Urease bacterial

Table 2 gives details of some conventional regimes (see British National Formulary, 2008). The efficacy of therapy can be checked by Radiocarbon-labelled urea breath testing (which depends upon release of labelled carbon dioxide by bacterial urease) or by testing gastric biopsy material for persistence of gastric urease, but should only be done after eradication therapy has been discontinued for at least a month, and whilst any anti-secretory treatment has been discontinued (because it tends to suppress but does not eradicate the organism). 

Fate of urea Urea diffuses from the liver, and is transported in the blood to the kidneys, where it is filtered and excreted in the urine. A portion of the urea diffuses from the blood into the intestine, and is cleaved to CO2 and NH3 by bacterial urease. This ammonia is partly lost in the feces, and is partly reabsorbed into the blood. In patients with kidney failure, plasma urea levels are elevated, promoting a greater transfer of urea from blood into the gut. The intestinal action of urease on this urea becomes a clinically important source of ammonia, contributing to the hyperam-... 

From bacterial action in the intestine Ammonia is formed fan urea by the action of bacterial urease in the lumen of the htes-tine. This ammonia is absorbed from the intestine by way of tie portal vein and is almost quantitatively removed by the liver ve conversion to urea. 

Bacterial urease. A major source of ammonia in liver (approximately 25%) is produced by the action of certain bacteria in the intestine that possess the enzyme urease. Urea present in the blood circulating through the lower digestive tract diffuses across cell membranes and into the intestinal lumen. Once urea is hydrolyzed by bacterial urease to form ammonia, the latter substance diffuses back into the blood, which transports it to the liver. 

X-ray structure determinations of three bacterial ureases have now been reported, as have those of a number of mutant enzymes and enzyme-inhibitor complexes. All structures of the native enzymes agree on the basic protein architecture the enzyme is a trimer of a/ 7 trimers with threefold symmetry.The a subunit (60.3 kDa) is the largest and consists of two domains, the larger of which is an (a//3)g barrel that contains the dinuclear nickel center. All three structures also contain a mobile flap that covers the active site and contains residues that interact with the active site. 

Urea entering the rumen is rapidly hydrolysed to ammonia by bacterial urease, and the rumen ammonia concentration is therefore liable to rise considerably. For this ammonia to be efficiently incorporated in microbial protein, two conditions... 

Illustration of protein synthesis as a function of fixed medium pH in the absence and then the presence of urea. Optimal protein synthesis is found at pH 7.0 and declines at pH 6.0 and is absent at pH 5.0 and 3.0 in the absence of urea. In the presence of urea, there is no effect at pH 7.0 but enhancement at pH 6.0 and a marked increase at pH 5.0 and 3.0. The role of urea and urease will be discussed in detail. UreA and UreB are the structural subunits of the bacterial urease. (SOD, superoxide dismutase.)... 

The fairly high content of urea in muscle tissue (1.3-2.1 g/kg) is characteristic of elasmobranchs (rays, sharks). The compound is decomposed to ammonia by bacterial urease during fish storage. 

The total amount of urea synthesized each day is several-fold higher than the amount that is excreted. Urea diffuses readily from the bloodstream into the large intestine, where it is hydrolysed by bacterial urease to carbon dioxide and ammonium. Much of the ammonium is reabsorbed and used in the liver for the synthesis of glutamate and glutamine, and then a variety of other nitrogenous compounds. Studies with urea show that a significant amount of label is found in essential amino acids. This may reflect intestinal bacterial synthesis of amino acids, or it may reflect the reversibility of the transamination of essential amino acids. 

Figure 10 shows that with the addition of urea at pH 7.4, there is no change in medium pH or cellular calcium or pH. At pH 6.5, there is a slight increase in medium pH and cell pH with a slow rise in cell calcium. However, at pH 5.8, where internal bacterial urease is activated, there is a rapid rise in medium pH, cell pH and cell calcium. These data are consistent with the expectation that internal bacterial urease is responsible for elevation of medium pH by activation of a urea transporter. Further, the increase of medium NH3 results in cell alkalinization and elevation of cell calcium, which may eventually result in cell damage or apoptosis. 

Figure 11. A hypothetical model of the causation of duodenal ulcer. Gastric acid activates bacterial urease in antral organisms, but most of the NH3 is converted to NH4. When the juice empties into the duodenum, more NH3 is present due to the higher pH. This permeant gas can then rapidly enter duodenal cells and can result in increased cell apoptosis and ulceration.

Bacterial catabolism of oral food residue is probably responsible for a higher [NHj] in the oral cavity than in the rest of the respiratory tract.Ammonia, the by-product of oral bacterial protein catabolism and subsequent ureolysis, desorbs from the fluid lining the oral cavity to the airstream.. Saliva, gingival crevicular fluids, and dental plaque supply urea to oral bacteria and may themselves be sites of bacterial NH3 production, based on the presence of urease in each of these materials.Consequently, oral cavity fNTi3)4 is controlled by factors that influence bacterial protein catabolism and ureolysis. Such factors may include the pH of the surface lining fluid, bacterial nutrient sources (food residue on teeth or on buccal surfaces), saliva production, saliva pH, and the effects of oral surface temperature on bacterial metabolism and wall blood flow. The role of teeth, as structures that facilitate bacterial colonization and food entrapment, in augmenting [NH3J4 is unknown. 

Cheng G, N Shapir, MJ Sadowsky, LP Wackett (2005) Allophanate hydrolase, not urease functions in bacterial cyannric acid metabolism. Appl Environ Microbiol 71 4437-4445. 

Both dietary and endogenous ammoniagenic substrates are removed from the intestinal lumen by the osmotic cathartic action of nonabsorbable disaccharides such as lactulose and lactitol. These compounds are currently the main therapeutic agents for chronic HE. The efficacy of oral lactulose for the treatment of HE has been established in controlled trials [41-43]. Besides having a cathartic effect, lactulose lowers the colonic pH as a result of the production of organic acids by bacterial fermentation. The decrease in pH creates an environment that is hostile to the survival of urease-producing intestinal bac-... 

For reactions in which one or more reactants or products is a gas, manometry (the measurement of pressure differences) can provide a convenient means for monitoring the course and kinetics of the reaction Thus, enzymes that can be assayed with this method include oxidases, urease, carbonic anhydrase, hydrogenase, and decarboxylases. For example, bacterial glutamate decarboxylase is readily assayed by utilizing a Warburg flask and measuring the volume of gas evolved at different times using a constant-pressure respirometer. ... 

In contrast to urease the nickel in other bacterial enzymes appears to have a redox function and to take up oxidation states Ni(I) and/or Ni(III). Fortunately these states have recently become better understood in inorganic systems (see the preceding review in this volume by... 

Ureases from several sources have been examined for enzymically active low molecular weight forms (47). It was noted that the 12 S forms from jack bean did not hybridize with that from B. pasteurii. It now seems probable that gastric urease is bacterial in origin (85). The oc-... 

Urease activity in soils has been found to reflect the bacterial count and content of organic matter. The urease isolated from an Australian forest soil (87) was crystallized and found to have a specific activity of 75 Sumner units (S.U.) per mg. The molecular weight species were estimated (sedimentation velocity) to be 42, 131, and 217 X 103. That urease activity persists in soils is shown by the finding that enzymic activities, including urease, could be demonstrated in soil samples over 8000 years old (88). 

Urease (urea amidohydrolase) is an enzyme first identified over a hundred years ago in bacterial extracts [22], The presence of urease is a virulence factor for some pathogenic bacteria [23,24], It is now known to occur also in plants, fungi, and invertebrates (see [24,25] for reviews). Urease from jack bean was the first enzyme to be crystallized, in 1926. Almost 50 years later its metal content was reexamined and it was found to contain two atoms of nickel per subunit [26]. Finally in 1995 the crystal structure of the enzyme from the enteric bacterium Klebsiella aerogenes was determined [27], Amino-acid sequence comparisons predict that the structures of the plant and bacterial enzymes are similar, although with different subunit arrangements. 

Prior to the crystallization of jack bean urease it was assumed by the biochemical community that enzymes had no ordered structure. In 1965 the first crystallographic evidence for the mechanism by which enzymes work when Phillips and his group solved the lysozyme structure [6], Details of the structure indicated how the enzyme could bind the oligosaccharides present in its target, bacterial cell wall peptidoglycans, and could respond to the binding event by changing its structure. 

Recently, Wada et al. (1999) have demonstrated that a daily oral dose of 10 mg of bovine LF for 3-4 weeks to H. pylori-infected mice significantly decreased the number of this bacterium colonising in the stomach. The authors suggested that the glycans present in LF may bind to the bacterial adhesins, thus interfering with the attachment of H. pylori to the epithelial cells. These findings are supported by another mouse model study (Dial et al., 1998), which showed that oral administration of 4 mg of bovine LF per day for 3 weeks reduces gastric urease activity and H. pylori colonisation in the stomach. 

Urea ((NH2)2CO) is excreted by larger organisms, can be a product of bacterial organic matter decomposition, and is a highly labile form of N for plankton nutrition (Bronk, 2002). Reports of concentrations in oceanic waters are relatively scarce, but are quite low (<0.5 pM Antia et al, 1991). There are currently two methods commonly used to measure urea concentrations—the urease method (McCarthy, 1970) and the monoxime method (Mulvenna and Savidge, 1992 Price and Harrison, 1987). 

CoUier, J. L., Brahamsha, B., and Palenik, B. (1999). The marine cyanobacterium Synechococcus sp. WH7805 requires urease (urea amidohydrolase, EC 3.5.1.5) to utilize urea as a nitrogen source molecular-genetic and biochemical analysis of the enzyme. Microbiol.—U.K. 145, 447—459. CoUos, Y. (1998). Covariation of ammonium and nitrate uptake in several marine areas Calculation artefact or indication of bacterial uptake Preliminary results from a review of 76 studies. In Integrated Marine System Analysis. Dehairs, F., Elskens, M., and Goeyens, L. (eds.). Vrije Universiteit, Brussel, pp. 121—138. 

Certain bacterial strains arc resistant to the action of mcthenamine because they elaborate urease, an enzyme that hydrolyzes urea to form ammonia. The resultant high urinary pH prevents the activation uf methenaminc. rendering it inc fcetivc. This problem can be overcome by the euadministra-tion uf the urease inhibitor acctohydroxamic acid (Litho.stat). 

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