Pyrroline-5-carboxylate synthesis from glutamate by rat intestinal mucosa. Subcellular localization and temperature stability

Citrulline, A Potential Biomarker of Radiation-Induced Small Intestine Damage

Radiation damage assessment of the small intestine is important in nuclear accidents or routine radiotherapy of abdominal tumors. This article reviews the clinical symptoms and molecular mechanisms of radiation-induced small intestinal damage and summarizes recent research on biomarkers of such damage. Citrulline is the most promising biomarker for the evaluation of radiation-induced small intestinal damage caused by radiotherapy and nuclear accidents. This article also summarizes the factors influencing plasma citrulline measurement investigated in the latest research, as well as new findings on the concentration of citrulline in saliva and urine after different types of radiation.

The Reactive Species Interactome: Evolutionary Emergence, Biological Significance, and Opportunities for Redox Metabolomics and Personalized Medicine

SIGNIFICANCE

Oxidative stress is thought to account for aberrant redox homeostasis and contribute to aging and disease. However, more often than not, administration of antioxidants is ineffective, suggesting that our current understanding of the underlying regulatory processes is incomplete. Recent Advances: Similar to reactive oxygen species and reactive nitrogen species, reactive sulfur species are now emerging as important signaling molecules, targeting regulatory cysteine redox switches in proteins, affecting gene regulation, ion transport, intermediary metabolism, and mitochondrial function. To rationalize the complexity of chemical interactions of reactive species with themselves and their targets and help define their role in systemic metabolic control, we here introduce a novel integrative concept defined as the reactive species interactome (RSI). The RSI is a primeval multilevel redox regulatory system whose architecture, together with the physicochemical characteristics of its constituents, allows efficient sensing and rapid adaptation to environmental changes and various other stressors to enhance fitness and resilience at the local and whole-organism level.

CRITICAL ISSUES

To better characterize the RSI-related processes that determine fluxes through specific pathways and enable integration, it is necessary to disentangle the chemical biology and activity of reactive species (including precursors and reaction products), their targets, communication systems, and effects on cellular, organ, and whole-organism bioenergetics using system-level/network analyses.

FUTURE DIRECTIONS

Understanding the mechanisms through which the RSI operates will enable a better appreciation of the possibilities to modulate the entire biological system; moreover, unveiling molecular signatures that characterize specific environmental challenges or other forms of stress will provide new prevention/intervention opportunities for personalized medicine. Antioxid. Redox Signal. 00, 000-000.

Idiosyncratic nutrient requirements of cats appear to be diet-induced evolutionary adaptations

Cats have obligatory requirements for dietary nutrients that are not essential for other mammals. The present review relates these idiosyncratic nutritional requirements to activities of enzymes involved in the metabolic pathways of these nutrients. The high protein requirement of cats is a consequence of the lack of regulation of the aminotransferases of dispensable N metabolism and of the urea cycle enzymes. The dietary requirements for taurine and arginine are consequences of low activities of two enzymes in the pathways of synthesis that have a negative multiplicative effect on the rate of synthesis. Cats have obligatory dietary requirements for vitamin D and niacin which are the result of high activities of enzymes that catabolise precursors of these vitamins to other compounds. The dietary requirement for pre-formed vitamin A appears to result from deletion of enzymes required for cleavage and oxidation of carotenoids. The n-3 polyunsaturated fatty acids (PUFA) requirements have not been defined but low activities of desaturase enzymes indicate that cats may have a dietary need for pre-formed PUFA in addition to those needed by other animals to maintain normal plasma concentrations. The nutrient requirements of domestic cats support the thesis that their idiosyncratic requirements arose from evolutionary pressures arising from a rigorous diet of animal tissue. These pressures may have favoured energy conservation through deletion of redundant enzymes and modification of enzyme activities to result in metabolites more suited to the cat's metabolism. However, this retrospective viewpoint allows only recognition of association rather than cause and effect.

Interaction between murine spf-ash mutation and genetic background yields different metabolic phenotypes

The spf-ash mutation in mice results in reduced hepatic and intestinal ornithine transcarbamylase. However, a reduction in enzyme activity only translates in reduced ureagenesis and hyperammonemia when an unbalanced nitrogen load is imposed. Six-week-old wild-type control and spf-ash mutant male mice from different genetic backgrounds (B6 and ICR) were infused intravenously with [(13)C(18)O]urea, l-[(15)N(2)]arginine, l-[5,5 D(2)]ornithine, l-[6-(13)C, 4,4,5,5, D(4)]citrulline, and l-[ring-D(5)]phenylalanine to investigate the interaction between genetic background and spf-ash mutation on ureagenesis, arginine metabolism, and nitric oxide production. ICR(spf-ash) mice maintained ureagenesis (5.5 +/- 0.3 mmol.kg(-1).h(-1)) and developed mild hyperammonemia (145 +/- 19 micromol/l) when an unbalanced nitrogen load was imposed; however, B6(spf-ash) mice became hyperammonemic (671 +/- 15 micromol/l) due to compromised ureagenesis (3.4 +/- 0.1 mmol.kg(-1).h(-1)). Ornithine supplementation restored ureagenesis and mitigated hyperammonemia. A reduction in citrulline entry rate was observed due to the mutation in both genetic backgrounds (wild-type: 128, spf-ash: 60; SE 4.0 micromol.kg(-1).h(-1)). Arginine entry rate was only reduced in B6(spf-ash) mice (B6(spf-ash): 332, ICR(spf-ash): 453; SE 20.6 micromol.kg(-1).h(-1)). Genetic background and mutation had an effect on nitric oxide production (B6: 3.4, B6(spf-ash): 2.8, ICR: 9.0, ICR(spf-ash): 4.6, SE 0.7 micromol.kg(-1).h(-1)). Protein breakdown was the main source of arginine during the postabsorptive state and was higher in ICR(spf-ash) than in B6(spf-ash) mice (phenylalanine entry rate 479 and 327, respectively; SE 18 micromol.kg(-1).h(-1)). Our results highlight the importance of the interaction between mutation and genetic background on ureagenesis, arginine metabolism, and nitric oxide production. These observations help explain the wide phenotypic variation of ornithine transcarbamylase deficiency in the human population.

Assay and subcellular localization of pyrroline-5-carboxylate dehydrogenase in rat liver

Delta1-pyrroline-5-carboxylate dehydrogenase (P5CDh) catalyzes the conversion of Delta1-pyrroline-5-carboxylate to glutamate in a reaction requiring NADP+ as a cofactor. Delta1-pyrroline-5-carboxylate is formed in liver from proline by proline oxidase (EC number not assigned) or from ornithine via ornithine aminotransferase. A spectrophotometric assay for P5CDh was shown to be valid if rotenone was included in the assay to prevent reoxidation of NADH. Using this new assay, liver was fractionated using differential centrifugation and the distribution of P5CDh was compared to that of appropriate marker enzymes. P5CDh is enriched only in the mitochondrial fractions, as are the mitochondrial enzymes, succinate cytochrome c reductase, proline oxidase, glutaminase, and ornithine aminotransferase. Thus, it can be concluded that P5CDh occurs only in mitochondria, not in both mitochondria and cytoplasm, as had previously been reported.

Endogenous synthesis of arginine plays an important role in maintaining arginine homeostasis in postweaning growing pigs

This study was conducted to determine whether endogenous synthesis of arginine plays a role in regulating arginine homeostasis in postweaning pigs. Pigs were fed a sorghum-based diet containing 0. 98% arginine and were used for studies at 75 d of age (28.4 kg body weight). Mitochondria were prepared from the jejunum and other major tissues for measuring the activities of Delta1-pyrroline-5-carboxylate (P5C) synthase and proline oxidase (enzymes catalyzing P5C synthesis from glutamate and proline, respectively) and of ornithine aminotransferase (OAT) (the enzyme catalyzing the interconversion of P5C into ornithine). For metabolic studies, jejunal enterocytes were incubated at 37 degrees C for 30 min in Krebs-Henseleit bicarbonate buffer containing 2 mmol/L L-glutamine, 2 mmol/L L-[U-14C]proline, and 0-200 micromol/L gabaculine (an inhibitor of OAT). The activities of P5C synthase, proline oxidase and OAT were greatest in enterocytes among all of the tissues studied. Incubation of enterocytes with gabaculine resulted in decreases (P < 0.05) in the synthesis of ornithine and citrulline from glutamine and proline. When gabaculine was orally administered to pigs (0.83 mg/kg body weight) to inhibit intestinal synthesis of citrulline from glutamine and proline, plasma concentrations of citrulline (-26%) and arginine (-22%) decreased (P < 0.05), whereas those of alanine (+21%), ornithine (+17%), proline (+107%), taurine (+56%) and branched-chain amino acids (+21-40%) increased (P < 0.05). On the basis of dietary arginine intake and estimated arginine utilization, the endogenous synthesis of arginine in the 28-kg pig provided >/=50.2% of total daily arginine requirement. Taken together, our results suggest an important role for endogenous synthesis of arginine in regulating arginine homeostasis in postweaning growing pigs, as previously shown in neonatal pigs.

Enzymes of delta 1-pyrroline-5-carboxylate metabolism in the camel tick Hyalomma dromedarii during embryogenesis. Purification and characterization of delta 1-pyrroline-5-carboxylate dehydrogenases

The activity of P5C metabolizing enzymes: OAT, P5CR, PO, and P5CD, in the camel tick Hyalomma dromedarii has been followed throughout embryogenesis. The profiles of enzymatic activity showed clear differences in the four enzymes as the embryos grew older. During purification of P5CD to homogeneity the ion exchange chromatography steps lead to two separate forms (termed A and B) with different molecular weights (60,000-59,000 and 50,000-52,000 for the native and denatured enzymes, respectively), amino acid composition, Km for P5C and coenzymes, varying dehydrogenase activities with different substrate specificity when supplied with various aldehyde substrates. Both P5CD A and B exhibited sharp optima at pH 7.5. The effect of different divalent cations and competitive and noncompetitive inhibitors was examined. The changes in P5C metabolizing enzymes during embryogenesis suggest that H. dromedarii has the metabolic potential to convert ornithine into proline and glutamate.

Enzymological evidence for the indispensability of small intestine in the synthesis of arginine from glutamate. I. Pyrroline-5-carboxylate synthase

The in vivo synthesis of arginine from glutamate in mammals requires seven enzymes to cooperate. Pyrroline-5-carboxylate synthase (PCS) is the first enzyme required. In order to establish the interorgan dependency of arginine synthesis, we quantitated PCS activity in as many as 32 rat tissues and found that the activity was concentrated only in the upper small intestine. Minor activity was found in pancreas, thymus, lymph node, and some other tissues: this was confirmed by the dependency on specific substrates, the loss of activity in the presence of an inhibitor, and identifying the reduced product as proline. No difference in activity was found between male and female rats on a milligram protein basis. The strict tissue localization of PCS and the localization of other enzymes of arginine synthesis previously reported clearly indicate that the upper small intestine is an indispensable tissue for the arginine synthesis from glutamate. Many of the tissues examined showed an activity to form an unknown product from glutamate. When assayed by the previously reported radiometric assay procedure using an AG1-X8 column (acetate), the product was not separated from PC and caused false-positive activities of PCS. An improved procedure was developed to overcome this technical difficulty. The new procedure enabled us to detect even 20 pmol PC without contamination by the adjoining unknown product. A preliminary characterization of the unknown product was achieved.

A radiochemical assay for a NADP+-specific gamma-glutamate semialdehyde dehydrogenase extracted from mitochondrial membrane of rat intestinal epithelial cells

A radiochemical assay has been developed for a NADP+-specific gamma-glutamate semialdehyde dehydrogenase from rat intestinal epithelial cells. The spectrophotometric assay utilized to measure the enzyme in bacterial cell homogenates is not sensitive enough for homogenates from rat mitochondria, which require an assay that can measure as little as 0.5 nmol NADPH formed/min/ml extract. The assay described here is sensitive to 0.1 nmol product formed/min/ml of extract and employs the use of [3H]pyrroline 5-carboxylate which is phosphorylated and oxidized by the enzyme to gamma-[3H]glutamyl phosphate, a product that decomposes to [3H]pyrrolidone 5-carboxylate. The latter product is separated from the substrate by ion-exchange chromatography. In order to correct for any product loss during separation by ion-exchange [14C]pyrrolidone 5-carboxylate is added as an internal standard to the deproteinized assay mixture. Under the assay conditions described mammalian gamma-glutamate semialdehyde dehydrogenase activity is linear with respect to time and protein concentration. Comparison between the kinetic parameters reported for the bacterial enzyme and those reported here for the mammalian enzyme indicate similarities in the pH optima as well as a requirement for phosphate. Kinetic studies on mammalian enzyme yield apparent Km values of 1.8 mM for pyrroline 5-carboxylate, 0.2 mM for NADP+, and 11.3 mM for phosphate.

Factors stabilizing pyrroline-5-carboxylate synthase of rat intestine mucosa at a physiological temperature

Mammalian pyrroline-5-carboxylate (PC) synthase in the mitochondrial membrane of rat small intestine mucosa possesses marked thermal instability at temperatures of 30 to 37 degrees C [Y. Wakabayashi, J. G. Henslee, and M. E. Jones (1983) J. Biol. Chem. 258, 3873-3882]. Factors stabilizing the enzyme activity at 37 degrees C were extensively examined by incubating the enzyme with various compounds before assay. In the presence of 60% sorbitol, the enzyme retained full activity for 30 min. Xylitol, glycerol, and fructose were also effective, although sucrose, ethylene glycol, polyethylene glycol and dimethyl sulfoxide were ineffective. AMP, GMP, IMP, and UMP (15 mM) were completely protective while ATP and adenosine were not. Phosphate and arsenate at 10 mM maintained 90 and 82%, respectively, of the original activity after 10 min. NADPH and NADP (3 mM) were protective whereas 3 mM NADH was not. The possibility that phosphate and NADPH are stabilizing PC synthase in vivo was discussed. Addition of 0.13 mM p-chloromercuriphenylsulfonic acid or 0.55 mM 5,5'-dithiobis-(2-nitrobenzoic acid) to the enzyme resulted in complete loss of activity, but prior addition of excess dithiothreitol to the enzyme prevented the inactivation, suggesting that a sulfhydryl group is involved in the activity.

A one-step assay for pyrroline-5-carboxylate synthase using a short AG1-X8 column

A rapid and simple method for assay of pyrroline-5-carboxylate synthase is presented. In this method, the incubation is terminated by raising the pH of incubation mixture to 10, and [14C]pyrroline 5-carboxylate produced from the substrate, [14C]glutamate, is first converted quantitatively to [14C]proline by reduction with NaBH4 at pH 10 and then the proline is allowed to pass through column of AG1-X8 anion exchanger under the conditions where the glutamate is completely retained by the column. Radioactive counting of the eluate gives the synthase activity. The entire procedure takes only one hour.