Molecular form and kinetic properties of phosphate-dependent glutaminase in the mitochondria isolated from the kidneys of normal and acidotic rats

Glutamine metabolism in metabolic acidosis

In chronic metabolic acidosis in the rat, there is increased ammoniagenesis, gluconeogenesis and renal extraction of glutamine with induction of renal phosphate-dependent glutaminase (PDG). Because the stimulus for these changes is not yet clear and also because acute acidosis is the more common clinical problem, the present study deals mainly with the metabolism of glutamine in acute metabolic acidosis. When acute metabolic acidosis is produced in rats by administration of mineral acid or by causing them to swim, thus inducing a severe lactic acidosis, a factor is found in the plasma which stimulates renal glutamine uptake and ammoniagenesis in vivo as well as in vitro. Acute acidosis does not induce synthesis of PDG in the kidney but causes a change in enzyme kinetics. The plasma factor not only enhances glutamine entry into cells, but apparently causes a conformational change in PDG, as shown by an increase in V1.0mM/Vmax. Intestinal metabolism of glutamine is also stimulated in vivo and in vitro by the plasma factor of acute acidosis.

Effects of mitochondrial swelling and calcium on phosphate-activated glutaminase in pig renal mitochondria

The effects of mitochondrial swelling and calcium have been used to study the possible function of the glutamine transporter in regulating glutamine hydrolysis. Salt-induced swelling of pig renal mitochondria and an iso-osmotic mixed salt solution and swelling caused by reducing the osmolarity of the incubation medium, are accompanied by activation of glutamine hydrolysis. Regulation of the glutaminase activity by salt-induced mitochondrial swelling is likely to have physiological importance, similar to the regulation of hepatic glutaminase by changing the matrix volume, that has been described by others. 0.1-1.0 mM calcium stimulates glutamine hydrolysis and the calcium activation curve follows Michaelis-Menten kinetics. The calcium activation is reversible, it is unaffected by phosphate, high glutamine and mitochondrial calcium uptake, as well as by sonication and the activation is calmodulin independent. The calcium activation is additive to that of swelling. Similar to calcium, hypo-osmotic swelling mainly increases the apparent Vmax for glutamine, whereas the apparent Km is little changed, indicating that the effects are primarily on the phosphate-activated glutaminase itself rather than on the glutamine transporter. Furthermore, calcium which activates glutamine hydrolysis, inhibits glutamine uptake into the mitochondria and so does alanine having no effect on glutamine hydrolysis. Therefore, it is indicative that glutamine transport is not rate limiting for glutamine hydrolysis.

Phosphate-activated glutaminase in the crude mitochondrial fraction (P2 fraction) from human brain cortex

The kinetics and other properties of phosphate-activated glutaminase have for the first time been studied in the crude mitochondrial fraction (P2 fraction) from human brain. The enzyme is for unexplained reasons inactivated postmortem. The enzyme activity decreases by storing the tissue or homogenate at 37 degrees C. The inactivation is not caused by formation of a dialysable inhibiting compound. No large proteolytic degradation has occurred, since the phosphate-activated glutaminase-like immunoreactive band did not disappear during the storage. The molecular weight of the subunit of the enzyme as determined by immunoblots of sodium dodecyl sulfate-treated homogenates from human brain is estimated to be approximately 64 K. The enzyme has been shown to have a pH optimum of 8.6; it is activated by phosphate, inhibited by glutamate, and partially inhibited by ammonia. Double-inverse plots of enzyme activity against phosphate are concave-upward, and more so in the presence of an inhibitor. The inhibition by glutamate appears to be noncompetitive with the substrate glutamine, and competitive with the activator phosphate. These kinetic properties are not significantly different from our earlier observations concerning phosphate-activated glutaminase from pig brain and pig kidney.

The orientation of phosphate-dependent glutaminase on the inner membrane of rat renal mitochondria

Phosphate-dependent glutaminase is associated with the inner membrane of rat renal mitochondria. The orientation of this enzyme was characterized by comparing its sensitivity in isolated mitochondria and in mitoplasts to two membrane impermeable inhibitors. Mitoplasts were prepared by repeated swelling of mitochondria in a hypotonic phosphate solution. This procedure released greater than 70% of the adenylate kinase from the intermembrane space, but less than 10 and 25% of the marker activities characteristic of the inner membrane and matrix compartments, respectively. The addition of 20 microM p-chloromercuriphenylsulfonate (pCMPS) caused a rapid inactivation of the purified glutaminase. In contrast, the glutaminase contained in isolated mitochondria and mitoplasts was only slightly affected by the addition of up to 2 mM pCMPS. Similarly, the activity in mitochondria and mitoplasts was not inhibited by the addition of an excess of inactivating Fab antibodies. However, a similar extent of inactivation occurred when either membrane fraction was incubated with concentrations of octylglucoside greater than 0.35%. Mitochondria were also treated with increasing concentrations of digitonin. At 0.4 mg digitonin/mg protein, all of the adenylate kinase was released but the glutaminase activity was either slightly inhibited or unaffected by the addition of pCMPS or the Fab antibodies, respectively. These studies establish that the glutaminase is localized on the inner surface of the inner membrane. Therefore, mitochondrial catabolism of glutamine must occur only within the matrix compartment.

The significance of the attachment of rat kidney glutaminase to the inner mitochondrial membrane

The inner mitochondrial membrane of rat kidney mitochondria was altered by 0.03% Triton X-100 treatment in such a way as to render it permeable to NAD and CoA molecules without release of phosphate-dependent glutaminase. A break of linearity in the Arrhenius plot of the enzyme activity was characteristic for a conformational change of a membrane-bound enzyme. The activity of phosphate-dependent glutaminase immobilized in the inner mitochondrial membrane, as studied in 0.03% Triton X-100-treated mitochondria, and solubilized, as in the supernatant of sonicated mitochondria, was hyperbolic with respect to glutamine concentration. Under optimal conditions (pH 8.6 and 100 mM phosphate) the Vmax and Km were 216 +/- 12 nmol/mg per min and 2.7 +/- 0.4 mM, respectively, for Triton X-100-treated mitochondria, and 121 +/- 8 nmol/mg per min and 15.9 +/- 1.8 mM for sonicated mitochondria. Under near physiological conditions (pH 7.8 and 20 mM phosphate), distinct differences in phosphate-dependent glutaminase kinetics were observed. The Vmax as 29.8 +/- 0.4 and 2.6 /- 0.3 nmol/mg per min and the apparent Km 1.55 +/- 0.06 and 24.5 +/- 6.6 mM for Triton X-100 and sonicated mitochondria, respectively. The sigmoidal activation by phosphate at pH 7.8 was significantly shifted to the left in Triton X-100-treated as compared to sonicated mitochondria. As opposed to the data obtained in sonicated mitochondria, the kinetics of phosphate-dependent glutaminase in 0.03% Triton X-100-treated mitochondria agreed quite well with those obtained in intact, rotenone-inhibited and metabolically active mitochondria. These results suggest that an attachment of phosphate-dependent glutaminase to the inner membrane of kidney mitochondria has a profound effect on its kinetics, particularly under near physiological conditions.

Fluorimetric assay of phosphate-activated glutaminase

A fluorimetric assay for the estimation of phosphate-activated glutaminase is presented. The liberated glutamate is separated from glutamine using a Dowex centrifugation technique allowing multiple samples to be rapidly analyzed. Glutamate is estimated fluorimetrically by reaction with o-phthaldialdehyde. Parameters for the assay were worker out based upon characterization of human frontal cortex glutaminase. High phosphate-activated glutaminase was found in cultured human skin fibroblasts and amniotic fluid cells and rat frontal cortex and striatum. Human caudate nucleus and frontal cortex activity was variable, but related in an exponential manner. Human and rat liver activity was markedly lower than brain activity.

Nitrogen metabolism in goldfish, Carassius auratus (L.) activities of amidases and amide synthetases in goldfish tissues

1. Activities of asparagine synthetase, asparaginase, glutamine synthetase and glutaminase have been determined in red muscle, white muscle, brain, kidney, liver and gills of goldfish. 2. Muscle and brain show a capacity for net amide synthesis, while liver and gills are capable of both amide synthesis and degradation. 3. These results are consistent with the hypothesis that amide synthesis and degradation functions as a mechanism controlling tissue ammonia levels and ammonia excretion rates.

Kinetic comparisons between soluble and membrane-bound glutaminase preparations from pig brain

The kinetic properties of membrane-bound glutaminase from pig brain have been compared with those of the soluble enzyme that had been preincubated with either Tris/HCl buffer or Tris/phosphate/borate buffer. The two preparations of the soluble enzyme were similar in the dependence of their activities on pH and in their inhibition by borate and some glutamine analogues. They differed markedly in their responses to phosphate and their inhibition by glutamate. The membrane-bound enzyme differed from the other preparations in showing a sigmoid dependence of initial velocity on glutamine concentration at higher pH values and being activated by borate ions. The apparently cooperative behaviour that has been previously reported for soluble preparations of glutaminase may have resulted from failure to measure the true initial rate of the reaction. The Km values for glutamine and the inhibitor and activator constants for glutamate and phosphate respectively were such as to indicate that changes in the concentrations of these compounds might affect the glutaminase activity in vivo. None of the enzyme preparations was inhibited significantly by ammonium ions at concentrations up to 50 mM. Considerable differences were found between the three preparations in their activation by compounds containing sulphate or sulphonate groups.

The stimulation of glutamine hydrolysis in isolated rat liver mitochondria by Mg2+ depletion and hypo-osmotic incubation conditions

1. In respiring rat liver mitochondria EDTA stimulates glutaminase activity measured in the presence of phosphate and HCO3- ions. The stimulation can be reversed by the addition of low concentrations of MgCl2. EGTA does not stimulate glutamine hydrolysis. 2. Glutaminase activity assayed in disrupted mitochondria is not significantly affected by EDTA or MgCl2. 3. The addition of EDTA results in a decrease in the concentration of phosphate required for half-maximal glutaminase activity. 4. Depletion of mitochondrial Mg2+ by the addition of the ionophore A23187 also stimulates glutamine hydrolysis in both the presence and the absence of EDTA. The effect of the ionophore can be abolished by the addition of MgCl2. 5. Hypo-osmotic incubation conditions increase the rate of mitochondrial glutamine hydrolysis. The effect of hypo-osmoticity on glutaminase is much less when EDTA is present. 6. It is suggested that glutaminase is partially and indirectly inhibited by endogenous mitochondrial Mg2+ and that the inner membrane may play a role in the regulation of glutaminase activity.

Localization and some properties of phosphate-dependent glutaminase in disrupted liver mitochondria

1. Glutaminase activity in frozen and thawed liver mitochondria was activated by NH4+, phosphate and HCO3-ions and also by ATP . 2. NH4+ and HCO3-ions decreased the requirement of the enzyme for phosphate. The activation by ATP was observed only in the presence of NH4+ or HCO3-ions. 3. In frozen-and-thawed mitochondria, the enzyme was loosely bound to the inner membrane, the Arrhenius plot showing a break at 23 degrees C. On sonication, glutaminase was detached from the membrane and the Arrhenius plot became linear. 4. The apparent Km for glutamine of the membrane-bound form was 6 mM, and that of the soluble form was 21 mM. 5. It is likely that the properties of glutaminase in the intact cell are dependent on the association of this enzyme with the mitochondrial membrane.

Mithcondrial glutamine permeability and renal ammonia production in metabolic acidosis

These experiments were undertaken to determine the correspondence between acidosis-induced in situ motochondrial glutamine uptake and the process by which glutamine moves across the mitochondrial membrane. Feeding rats 1.5% NH4Cl for 2 wk accelerated the in situ uptake rate from 0.12 +/- 0.08 to 1.89 +/- 0.28 mu mol/min or some 16-fold. To determine glutamine uptake independent of its metabolic conversion, D-glutamine was employed. In isolated mitochondria from non-acidotic rat kidneys, D-glutamine diffused into 71 +/- 10% of the mitochondria water volume; in acidotic mitochondria the diffusion volume increased to 127 +/- 16%. The reflection coefficients (sigma) for a series of amides, including glutamine, were determined by gravimetrically following volume decrements in increasing concentrations of solute; D-glutamine's sigma fell from 1.05 +/- 0.08 to 0.50 +/- 0.06 in acidotic rat kidney mitchondria, The permeability coefficients corresponding to the measured sigma were 10(-7) cm/s and 10(-5) cm/s in nonacidosis and acidosis, respectively. When viewed in situ proximal tubule mitochondria undergo dramatic alterations during chronic acidosis. These involved an enlargement in the mitochondria particularly in the basal region of the cell and a reduction in number. Furthermore, numerous autophagic vacuoles, containing mitochondria, appear in the basal region. The findings are consistent availability that becomes activated during acidosis as a consequence of mitochondrial glutamine permeability resulting in increased ammoniagenesis and accelerated organelle turnover.