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

Down-regulation of hepatic urea synthesis by oxypurines: xanthine and uric acid inhibit N-acetylglutamate synthase

We previously reported that isobutylmethylxanthine (IBMX), a derivative of oxypurine, inhibits citrulline synthesis by an as yet unknown mechanism. Here, we demonstrate that IBMX and other oxypurines containing a 2,6-dione group interfere with the binding of glutamate to the active site of N-acetylglutamate synthetase (NAGS), thereby decreasing synthesis of N-acetylglutamate, the obligatory activator of carbamoyl phosphate synthase-1 (CPS1). The result is reduction of citrulline and urea synthesis. Experiments were performed with (15)N-labeled substrates, purified hepatic CPS1, and recombinant mouse NAGS as well as isolated mitochondria. We also used isolated hepatocytes to examine the action of various oxypurines on ureagenesis and to assess the ameliorating affect of N-carbamylglutamate and/or l-arginine on NAGS inhibition. Among various oxypurines tested, only IBMX, xanthine, or uric acid significantly increased the apparent K(m) for glutamate and decreased velocity of NAGS, with little effect on CPS1. The inhibition of NAGS is time- and dose-dependent and leads to decreased formation of the CPS1-N-acetylglutamate complex and consequent inhibition of citrulline and urea synthesis. However, such inhibition was reversed by supplementation with N-carbamylglutamate. The data demonstrate that xanthine and uric acid, both physiologically occurring oxypurines, inhibit the hepatic synthesis of N-acetylglutamate. An important and novel concept emerging from this study is that xanthine and/or uric acid may have a role in the regulation of ureagenesis and, thus, nitrogen homeostasis in normal and disease states.

Mitochondrial dysfunction by gamma-irradiation accompanies the induction of cytochrome P450 2E1 (CYP2E1) in rat liver

Multiple biological effects are induced by ionizing radiation through dysfunction of cellular organelles, direct interaction with nucleic acids and production of free radical species. The expression of cytochrome P450s was assessed in the livers of 60Co gamma-irradiated rats. Three gray (G) of gamma-irradiation caused CYP2E1 induction with a 3.6-fold increase in the mRNA at 24 h, whereas the expression of CYP1A2 and CYP3A was not changed. Pharmacokinetics of chlorzoxazone, a specific substrate of CYP2E1, was studied in 3 G-irradiated rats. The area under the plasma concentration-time curve from time zero to infinity of 6-hydroxychlorzoxazone and the amount of 6-hydroxychlorzoxazone excreted in 8 h urine were both significantly greater than those in control rats. Hepatic CYP2E1 was not induced in rats exposed to 0.5-1 G of gamma-rays. Rats irradiated at 6-9 G accumulated doses of gamma-rays exhibited smaller increases in the mRNA due to liver injury than those irradiated at a single dose of 3 G gamma-rays. The plasma glucose and insulin levels were not altered in rats with 3 G of gamma-irradiation. As the exposure level of gamma-irradiation increased, the activity of hepatic aconitase, a key enzyme in energy metabolism in mitochondria, was 30-90% decreased. The amount of mitochondrial DNA per gram of wet liver was 50% decreased in rats exposed to 3 G of gamma-rays. These results demonstrated that gamma-ray irradiation at the exposure level inducing organelle dysfunction induced CYP2E1 in the liver, which might be associated with mitochondrial damage, but not with alterations in glucose or insulin levels.

Regulation of [15N]urea synthesis from [5-15N]glutamine. Role of pH, hormones, and pyruvate

We have utilized both [5-15N]glutamine and [3-13C] pyruvate as metabolic tracers in order to: (i) examine the effect of pH, glucagon (GLU), or insulin on the precursor-product relationship between 15NH3, [15N]citrulline, and, thereby, [15N]urea synthesis and (ii) elucidate the mechanism(s) by which pyruvate stimulates [15N] urea synthesis. Hepatocytes isolated from rat were incubated at pH 6.8, 7.4, or 7.6 with 1 mM [5-15N]glutamine and 0.1 mM 14NH4Cl in the presence or the absence of [3-13C] pyruvate (2 mM). A separate series of experiments was performed at pH 7.4 in the presence of insulin or GLU. 15NH3 enrichment exceeded or was equal to that of [15N]citrulline under all conditions except for pH 7.6, when the 15N enrichment in citrulline exceeded that in ammonia. The formation of [15N]citrulline (atom % excess) was increased with higher pH. Flux through phosphate-dependent glutaminase (PDG) and [15N]urea synthesis were stimulated (p < 0.05) at pH 7.6 or with GLU and decreased (p < 0.05) at pH 6.8. Insulin had no significant effect on flux through PDG or on [15N]urea synthesis. Decreased [15N]urea production at pH 6.8 was associated with depleted aspartate and glutamate levels. Pyruvate attenuated this decrease in the aspartate and glutamate pools and stimulated [15N]urea synthesis. Production of Asp from pyruvate was increased with increasing medium pH. Approximately 80% of Asp was derived from [3-13C]pyruvate regardless of incubation pH or addition of hormone. Furthermore, approximately 20, 40, and 50% of the mitochondrial N-acetylglutamate (NAG) pool was derived from [3-13C]pyruvate at pH 6.8, 7.4, and 7.6, respectively. Both the concentration and formation of [13C]NAG from [3-13C]pyruvate were increased (p < 0.05) with glucagon and decreased (p < 0.05) with insulin or at pH 6.8. The data suggest a correlation between changes in [15N]urea synthesis and alterations in the level and synthesis of [13C]NAG from pyruvate. The current observations suggest that the stimulation of [15N]urea synthesis in acute alkalosis is mediated via increased flux through PDG and subsequent increased utilization of [5-15N] of glutamine for [15N]citrulline synthesis and/or increased synthesis of NAG from glutamate and pyruvate. The opposite may have occurred in acute acidosis. Glucagon, but not insulin, stimulated [15N]urea synthesis via increased flux through PDG and synthesis of NAG. Pyruvate stimulated urea synthesis via increased availability of aspartate and/or increased synthesis of NAG. The formation of NAG and aspartate from pyruvate are both pH-sensitive processes.

Regulation of urea and citrulline synthesis under physiological conditions

Information on the regulation of urea synthesis in vivo was obtained by examining the relationship between ureagenesis in vivo, citrulline synthesis in vitro, and two factors currently hypothesized to exert short-term regulation of this pathway: the liver mitochondrial content of N-acetylglutamate (NAG) and substrate availability. Rats meal-fed for 4 h every day (4-20 schedule) or for 8 h every other day (8-40 schedule) were used. (1) The citrulline-synthesizing capacity of mitochondria from livers of rats on the 8-40 schedule exceeded the corresponding velocity of urea synthesis in vivo at all time points studied. (2) Mitochondrial NAG in these livers increased from 127 +/- 32 pmol/mg of protein at 0 h to 486 +/- 205 pmol/mg at 3 h after the start of a meal, and decreased thereafter, but the correlation between NAG content and the velocity of citrulline synthesis was not simple, suggesting that NAG is not the only determinant of the state of activation of carbamoyl phosphate synthase I. (3) In rats on the 4-20 schedule killed 1 h after the start of the meal, the liver content of ornithine, citrulline, arginine, glutamate, alanine and urea increased 2.1-12-fold with respect to the values at 0 h; glutamine decreased by 39%. (4) The combined findings indicate that in vivo, moment-to-moment control of the velocity of urea synthesis is exerted by substrate availability. (5) Digestion limits the supply of substrate to the liver, and prevents its ureagenic capacity from being overwhelmed following a protein-containing meal.

Kinetic properties of carbamoyl-phosphate synthase (ammonia) and ornithine carbamoyltransferase in permeabilized mitochondria

Previous studies using intact rat liver mitochondria have shown that the soluble matrix enzymes carbamoyl-phosphate synthase (ammonia) (CPS) and ornithine carbamoyltransferase (OCT) display some kinetic properties which would not be observed if they were homogeneously distributed in the matrix. In the present work we have extended these studies, using toluene-treated mitochondria which are fully permeable to substrates and inhibitors, yet retain 90% of their soluble enzymes. The results provide evidence of functional organization of CPS and OCT in situ. The major findings are as follows. (1) The apparent Km values of matrix OCT for carbamoyl phosphate and ornithine are respectively 8 and 2 times those measured for the soluble enzyme. delta-N-Phosphonacetyl-L-ornithine inhibits OCT in situ less than in solution, especially when carbamoyl phosphate is synthesized in the mitochondria rather than added to the medium. (2) During citrulline synthesis from endogenously generated carbamoyl phosphate, the concentration of the latter in permeabilized mitochondria is more than 10 times that in the medium, although the mitochondria are freely permeable to added molecules of this size. (3) Endogenously formed carbamoyl phosphate is used preferentially by OCT in situ; addition of a 200-fold excess of unlabelled carbamoyl phosphate has little effect on the conversion of labelled endogenously formed carbamoyl phosphate into citrulline by matrix OCT. (4) The synthesis de novo of carbamoyl phosphate from NH3, HCO3- and ATPMg is the same in the presence and absence of ornithine. (5) Studies with co-immobilized CPS and OCT gave results concordant with some of the above observations and with previous ones with intact mitochondria.

Effect of level of dietary protein on arginine-stimulated citrulline synthesis. Correlation with mitochondrial N-acetylglutamate concentrations

Increases in dietary protein have been reported to increase the rate of citrulline synthesis and the level of N-acetylglutamate in liver. We have confirmed this effect of diet on citrulline synthesis in rat liver mitochondria and show parallel increases in N-acetylglutamate concentration. The magnitude of the effect of arginine in the suspending medium on citrulline synthesis was also dependent on dietary protein content. Mitochondria from rats fed on a protein-free diet initially contained low levels of N-acetylglutamate, and addition of arginine increased the rate of its synthesis. Citrulline synthesis and acetylglutamate content in these mitochondria increased more than 5-fold when 1 mM-arginine was added. A diet high in protein results in mitochondria with increased N-acetylglutamate and a high rate of citrulline synthesis; 1 mM-arginine increased citrulline synthesis in such mitochondria by only 36%. The concentration of arginine in portal blood was 47 microM in rats fed on a diet lacking protein, and 182 microM in rats fed on a diet containing 60% protein, suggesting that arginine may be a regulatory signal to the liver concerning the dietary protein intake. The rates of citrulline synthesis were proportional to the mitochondrial content of acetylglutamate in mitochondria obtained from rats fed on diets containing 0, 24, or 60% protein, whether incubated in the absence or presence of arginine. Although the effector concentrations are higher than the Ka for the enzymes, these results support the view that concentrations of both arginine and acetylglutamate are important in the regulation of synthesis of citrulline and urea. Additionally, the effects of dietary protein level (and of arginine) are exerted in large part by way of modulation of the concentration of acetylglutamate.

Altered enzyme activities and citrulline synthesis in liver mitochondria from ornithine carbamoyltransferase-deficient sparse-furash mice

Male mice carrying the spfash mutation have 5-10% of the normal activity of ornithine carbamoyltransferase, yet are only slightly hyperammonaemic and develop quite well. A study of liver mitochondria from normal and spfash males showed that they differ in important ways. (1) The spfash liver contains about 33% more mitochondrial protein per g than does normal liver. (2) The specific activities of carbamoyl-phosphate synthetase (ammonia) and glutamate dehydrogenase are about 15% lower than normal in mitochondria from spfash mice, whereas those of beta-hydroxybutyrate dehydrogenase and cytochrome oxidase are 22% higher and 30% lower respectively. (3) In the presence of 10 mM-ornithine and the substrates for carbamoyl phosphate synthesis, coupled and uncoupled mitochondria from spfash mice synthesize citrulline at unexpectedly high rates, about 25 and 44 nmol/min per mg respectively. Though these are somewhat lower than the corresponding rates obtained with normal mitochondria, the difference does not arise from the deficiency in ornithine carbamoyltransferase, but from the lower carbamoyl-phosphate synthetase activity of the mutant mitochondria. (4) At lower external [ornithine] (less than 2 mM), a smaller fraction of the carbamoyl phosphate synthesized is converted into citrulline in spfash than in normal mitochondria. These studies show that what appears to be a single mutation brings about major adaptations in the mitochondrial component of liver. In addition, they clarify the role of ornithine transport and of protein-protein interactions in citrulline synthesis in normal mitochondria.

Measurements of mitochondrial volumes are affected by the amount of mitochondria used in the determinations

Mitochondrial water spaces were determined by centrifugal filtration, by using 3H2O and [14C]-sucrose, -mannitol, -inulin and -dextran. The volume (in microliter/mg of mitochondrial protein) of each of the spaces was inversely proportional to the amount of mitochondria (mg of protein) centrifuged. The dextran space (representing extramitochondrial water carried down with the mitochondria) decreased the most, and accounted for most of the changes observed in the other spaces. However, the calculated matrix and intermembrane spaces also decreased when increasing amounts of mitochondria were centrifuged. For each space, the same value was obtained when centrifugal filtration was done at 8000 and at 15,600 g, and when the mitochondria were incubated with the markers for 15 s to 5 min, indicating that sucrose, mannitol and inulin do not penetrate the matrix, nor does dextran penetrate the intermembrane space, under the incubation and centrifugation conditions generally used to measure mitochondrial spaces.

The apparent Km of ammonia for carbamoyl phosphate synthetase (ammonia) in situ

Experiments with carbamoyl phosphate synthetase (ammonia) in solution and in isolated mitochondria are reported which show the following. NH3 rather than NH4+ is the substrate of the enzyme. The apparent Km of NH3 for the purified enzyme is about 38 microM. The apparent Km for NH3 measured in intact isolated mitochondria is about 13 microM. This value was obtained for both coupled and uncoupled mitochondria and was unchanged when the rate of carbamoyl phosphate synthesis was increased 2-fold by incubating uncoupled mitochondria in the presence of 5 mM-N-acetylglutamate. According to the literature, the concentration of NH3 in liver is well below the measured apparent Km. On the basis of this and previous work we conclude that, quantitatively, changes in liver [NH3] and [ornithine] are likely to be the most important factors in the fast regulation of synthesis of carbamoyl phosphate and urea. This conclusion is consistent with all available evidence obtained with isolated mitochondria, isolated hepatocytes, perfused liver and whole animals.