approx. 82%) than without canaline (approx. 48%). Inhibition of urea synthesis from NH4Cl + lactate by alpha-methylisocitrate was partially prevented by oleate, octanoate, or 3-hydroxybutyrate. When the NADH content of hepatocytes was increased by 3-hydroxybutyrate, the addition of NH4Cl and/or alpha-methylisocitrate caused a decline in NADH (and NADPH) content, suggesting that reducing equivalents from NADH as well as from NADPH can support net reductive amination of 2-oxoglutarate when required for urea synthesis.     

Induction of ornithine decarboxylase by biliverdin in rat liver.

The administration of biliverdin (0.1mg/g of body weight) into the peritoneal cavity of rats resulted in the induction of ornithine decarboxylase in the liver. When the temporal relationships between the changes in intracellular adenosine 3', 5'-cyclic monophosphate (cyclic AMP) level, cyclic AMP-dependent protein kinase activity and the induction of ornithine decarboxylase were investigated, the concentration of cyclic AMP increased significantly 2 h after the administration of biliverdin, while cyclic AMP-dependent protein kinase was activated after 2–4 h. The hepatic ornithine decarboxylase activity began to increase 4 h after biliverdin injection. These results suggest that there is some sequential relationship between the increase of cyclic AMP, the activation of cyclic AMP-dependent protein kinase and the induction of ornithine decarboxylase although the direct correlation of these three events remains to be elucidated.     

Uptake of polyamines by the unicellular green alga Chlamydomonas reinhardtii and their effect on ornithine decarboxylase activity

Uptake of exogenous polyamines by the unicellular green alga Chlamydomonas reinhardtii and their effects on polyamine metabolism were investigated. Our data show that, in contrast to mammalian cells, Chlamydomonas reinhardtii does not contain short-living, high-affinity polyamine transporters whose cellular level is dependent on the polyamine concentration. However, exogenous polyamines affect polyamine metabolism in Chlamydomonas cells. Exogenous putrescine caused a slow increase of both putrescine and spermidine and, vice versa, exogenous spermidine also led to an increase of the intracellular levels of both spermidine and putrescine. No intracellular spermine was detected under any conditions. Exogenous spermine was taken up by the cells and caused a decrease in their putrescine and spermidine levels. As in other organisms, exogenous polyamines led to a decrease in the activity of ornithine decarboxylase, a key enzyme of polyamine synthesis. In contrast to mammalian cells, this polyamine-induced decrease in ornithine decarboxylase activity is not mediated by a polyamine-dependent degradation or inactivation, but exclusively due to a decreased synthesis of ornithine decarboxylase. Translation of ornithine decarboxylase mRNA, but not overall protein biosynthesis is slowed by increased polyamine levels.     

In vivo 13C nuclear magnetic resonance spectroscopy using a solenoid transmit/receiver coil

The administration of cadmium (1.25 mg as Cd2+/kg, ip.) to male rats resulted in a significant increase of hepatic and renal ornithine decarboxylase activity. The maximum increase of ornithine decarboxylase activity to about 10-fold of the controls was seen at 4 hr after the administration of cadmium, and the increased enzyme activity was returned to control levels by 12 hr. Cadmium produced somewhat dose-dependently the increase of ornithine decarboxylase activity. The increase of ornithine decarboxylase seen on the administration of cadmium was cancelled by pretreatment of rats with cycloheximide. The treatment of female rats with cadmium also caused the increase of hepatic ornithine decarboxylase activity, but not renal enzyme activity.     

Regulation of the activity of ornithine decarboxylase and S-adenosylmethionine decarboxylase in mammary gland and liver of lactating rats. Effects of starvation, prolactin and insulin deficiency.

1. Starvation caused a marked decrease in the activity of ornithine decarboxylase in mammary gland, together with a lesser decrease in the activity of S-adenosylmethionine decarboxylase and a marked fall in milk production. Liver ornithine decarboxylase and S-adenosylmethionine decarboxylase activities were unaffected. 2. Refeeding for 2.5 h was without effect on ornithine decarboxylase in mammary gland, but it returned the S-adenosylmethionine decarboxylase activity in mammary gland to control values and elevated both ornithine decarboxylase and S-adenosylmethionine decarboxylase in liver. 3. Refeeding for 5 h returned the activity of ornithine decarboxylase in mammary gland to fed-state values and resulted in further increases in S-adenosylmethionine decarboxylase in mammary gland and liver and in ornithine decarboxylase in liver. 4. Prolactin deficiency in fed rats resulted in decreased milk production and decreased activity of ornithine decarboxylase in mammary gland. The increase in ornithine decarboxylase activity normally seen after refeeding starved rats for 5 h was completely blocked by prolactin deficiency. 5. In fed rats, injection of streptozotocin 2.5 h before death caused a decrease in the activities of ornithine decarboxylase and S-adenosylmethionine decarboxylase in mammary gland, which could be reversed by simultaneous injection of insulin. Insulin deficiency also prevented the increase in S-adenosylmethionine decarboxylase in liver and mammary gland normally observed after refeeding starved rats for 2.5 h.     

Modulation of ornithine decarboxylase activity and ornithine decarboxylase-antizyme complex in rat heart by hormone and putrescine treatment

Ornithine decarboxylase was present in a cryptic, complexed form in an amount approximately equivalent to that of free ornithine decarboxylase activity in adult rat heart. Addition of isoproterenol (10 mg/kg) caused a notable rise in ornithine decarboxylase activity and a simultaneous decrease in the amount of the complexed enzyme. During the period of ornithine decarboxylase decay, when cardiac putrescine content had reached high values, the level of the complex increased above that of the control. Administration of putrescine (1.5 mmol/kg, twice) or dexamethasone (4 mg/kg) produced a decrease of heart ornithine decarboxylase activity, while it did not remarkably affect the level of complexed ornithine decarboxylase, therefore raising significantly the ratio of bound to total ornithine decarboxylase. Putrescine also elicited the appearance of free antizyme, concomitantly with the disappearance of free ornithine decarboxylase activity after 3-4 h of treatment. These results indicate that a significant amount of ornithine decarboxylase occurs in an inactive form in the heart under physiological conditions and that its absolute and relative levels may vary following stimuli which affect heart ornithine decarboxylase activity.     

Molecular mechanisms of the synergistic induction of ornithine decarboxylase by asparagine and glucagon in primary cultured hepatocytes

In primary cultures of adult rat hepatocytes maintained in a salts/glucose medium, a more than 100-fold increase in ornithine decarboxylase (EC 4.1.1.17) activity was caused by asparagine and glucagon in a synergistic manner. The synthesis rate of ornithine decarboxylase was determined by [35S]methionine incorporation into the enzyme protein, and the amount of ornithine decarboxylase-mRNA was measured by hybridization with a cloned rat liver ornithine decarboxylase-cDNA. The synthesis rate of ornithine decarboxylase was stimulated more than 20-fold by asparagine and glucagon together, but the amount of ornithine decarboxylase-mRNA was increased only 3-4-fold, indicating that translational stimulation was involved in the induction process. Asparagine alone stimulated the synthesis of ornithine decarboxylase without substantial effect on the amount of ornithine decarboxylase-mRNA, whereas glucagon alone increased the amount of ornithine decarboxylase-mRNA about 3-fold without a detectable change in either enzyme activity or enzyme synthesis. Asparagine, at least in part, also suppressed degradation of ornithine decarboxylase.     

Diamine-induced inhibition of liver ornithine decarboxylase

Repeated injections of 1,3-diaminopropane, a potent inhibitor of mammalian ornithine decarboxylase, induced protein-synthesis-dependent formation of macromolecular inhibitors or ;antienzymes' [Heller, Fong & Canellakis (1976) Proc. Natl. Acad. Sci. U.S.A.73, 1858-1862] to ornithine decarboxylase in normal rat liver. Addition of the macromolecular inhibitors, produced in response to repeated injections of diaminopropane, to active ornithine decarboxylase in vitro resulted in a profound loss of the enzyme activity, which, however, could be partly recovered after passage of the enzyme-inhibitor mixture through a Sephadex G-75 columin in the presence of 0.4m-NaCl. This treatment also resulted in the appearance of free inhibitor. In contrast with the separation of the enzyme and inhibitory activity after combination in vitro, it was not possible to re-activate, by using identical conditions of molecular sieving, any inhibited ornithine decarboxylase from cytosol fractions obtained from animals injected with diaminopropane. However, the idea that injection of various diamines, also in vivo, induces acute formation of macromolecular inhibitors, which reversibly combine with the enzyme, was supported by the finding that the ornithine decarboxylase activity remaining after diaminopropane injection appeared to be more stable to increased ionic strength than the enzyme activity obtained from somatotropin-treated rats. Incubation of the inhibitory cytosol fractions with antiserum to ornithine decarboxylase did not completely abolish the inhibitory action of either the cytosolic inhibitor or the antibody. A single injection of diaminopropane produced an extremely rapid decay of liver ornithine decarboxylase activity (half-life about 12min), which was comparable with, or swifter than, that induced by cycloheximide. However, although after cycloheximide treatment the amount of immunotitrable ornithine decarboxylase decreased only slightly more slowly than the enzyme activity, diaminopropane injection did not decrease the amount of the immunoreactive protein, but, on the contrary, invariably caused a marked increase in the apparent amount of antigen, after some lag period. The diamine-induced increase in the amount of the immunoreactive enzyme protein could be totally prevented by a simultaneous injection of cycloheximide. These results are in accord with the hypothesis that various diamines may result in rapid formation of macromolecular inhibitors to ornithine decarboxylase in vivo, which, after combination with the enzyme, abolish the catalytic activity but at the same time prevent the intracellular degradation of the enzyme protein.     

Nuclear ornithine decarboxylase. Electron microscope autoradiographic identification of the active enzyme using alpha-[5-3H]difluoromethylornithine

alpha-Difluoromethylornithine (DFMO) is an enzyme-activated irreversible inhibitor of ornithine decarboxylase, that forms a covalent bond with the active enzyme. The highly selective binding of tritium-labeled DFMO to ornithine decarboxylase in vivo, as identified by electron microscope autoradiography, was used to determine the intracellular distribution of the enzyme in the germ cells of a polychaete (Ophryotrocha labronica). In mid-oogenesis ornithine decarboxylase was predominantly located in the nurse cells, which are actively supporting growth of the oocytes. On the basis of biochemical analyses ornithine decarboxylase has been considered mainly cytoplasmic in its distribution. However, in metabolically active polychaete cells (oocytes, nurse cells, intestinal and body wall cells), binding sites for tritiated DMFO, indicating the presence of active ornithine decarboxylase, were as abundant in the nucleus. The nucleolus was the most densely labeled organelle in nurse cells and oocytes.     

Polyamine and ornithine metabolism during the germination of conidia of Aspergillus nidulans.

1. The activities of ornithine decarboxylase, S-adenosylmethionine decarboxylase and ornithine-2-oxoglutarate aminotransferase were studied during the first 24 h of conidial germination in Aspergillus nidulans. 2. Increases (over 100-fold) in the activities of ornithine decarboxylase and S-adenosylmethionine decarboxylase occurred during the emergence of the germ-tube and before the doubling of DNA and this was followed by a sharp fall in the activities of both enzymes by 16h. 3. The increase in ornithine decarboxylase could be largely suppressed if 0.6 mM-putrescine was added to the growth medium. 4. Low concentrations of cycloheximide, which delayed germination by 2h, caused a corresponding delay in the changes in ornithine decarboxylase activity. 5. Ornithine-2-oxoglutarate aminotransferase activity increased steadily during the first 24h of germination. 6. Ornithine or arginine in the growth medium induced higher activity of ornithine-2-oxoglutarate aminotransferase, but did not affect ornithine decarboxylase activity. 7. The significance of these enzyme changes during germination is discussed.     

Synergistic induction of ornithine decarboxylase by diacylglycerol, A23187, and cholera toxin in guinea pig lymphocytes

When guinea pig lymphocytes were cultured with 1-oleoyl-2-acetyl-glycerol (OAG), A23187, and cholera toxin, ornithine decarboxylase activity was induced synergistically, peaking at 6 h. Addition of 12-O-tetradecanoyl-phorbol 13-acetate (TPA), A23187, and dibutyryl cAMP caused the same kind of induction. Cholera toxin potentiated the ability of A23187 to induce ornithine decarboxylase, but not that of OAG. Dibutyryl cAMP augmented the induction caused by A23187 but not by TPA. These results suggest that both the activation of Ca++-sensitive, phospholipid-dependent protein kinase (protein kinase C) and the increase in intracellular levels of Ca++ and cAMP are necessary for this induction. cAMP may potentiate the induction by modulating a Ca++ messenger system other than that for protein kinase C activation.     

Regulation of ornithine decarboxylase activity by putrescine and spermidine in rat liver

The marked enhancement of the activity of ornithine decarboxylase (EC 4.1.1.17) in rat liver at 4 h following partial hepatectomy or the treatment with growth hormone could be almost completely prevented by intraperitoneal administration of putrescine. A single injection of putrescine to partially hepatectomized rats caused a remarkably rapid decline in the activity of liver ornithine decarboxylase with an apparent half-life of only 30 min, which is almost as rapid as the decay of the enzyme activity after the administration of inhibitors of protein synthesis. Under similar conditions putrescine did not have any inhibitory effect on the activity of adenosylmethionine decarboxylase (EC 4.1.1.50) or tyrosine aminotransferase (EC 2.6.1.5). Spermidine given at the time of partial hepatectomy or 2 h later also markedly inhibited ornithine decarboxylase activity at 4 h after the operation and, in addition, also caused a slight inhibition of the activity of adenosylmethionine decarboxylase.     

Development of a robust system for high-throughput colorimetric assay of diverse amino acid decarboxylases

Amino acid decarboxylases catalyze decarboxylation of amino acids into amines that possess wide industrial applications. As key enzymes in biobased production of industrially important amines such as cadaverine, putrescine and β-alanine, lysine decarboxylase, ornithine decarboxylase and aspartic acid decarboxylase have attracted increasing attention. To develop enzyme variants with superior catalytic properties, there is a great need for high-throughput assay of these decarboxylases. Here we report the development of assays based on the color change of pH indicator – chlorophenol red (CPR) or bromothymol blue (BTB) – in decarboxylation reactions, in which one proton was consumed per carboxylic group decarboxylated resulting in an increase in pH. First, two buffer-indicator pairs, 4-morpholineethanesulfonic acid (MES)-CPR and 3-morpholinopropanesulfonic acid (MOPS)-BTB, were chosen on the basis of their similar pK_a values at approximately pH 6.0 and 7.0, both of which are physiologically relevant. Next, the effects of buffer strength and indicator concentration on absorbance changes were examined in assay mixtures with NaOH titration, which mimicked proton consumption in decarboxylation reactions. Finally, high-throughput quantification of lysine decarboxylase, ornithine decarboxylase and aspartic acid decarboxylase was achieved using a microplate format. These results suggest that our indicator assay system may have potential applications for screening diverse decarboxylases.     

Ornithine decarboxylase activity from an extremely thermophilic bacterium,Clostridium thermohydrosulfuricum effect of GTP analogues on enzyme activity

The activity of ornithine decarboxylase has been detected for the first time in extracts of a thermophilic bacterium, Clostridium thermohydrosulfuricum. The temperature optimum of the thermoresistant ornithine decarboxylase was 55°C and the pH optimum was 7.5. It required pyridoxal phosphate and a thiol (dithiothreitol) for activity. The activity of the enzyme was closely connected to the growth of the thermophilic bacteria, since the activity was highest during the logarithmic growth. The enzyme was not inhibited (in contrast to the enzyme from Escherichia coli) by putrescine, spermidine or other naturally occurring polyamines. When the effect of GTP and a number of GTP analogues was tested on the activity of the enzyme, it was observed that GTP or dGTP was necessary for the full activity. The modification of either the purine base or 5′-phosphate chain of GTP leads to a stimulation smaller than that caused by GTP. Modification of the 3′-carbon of the ribose part of GTP (magic spot I and II of Cashel and Gallant, Nature 221 (1969) 838–841) caused a distinct inhibition of the enzyme activity, indicating that ornithine decarboxylase contains at least two domains for binding of GTP. The enzyme was inhibited irreversibly by high concentrations (50 mM) of difluoromethylornithine. Extracts of the bacterium contained also arginine decarboxylase, but its activity was always very much lower than that of ornithine decarboxylase. The activity of arginine decarboxylase was inhibited irreversibly by difluoromethylarginine, which is an irreversible suicide inhibitor of bacterial arginine decarboxylase (Kallio, A., McCann, P.P. and Bey, P. (1981) Biochemistry 20, 3163–3166).     

Limitations of the Moeller lysine and ornithine decarboxylase tests.

A total of 40 fecal and environmental isolates, including 26 Escherichia coli strains, 9 members of the genus Klebsiella, and 5 members of the genus Enterobacter, were tested by enzyme assay for their endogenous and induced levels of lysine decarboxylase and ornithine decarboxylase when grown in Moeller decarboxylase medium. All of the coliforms examined had measurable lysine decarboxylase and ornithine decarboxylase activities whether or not they were positive in the Moeller test. In general, the Moeller lysine decarboxylase test reflected the inducibility of lysine decarboxylase whereas the Moeller ornithine decarboxylase test did not relect the inducibility of ornithine decarboxylase. Neither test measured the amount of intracellular enzyme; rather, they indicated whether the amount of polyamine liberated was sufficient to raise the pH of the culture medium above 7. Changing the growth conditions (i.e., the concentrations of glucose, lysine, and amino acids other than lysine) greatly influenced the lysine decarboxylase activity in coliforms. The limitations on the interpretation of the Moeller test results are discussed.     

Ammonium chloride-induced acidification in renal TALH SVE.1 cells monitored by 31P-NMR.

The aim of this study was to investigate the effect of NH4+ on the intracellular pH in TALH SVE.1 cells derived from the medullary thick ascending limb of Henle's loop (TALH) of rabbit kidney. These cells are specialized to perform NH4+ transport in vivo. Intracellular pH was monitored by 31P-NMR. The steady state intracellular pH (pHi) under standard conditions was 7.24 +/- 0.04 (n = 46). Exposure to NH4Cl resulted in an initial intracellular acidification of the TALH SVE.1 cells, followed by a recovery to the initial steady-state pHi value. The NH4(+)-induced acidification followed saturation kinetics up to 20 mM NH4Cl (delta pHmax = 0.2 pHunits). Half-maximal acidification was observed at 0.6 mmol/l. The intracellular acidification due to NH4Cl exposure was completely inhibited by 0.1 mM of the diuretic bumetanide, an inhibitor of the Na+/K+/2Cl- cotransporter. The effect of bumetanide was dose-dependent and a Ki value of 8.10(-7) M was calculated. NH4+ influx via K+ channels or the (Na+ + K+)ATPase could not be detected. pHi recovery to the initial value was caused mainly by amiloride-sensitive Na+/H+ exchange and to a lesser extent by an amiloride-insensitive system, which was not studied in detail. In the presence of bumetanide, pulses of high concentrations of NH4Cl induced small intracellular alkalinizations. From these experiments, an intrinsic buffer capacity (beta i) in TALH SVE.1 cells of 26 +/- 3 mM x pH-1 (pHi = 7.65) was determined. It could also be shown that the TALH SVE.1 cells exhibit maximal 'functional buffer capability' between pHout 6.9 and 7.3. Within these limits the cells can maintain their intracellular pH at a constant level, even though the extracellular pH changes. These data strongly suggest that the Na+/K+/2Cl- cotransporter is the main site of NH4+ entry into rabbit thick ascending limb cells in culture. A high intracellular buffer capacity and potent acid extrusion mechanism cooperate in counteracting the intracellular acidification caused by NH4+ influx into the cell.