Subunit interaction elicited by partial inactivation with L-methionine sulfoximine and ATP differently affects the biosynthetic and gamma-glutamyltransferase reactions catalyzed by yeast glutamine synthetase

Inhibition of human glutamine synthetase by L-methionine-S,R-sulfoximine-relevance to the treatment of neurological diseases

At high concentrations, the glutamine synthetase inhibitor L-methionine-S,R-sulfoximine (MSO) is a convulsant, especially in dogs. Nevertheless, sub-convulsive doses of MSO are neuroprotective in rodent models of hyperammonemia, acute liver disease, and amyotrophic lateral sclerosis and suggest MSO may be clinically useful. Previous work has also shown that much lower doses of MSO are required to produce convulsions in dogs than in primates. Evidence from the mid-20th century suggests that humans are also less sensitive. In the present work, the inhibition of recombinant human glutamine synthetase by MSO is shown to be biphasic-an initial reversible competitive inhibition (K i 1.19 mM) is followed by rapid irreversible inactivation. This K i value for the human enzyme accounts, in part, for relative insensitivity of primates to MSO and suggests that this inhibitor could be used to safely inhibit glutamine synthetase activity in humans.

Phosphinothricin resistance in Aspergillus niger and its utility as a selectable transformation marker

Aspergillus niger is moderately susceptible to inhibition by phosphinothricin (PPT)-a potent inhibitor of glutamine synthetase. This growth inhibition was relieved by L-glutamine. PPT inhibited A. niger glutamine synthetase in vitro (K(I), 54 microM) and the inhibition was competitive with L-glutamate. The bar gene, imparting resistance to PPT, was successfully exploited as a dominant marker to transform this fungus. Very high PPT concentrations were required in the overlay for selection. Apart from bar transformants, colonies spontaneously resistant to PPT were frequently encountered on selection media. Reasons for such spontaneous resistance, albeit of moderate growth phenotype, were sought using one such isolate (SRPPT). The SRPPT isolate showed a 2-3-fold decrease in its glutamate uptake rate. Elevated external glutamate levels further suppressed the PPT-induced growth inhibition. Cellular entry of PPT could be through the L-glutamate uptake system thereby accounting for the observed spontaneous resistant phenotype. These results were useful in the fine-tuning of bar-selection in A. niger.

Ammonium permease-based sensing mechanism for rapid ammonium activation of the protein kinase A pathway in yeast

In the yeast Saccharomyces cerevisiae starvation for nitrogen on a glucose-containing medium causes entrance into G0 and downregulation of all targets of the PKA pathway. Re-addition of a nitrogen source in the presence of glucose causes rapid activation of trehalase and other PKA targets. Trehalase activation upon ammonium re-supplementation is dependent on PKA activity, but not on its regulatory subunit nor is it associated with an increase in cAMP. In nitrogen-starved cells, ammonium transport and activation of trehalase are most active in strains expressing either the Mep2 or Mep1 ammonium permease, as opposed to Mep3. The non-metabolizable ammonium analogue, methylamine, also triggers activation of trehalase when transported by Mep2 but not when taken up by diffusion. Inhibition of ammonium incorporation into metabolism did not prevent signalling. Extensive site-directed mutagenesis of Mep2 showed that transport and signalling were generally affected in a similar way, although they could be separated partially by specific mutations. Our results suggest an ammonium permease-based sensing mechanism for rapid activation of the PKA pathway. Mutagenesis of Asn246 to Ala in Mep2 abolished transport and signalling with methylamine but had no effect with ammonium. The plant AtAmt1;1, AtAmt1;2, AtAmt1;3 and AtAmt2 ammonium transporters sustained transport and trehalase activation to different extents. Specific mutations in Mep2 affected the activation of trehalase differently from induction of pseudohyphal differentiation. We also show that Mep permease involvement in PKA control is different from their role in haploid invasive growth, in which Mep1 sustains and Mep2 inhibits, in a way independent of the ammonium level in the medium.

Regulation of glutamine synthetase from the white button mushroom Agaricus bisporus

The regulation of glutamine synthetase (GS) from Agaricus bisporus was studied at the posttranscriptional level using a specific antibody fraction directed against purified GS. The cross-reactivity of the antiserum against various Agaricus species and other fungi was tested and low reactivity with the Ascomycetes was found. GS protein and activity levels were measured in cell-free extracts of mycelium grown on different N sources. In mycelium grown on glutamine or ammonium as N source, the biosynthetic GS activity is higher than the transferase activity. Moreover, the results show a correlation between GS biosynthetic activity, GS protein, and previously reported mRNA levels. Also, after addition of ammonium or glutamine to glutamate-utilizing cultures, transferase activity decreased more rapidly than biosynthetic activity and GS protein level. This suggests a conformational modification which only affects transferase activity.

Uptake of glutamate, not glutamine synthetase, regulates adaptation of mammalian cells to glutamine-free medium

Two cell lines (McCoy and MDCK) were studied in an attempt to understand the metabolic changes associated with adaptation to glutamine-free medium (GMEM + gmate). McCoy cells assumed normal growth rates after 2–3 passages in this medium whereas MDCK cells showed no growth in GMEM + gmate. The glutamine synthetase (GS) activity of both cell lines was elevated (up to × 9) as glutamine was depleted from normal media (GMEM + gmine). The high activity of GS was maintained during McCoy cell growth in GMEM + gmate. However, there was no apparent significant difference between the two cell lines in the pattern of changes of GS activity in response to glutamine. The cellular uptake rates of glutamine and glutamate from the medium differed significantly between the two cell lines. During the adaptation of McCoy cells to GMEM + gmate, the rate of glutamate uptake doubled to a value of 0.54 nmol/min per mg cell protein whereas the maximum value for MDCK cells was considerably lower (0.04 nmol/min per mg cell protein). We propose that the difference in intrinsic ability for glutamate transport accounts for the difference in growth response between the two cell lines in the glutamine-free medium.

Kinetic characterization and regulation of purified glutamine synthetase from brain of Clarias batrachus

Glutamine synthetase (GS) from brain of Clarias batrachus is purified to about 42-fold and characterized at optimum pH and temperature with respect to its kinetic parameters. Values for apparent Michaelis constant of the enzyme for L-glutamine, hydroxylamine and ADP are 50, 62.5 and 0.833 mM respectively. The very low apparent Km for ADP may be specially related to the expression of GS action under high energy bond and also be evidenced by the requirement of enzyme for a high ionic strength of the ADP. The study is extended by examining the effect of various amino acids and metabolites on GS activity in order to gain further understanding of the changes in kinetics and regulation. It reveals that whereas uridine monophosphate and glutamate act competitively with respect to L-glutamine, carbamylphosphate and asparagine act non-competitively. GS activity is markedly inhibited by leucine, aspartic acid and AMP but not by lysine. ATP and methionine sulfoximine behaved as potent inhibitors of the enzyme in vitro. It is suggested that the teleostean GS has most of the properties similar to those reported for mammalian and avian glutamine synthetases. However, it is proposed that kinetic regulation of this enzyme may play a significant role in ammonia detoxication and rate of formation of glutamine-derived neurotransmitters in fish brain.

Ammonia assimilating enzymes from cyanobacteria: in situ and in vitro assay using high-performance liquid chromatography

Assay systems for ammonia assimilating enzymes in cyanobacteria are reported. Glutamine synthetase, glutamate synthase, and glutamate dehydrogenase can be easily assayed in situ, after the cells are made permeable to the reagents, or in vitro. The method is based upon the quantitation of glutamine or glutamate after the separation, when needed, of their o-phthaldialdehyde derivatives by reverse-phase high-performance liquid chromatography on a C18 column. The isocratic elution and the fluorometric detection of the amino acid derivatives make the method fast, simple, sensitive, and free of the assay artifacts which can be produced in coupled assays or when spectrophotometric measurements are carried out in the turbid samples employed for in situ assays.