Citrulline synthesis in rat-liver mitochondria

Maintenance of energy-linked functions in rat liver mitochondria

EGTA and EDTA are compared with respect to their ability to preserve ATP-requiring reactions in rat liver mitochondria. The presence of EGTA sustains the high ATP requirement of citrulline synthesis. EDTA does not, even with excess Mg2+. Carboxylation of pyruvate, which has a lower ATP demand, is not influenced by the type of chelating agent. In mitochondria stored for 24 h at 4 degrees C, EGTA is more effective than EDTA in preventing loss of these energy-linked functions.

Aroclor 1254 treatment and fasting influences on rat liver mitochondrial carbamoyl phosphate synthesis with ADP and ATP

Previous studies have shown that the polychlorinated biphenyl mixture, Aroclor 1254 (ARO), -induced wasting in male rats is associated with increased permeability of hepatic mitochondria. This was correlated with hyperuremia and stimulated urea synthesis, hypoglycemia and suppressed glucogenesis after an ammonium acetate injection, and decreased retention of assimilated nitrogen and food intake. For ARO-toxic rats (100 mg/kg, ip, for 1, 2, and 4 days) versus Tween 80-treated, ad libitum-fed controls, mitochondrial carbamoyl phosphate (CP) formation (the initial step in urea synthesis from NH4+) was progressively stimulated for the duration of treatment from NH4+ and ATP but not from NH4+ and ADP. ARO maximal stimulation of CP formation also correlated with significant loss in body weight. Mitochondrial ornithine transcarbamoylase synthesis of citrulline from ornithine and carbamoyl phosphate was also stimulated. In comparison to fasted rats (24 hr), mitochondrial CP synthesis from NH4+ was enhanced with ADP but not with ATP. This ARO uncoupling of mitochondrial NH4+ metabolism and stimulation of CP formation with exogenous ATP and citrulline synthesis may have resulted from increased availability of substrates and cofactors in the matrix space, leakage of enzymes from the matrix, or a combination of these effects. These results are consistent with an increased inner membrane permeability and fragility during isolation and assays. In agreement with our previous studies, the data show that ARO exposure poises hepatic mitochondria toward the synthesis of urea intermediates.

Modified methods for measuring citrulline and carbamoyl-beta-alanine with reduced light sensitivity and sucrose interference

Two modified procedures for the colorimetric quantitation of citrulline aimed primarily at the assay of ornithine transcarbamylase were developed. Both methods give highly reproducible results in a short period of time since color is developed in 25 min at 100 degrees C. One method is more sensitive than previous methods and the other is as sensitive but requires less than one-fifth as much acid for color development. The reduction in acid concentration results in the stability of the colored complex for at least 50 min under the laboratory lighting conditions and allows for the presence of 5 mumol sucrose and 0.03 mumol mercaptoethanol in the assay. A low-acid modification for quantitating carbamoyl-beta-alanine is described also and may be applicable to the assay of dihydropyrimidinase.

Gut-liver interaction in glutamine homeostasis: portal ammonia role in uptake and metabolism

The role of ammonia released by the gut on hepatic glutamine handling and metabolism was studied in postabsorptive anesthesized male Sprague-Dawley rats at spontaneous and elevated arterial glutamine concentrations. Glutamine handling and metabolite release across both organ beds were studied using arteriovenous concentration differences and simultaneously measured portal and hepatic venous plasma flows. At the spontaneous arterial glutamine load, fractional glutamine extraction, FE-Gln, by the gut and the liver was 24 and 10%, respectively. At the elevated glutamine load, gut glutamine uptake doubled, while FE-Gln remained at 24%; however, portal ammonia and alanine increased and decreased, respectively. In response, hepatic FE-Gln increased to 28% with a large release of glutamate and urea. The role of portal ammonia in modulating hepatic glutamine uptake was studied by infusing NH4HCO3 directly into the portal vein. Increasing the portal load promptly stimulated hepatic glutamine uptake and glutamate and urea release. Mitochondria isolated from these livers produced more glutamate from glutamine, suggesting ammonia activation of hepatic glutaminase flux; in addition, citrulline formation increased, suggesting a coupling of glutaminase flux to urea synthesis. The results are consistent with portal ammonia release acting as a key informational molecule in interorgan glutamine flow.

Differential effects of N-acetylglutamate on citrulline synthesis by coupled and uncoupled mitochondria

When rats were placed on a low-protein (5%) diet for 24 h or less, liver mitochondrial acetylglutamate decreased rapidly, carbamyl phosphate synthetase (ammonia) and ornithine transcarbamylase decreased little, and carbamyl phosphate synthesis (measured as citrulline) by isolated mitochondria occurred at very low rates. The matrix acetylglutamate content of these mitochondria, whether coupled or uncoupled, was increased similarly by preincubating them with added acetylglutamate, but citrulline synthesis increased from less than 1 to 2.3 nmol min-1 mg-1 in the coupled state, and from less than 1 to 35 nmol min-1 mg-1 in the uncoupled state. However, when coupled mitochondria were incubated with the substrates required for the synthesis of acetylglutamate in the matrix, citrulline synthesis increased to 48 nmol min-1 mg-1; this rate was similar to that of mitochondria from control rats (fed a normal diet). When mitochondria from controls were incubated with up to 5mM acetylglutamate, citrulline synthesis by coupled mitochondria was increased by 10 to 40%, while synthesis by uncoupled mitochondria was 1.5 to 4 times higher than that observed with the coupled mitochondria; matrix acetylglutamate in both conditions rose to levels similar to those in the medium. The reason for the different behavior of carbamyl phosphate synthetase (ammonia) in coupled and uncoupled mitochondria was not apparent; neither oxidative phosphorylation nor ornithine transport were limiting in the coupled system. These observations are an example of the restrictions imposed upon enzymatic systems by the conditions existing in the mitochondrial matrix, and of the different behavior of carbamyl phosphate synthetase in situ and in solution. In addition, they show that conclusions about the characteristics of the enzyme in coupled mitochondria based on observations made in uncoupled mitochondria are not necessarily justified.

The relationship between intramitochondrial N-acetylglutamate and activity of carbamoyl-phosphate synthetase (ammonia). The effect of glucagon

1. The relationship between urea synthesis, intracellular N-acetylglutamate and the capacity of rat-liver mitochondria to synthesize citrulline was investigated. 2. Treatment of rats with glucagon prior to killing results not only in an increased intramitochondrial ATP concentration and an increased capacity of the mitochondria to synthesize citrulline, but also in an increased concentration of intramitochondrial N-acetylglutamate. 3. Comparison of the rate of citrulline synthesis in mitochondria from glucagon-treated and from control rats, incubated under different conditions, shows that the increased N-acetylglutamate concentration after glucagon treatment is at least in part responsible for the observed increased capacity of the mitochondria to synthesize citrulline. 4. Ureogenic flux in isolated hepatocytes under different incubation conditions correlated with the intracellular concentration of N-acetylglutamate and with the capacity of the mitochondria to synthesize citrulline. 5. When isolated hepatocytes were incubated with NH3, ornithine, lactate and oleate, intracellular N-acetylglutamate increased about eightfold in the first 10 min; during this period the rate of urea synthesis increased considerably. 6. It is concluded that the concentration of intramitochondrial N-acetylglutamate plays an important role in the short-term control of flux through the urea cycle under different nutritional and hormonal conditions.

Relationship between intramitochondrial citrate and the activity of carbamoyl-phosphate synthase (ammonia)

The possibility of control of the activity of carbamoyl-phosphate synthase (ammonia) (EC 2.7.2.5) in rat-liver mitochondria by variation in the intramitochondrial free Mg2+ concentration has been investigated. Carbamoyl-phosphate synthase activity was measured by coupling the formation of carbamoylphosphate to the synthesis of citrulline in a reaction mixture containing ammonia, bicarbonate, a source of ATP, and ornithine. The synthesis of citrulline was inhibited by lowering the concentration of intramitochondrial free Mg2+. This could be achieved not only by depleting the mitochondria of Mg2+ (by adding the ionophore A23187), but also by increasing the intramitochondrial concentration of citrate. Under various conditions an inverse relationship between the rate of citrulline synthesis and the magnitude of the intramitochondrial concentration of citrate was observed. Inhibition of citrulline synthesis by intramitochondrial citrate could be partly reversed by addition of Mg2+ in the presence of A23187. Possible implications of the regulation of carbamoyl-phosphate synthase (ammonia) activity by intramitochondrial citrate for nitrogen metabolism in the liver are discussed.