Urease-catalyzed urea synthesis.

The equilibrium of hydrolytic reactions can be shifted toward condensation by carrying out the reaction at low water concentration. The rate and yield of urease-catalyzed urea synthesis from (NH₄)₂CO₃ or NH₄HCO₃ has been examined as a function of water concentration (in mixtures with organic solvents), substrate and H⁺ concentration, and polarity of the nonaqueous component of the solvent. Similar effects of organic solvents are observed on the reaction rate in both directions; the results suggest that at least in some conditions the reaction proceeds through nonenzymically formed carbamate. The equilibrium concentration of urea, in 50% ($\text{v}\text{v}$) water, varies over 10-fold, depending on the nature of the nonaqueous component of the solvent; nonhydroxylic solvents such as acetone given the highest yield. Solubility measurements suggest that the interactions of the solvent mixtures with (NH₄)₂CO₃ (or carbamate), rather than urea, are responsible for the variations in urea yield. Activities of water and the ionic components of the equilibrium are strongly influenced by the nature of the nonaqueous component of the solvent, as well as its concentration.     

Sources of ammonia for mammalian urea synthesis.

The initial rate of incorporation of [15N]alanine into the 6-amino group of the adenine nucleotides in rat hepatocytes was about one-eighteenth of the rate of incorporation into urea. Thus the purine nucleotide cycle cannot provide most of the ammonia needed in urea synthesis for the carbamoyl phosphate synthase reaction (EC 2.7.2.5). On the other hand, contrary to the view expressed by McGivan & Chappell [(1975) FEBS Lett. 52, 1--7], the experiments support the view that hepatic glutamate dehydrogenase can supply the required ammonia.     

Effects of adrenalectomy on urea synthesis in rats

The effect of depletion of glucocorticoids on the dynamics of hepatic amino-N conversion was examined 2 and 7 days after adrenalectomy in a total of 22 rats substituted by adrenaline. The capacity of urea synthesis was studied by infusion of alanine under steady state conditions with arterial concentrations of alanine between 7.3 and 11.6 mmol/l. The animals were nephrectomized and the capacity was calculated as accumulation of urea in total body water corrected for intestinal hydrolysis. Adrenalectomy reduced the capacity of urea synthesis to 55% of the capacity for control rats and reduced the alanine metabolic rate to 60%. In control rats the urea synthesis exceeded the alanine infusion by indicating an extrahepatic tissue release of amino acids. This difference disappeared after adrenalectomy. The body weight and food intake did not change during the study period. Thus lack of glucocorticoids influences the in vivo nitrogen economy both by decreasing the liver function as to conversion of amino-nitrogen and by decreasing release of tissue amino-nitrogen.     

Helicobacter pylori utilises urea for amino acid synthesis

Abstract Helicobacter pylori has one of the highest urease activities of all known bacteria. Its enzymatic production of ammonia protects the organism from acid damage by gastric juice. The possibility that the urease activity allows the bacterium to utilise urea as a nitrogen source for the synthesis of amino acids was investigated. H. pylori (NCTC 11638) was incubated with 50 mM urea, enriched to 5 atom% excess ¹⁵N, that is the excess enrichment of ¹⁵N above the normal background, in the presence of either NaCl pH 6.0, or 0.2M citrate pH 6.0. E. coli (NCTC 9001) was used as a urease-negative control. ¹⁵N enrichment was detected by isotope ratio mass spectrometry. H. pylori showed intracellular incorporation of ¹⁵N in the presence of citrate buffer pH 6.0 but there was no significant incorporation of ¹⁵N in unbuffered saline or by E. coli in either pH 6.0 citrate buffer or unbuffered saline. The intracellular fate of the urea-nitrogen was determined by means of gas chromatography/mass spectrometry following incubation with ¹⁵N enriched 5 mM urea in the presence of either 0.2 M citrate buffer pH 6.0 or 0.2 M acetate buffer pH 6.0. After 5 min incubation in either buffer the ¹⁵n label appeared in glutamate, glutamine, phenylalanine, aspartate and alanine. It appears, therefore, that at pH and urea concentrations typical of the gastric mucosal surface, H. pylori utilises exogenous urea as a nitrogen source for amino acid synthesis. The ammonia produced by H. pylori urease activity thus facilitates the organism's nitrogen metabolism at neutral pH as well as protecting it from acid damage at low pH.     

Control by pH of urea synthesis in isolated rat hepatocytes

Control by pH of urea synthesis has been studied in isolated rat hepatocytes incubated with a physiological mixture of amino acids. Inhibition of urea synthesis by decreasing the pH of the medium was caused by diminished production of ammonia and not, as suggested in the literature, by inhibition of entry of ammonia into the ornithine cycle. The decrease by low pH of the rate of degradation of the added amino acids, that of alanine being quantitatively the most important, was accompanied by a decrease in their intracellular concentration. It is concluded that inhibited transport of amino acids across the plasma membrane of the hepatocyte is responsible, at least in part, for the fall in urea synthesis with decreasing pH. It is proposed that inhibition by low pH of other steps in the ureogenic pathway, distal to the production of ammonia, does not affect flux through the ornithine cycle per se, but rather contributes to the buffering of the intrahepatic concentration of ammonia.