Mitochondrial ammonia metabolism and the proton-neutral theory of hepatic ammonia detoxication

Dietary manganese nano-particles improves gene regulation and biochemical attributes for mitigation of lead and ammonia toxicity in fish

In the present study, we explored the capability of manganese nanoparticles (Mn-NPs) to alleviate the toxicity induced by lead (Pb) and ammonia (NH3) toxicity in Oreochromis niloticus (GIFT strain). The experiment followed a completely randomized design, including a control group (Mn-NPs-0 mg kg-1 diet) and groups exposed to Pb and NH3 alongwith Mn-NPs at 2 and 3 mg kg-1. Cortisol levels were significantly elevated in Pb + NH3 group whereas reduced by Mn-NPs diets. Gene expressions of HSP 70, iNOS, CYP 450, and Cas 3a were notably upregulated by Pb + NH3 group and downregulated by Mn-NPs diets. The cellular metabolic enzymes were affected by Pb + NH3 exposure and mitigated by Mn-NPs diets. The liver and kidney exhibited reduced activities of catalase, superoxide dismutase, and glutathione-s-transferase with Mn-NPs diets. Concurrently, immune-related genes such as total immunoglobulin (Ig) and tumor necrosis factor (TNFα) were upregulated in the Mn-NPs-fed groups. Growth performance indicators, including weight gain %, feed conversion ratio, specific growth rate, protein efficiency ratio, and relative feed intake were adversely affected by Pb + NH3 stress but improvement with Mn-NPs diets. Genes associated with growth performance, such as growth hormone (GH), growth hormone regulatory (GHR1), and myostatin, exhibited enhancements in response to Mn-NPs diets. Digestive enzymes, including protease and amylase were also enhanced by Mn-NPs diets. Additionally, Mn-NPs diets led to a reduction in the bioaccumulation of lead. This study aims to investigate the role of Mn-NPs in mitigating the effects of lead and ammonia toxicity on fish by examining various biochemical and gene regulatory factors to enhance fish wellbeing.

The nuclear factor kappa B (NF-κB) signaling pathway is involved in ammonia-induced mitochondrial dysfunction

Hyperammonemia is very toxic to the brain, leading to inflammation, disruption of brain cellular energy metabolism and cognitive function. However, the underlying mechanism(s) for these impairments is still not fully understood. This study investigated the effects of ammonia in hippocampal astroglia derived from C57BL/6 mice. Parameters measured included oxygen consumption rates (OCR), ATP, cytochrome c oxidase (COX) activity, alterations in oxidative phosphorylation (OXPHOS), nuclear factor kappa B (NF-κB) subunits, key regulators of mitochondrial biogenesis (peroxisome proliferator-activated receptor gamma coactivator1-alpha (PGC-1α), calcium/calmodulin-dependent protein kinase II (CaMKII), cAMP-response element binding protein (CREB), nuclear factor (erythroid-derived 2)-like 2 (Nrf2), early growth response (Egr) factor family of proteins, and mitochondrial transcription factor A (TFAM). Ammonia was found to decrease mitochondrial numbers, potentially through a CaMKII-CREB-PGC1α-Nrf2 pathway in astroglia. Ammonia did not alter the levels of Egrs and TFAM in astroglia. Ammonia decreased OCR, ATP, COX, and OXPHOS levels in astroglia. To assess whether energy metabolism is reduced by ammonia through NF-κB associated pathways, astroglia were treated with ammonia alone or with NF-κB inhibitors such as Bay11-7082 or SN50. Mitochondrial OCR levels were reduced in the presence of NF-κB inhibitors; however co-treatment of NF-κB inhibitors and ammonia reversed mitochondrial deficits. Further, ammonia increased translocation of the NF-κB p65 into the nucleus of astroglia that correlates with an increased activity of NF-κB. These findings suggest that the NF-κB signaling pathway is putatively involved in ammonia-induced changes in bioenergetics in astroglia. Such research has critical implications for the treatment of disorders in which brain bioenergetics is compromised.

Excretory nitrogen metabolism and defence against ammonia toxicity in air-breathing fishes

With the development of air-breathing capabilities, some fishes can emerge from water, make excursions onto land or even burrow into mud during droughts. Air-breathing fishes have modified gill morphology and morphometry and accessory breathing organs, which would tend to reduce branchial ammonia excretion. As ammonia is toxic, air-breathing fishes, especially amphibious ones, are equipped with various strategies to ameliorate ammonia toxicity during emersion or ammonia exposure. These strategies can be categorized into (1) enhancement of ammonia excretion and reduction of ammonia entry, (2) conversion of ammonia to a less toxic product for accumulation and subsequent excretion, (3) reduction of ammonia production and avoidance of ammonia accumulation and (4) tolerance of ammonia at cellular and tissue levels. Active ammonia excretion, operating in conjunction with lowering of ambient pH and reduction in branchial and cutaneous NH₃ permeability, is theoretically the most effective strategy to maintain low internal ammonia concentrations. NH₃ volatilization involves the alkalization of certain epithelial surfaces and requires mechanisms to prevent NH₃ back flux. Urea synthesis is an energy-intensive process and hence uncommon among air-breathing teleosts. Aestivating African lungfishes detoxify ammonia to urea and the accumulated urea is excreted following arousal. Reduction in ammonia production is achieved in some air-breathing fishes through suppression of amino acid catabolism and proteolysis, or through partial amino acid catabolism leading to alanine formation. Others can slow down ammonia accumulation through increased glutamine synthesis in the liver and muscle. Yet, some others develop high tolerance of ammonia at cellular and tissue levels, including tissues in the brain. In summary, the responses of air-breathing fishes to ameliorate ammonia toxicity are many and varied, determined by the behaviour of the species and the nature of the environment in which it lives.

Ammonia production, excretion, toxicity, and defense in fish: a review

Many fishes are ammonotelic but some species can detoxify ammonia to glutamine or urea. Certain fish species can accumulate high levels of ammonia in the brain or defense against ammonia toxicity by enhancing the effectiveness of ammonia excretion through active NH4+transport, manipulation of ambient pH, or reduction in ammonia permeability through the branchial and cutaneous epithelia. Recent reports on ammonia toxicity in mammalian brain reveal the importance of permeation of ammonia through the blood-brain barrier and passages of ammonia and water through transporters in the plasmalemma of brain cells. Additionally, brain ammonia toxicity could be related to the passage of glutamine through the mitochondrial membranes into the mitochondrial matrix. On the other hand, recent reports on ammonia excretion in fish confirm the involvement of Rhesus glycoproteins in the branchial and cutaneous epithelia. Therefore, this review focuses on both the earlier literature and the up-to-date information on the problems and mechanisms concerning the permeation of ammonia, as NH(3), NH4+ or proton-neutral nitrogenous compounds, across mitochondrial membranes, the blood-brain barrier, the plasmalemma of neurons, and the branchial and cutaneous epithelia of fish. It also addresses how certain fishes with high ammonia tolerance defend against ammonia toxicity through the regulation of the permeation of ammonia and related nitrogenous compounds through various types of membranes. It is hoped that this review would revive the interests in investigations on the passage of ammonia through the mitochondrial membranes and the blood-brain barrier of ammonotelic fishes and fishes with high brain ammonia tolerance, respectively.

Discovery of an alternate metabolic pathway for urea synthesis in adult Aedes aegypti mosquitoes

We demonstrate the presence of an alternate metabolic pathway for urea synthesis in Aedes aegypti mosquitoes that converts uric acid to urea via an amphibian-like uricolytic pathway. For these studies, female mosquitoes were fed a sucrose solution containing (15)NH4Cl, [5-(15)N]-glutamine, [(15)N]-proline, allantoin, or allantoic acid. At 24 h after feeding, the feces were collected and analyzed in a mass spectrometer. Specific enzyme inhibitors confirmed that mosquitoes incorporate (15)N from (15)NH4Cl into [5-(15)N]-glutamine and use the (15)N of the amide group of glutamine to produce labeled uric acid. More importantly, we found that [(15)N2]-uric acid can be metabolized to [(15)N]-urea and be excreted as nitrogenous waste through an uricolytic pathway. Ae. aegypti express all three genes in this pathway, namely, urate oxidase, allantoinase, and allantoicase. The functional relevance of these genes in mosquitoes was shown by feeding allantoin or allantoic acid, which significantly increased unlabeled urea levels in the feces. Moreover, knockdown of urate oxidase expression by RNA interference demonstrated that this pathway is active in females fed blood or (15)NH4Cl based on a significant increase in uric acid levels in whole-body extracts and a reduction in [(15)N]-urea excretion, respectively. These unexpected findings could lead to the development of metabolism-based strategies for mosquito control.

Ammonia metabolism in Aedes aegypti

We investigated the mechanisms by which Aedes aegypti mosquitoes are able to metabolize ammonia. When females were given access to solutions containing NH(4)Cl or to a blood meal, hemolymph glutamine and proline concentrations increased markedly, indicating that ammonium/ammonia can be removed from the body through the synthesis of these two amino acids. The importance of glutamine synthetase was shown when an inhibitor of the enzyme was added to the meal causing the glutamine concentration in hemolymph to decrease significantly, while the proline concentration increased dramatically. Unexpectedly, we found an important role for glutamate synthase. When mosquitoes were fed azaserine, an inhibitor of glutamate synthase, the glutamine concentration increased and the proline concentration decreased significantly. This confirms the presence of glutamate synthase in mosquitoes and suggests that this enzyme contributes to the production of glutamate for proline synthesis. Several key enzymes related to ammonium/ammonia metabolism showed activity in homogenates of mosquito fat body and midgut. The mosquito genes encoding glutamate dehydrogenase, glutamine synthetase, glutamate synthase, pyrroline-5-carboxylate synthase were cloned and sequenced. The mRNA expression patterns of these genes were examined by a real-time RT-PCR in fat body and midgut. The results show that female mosquitoes have evolved efficient mechanisms to detoxify large loads of ammonium/ammonia.

Arginine metabolism: nitric oxide and beyond

Arginine is one of the most versatile amino acids in animal cells, serving as a precursor for the synthesis not only of proteins but also of nitric oxide, urea, polyamines, proline, glutamate, creatine and agmatine. Of the enzymes that catalyse rate-controlling steps in arginine synthesis and catabolism, argininosuccinate synthase, the two arginase isoenzymes, the three nitric oxide synthase isoenzymes and arginine decarboxylase have been recognized in recent years as key factors in regulating newly identified aspects of arginine metabolism. In particular, changes in the activities of argininosuccinate synthase, the arginases, the inducible isoenzyme of nitric oxide synthase and also cationic amino acid transporters play major roles in determining the metabolic fates of arginine in health and disease, and recent studies have identified complex patterns of interaction among these enzymes. There is growing interest in the potential roles of the arginase isoenzymes as regulators of the synthesis of nitric oxide, polyamines, proline and glutamate. Physiological roles and relationships between the pathways of arginine synthesis and catabolism in vivo are complex and difficult to analyse, owing to compartmentalized expression of various enzymes at both organ (e.g. liver, small intestine and kidney) and subcellular (cytosol and mitochondria) levels, as well as to changes in expression during development and in response to diet, hormones and cytokines. The ongoing development of new cell lines and animal models using cDNA clones and genes for key arginine metabolic enzymes will provide new approaches more clearly elucidating the physiological roles of these enzymes.

Distribution of phosphate-activated glutaminase isozymes in the chicken: absence from liver but presence of high activity in pectoralis muscle

The distribution of glutaminase expression in a uricotelic species, the chicken, has been examined using cDNA probes to the rat isozymes. The results suggest that chickens do not possess a glutaminase isozyme equivalent to the liver-type isozyme of mammalian liver. Measurements of enzymic activity also showed very low glutaminase activity in chicken liver. Extra-hepatic tissues in the chicken do express a glutaminase isozyme mRNA which is detected by rat kidney-type glutaminase cDNA. The abundance of this mRNA was highest in kidney and breast muscle and relatively abundant in brain, spleen and adipose tissue. Chicken small intestine expressed relatively low levels of the mRNA. The high level of glutaminase mRNA in chicken pectoralis muscle was accompanied by high glutaminase enzymic activity. In contrast, in mixed leg muscle glutaminase mRNA was barely detectable by Northern blot and glutaminase activity was relatively low. Starvation for 48 h resulted in a slight decrease in the activity of glutaminase in pectoralis muscle, but a large decrease in the relative abundance of the mRNA. The results suggest that in the chicken, hepatic glutamine hydrolysis is not quantitatively important, but skeletal muscle may be a major site of glutamine catabolism.

Compartmentation and kinetics of urea cycle enzymes in porcine enterocytes

We have recently reported the synthesis of urea from ammonia, glutamine and arginine in enterocytes of postweaning pigs. The present study was conducted to determine the compartmentation and kinetics of urea cycle enzymes in these cells. Carbamoyl phosphate synthase I (CPS I) and ornithine carbamoyltransferase (OCT) were located exclusively in mitochondria, whereas argininosuccinate synthase (ASS) and argininosuccinate lyase (ASL) were found in the cytosol. Arginase isozymes were present in both the cytosol and mitochondria of enterocytes, and differed in their sensitivity to heat inactivation. Except for OCT, Vmax values of urea cycle enzymes were much lower in enterocytes than in the liver of pigs, and vice versa for their Km values. Because of a low rate of ureagenesis in enterocytes compared with the liver, intestinal urea cycle enzymes may function primarily to synthesize citrulline. The co-localization of CPS I and OCT and a high activity of OCT in enterocyte mitochondria favors the intestinal synthesis of citrulline from ammonia, HCO3- and ornithine. Low activities of cytosolic ASS and ASL minimize the conversion of citrulline into arginine and therefore, the recycling of citrulline into ornithine via arginase in postweaning-pig enterocytes. These kinetic properties of intestinal urea cycle enzymes maximize the net synthesis of citrulline from glutamine and explain the release of large amounts of citrulline by the pig small intestine. The two compartmentally separated arginase isozymes in enterocytes may play an important role in regulating the intestinal metabolism of proline, nitric oxide and polyamines.