Urea

About: Urea is a research topic. Over the lifetime, 21394 publications have been published within this topic receiving 382444 citations. The topic is also known as: carbamide & carbonic acid diamide.

Whole body nitric oxide synthesis in healthy men determined from [15N] arginine-to-[15N]citrulline labeling.

Abstract: The rates of whole body nitric oxide (NO) synthesis, plasma arginine flux, and de novo arginine synthesis and their relationships to urea production, were examined in a total of seven healthy adults receiving an L-amino acid diet for 6 days. NO synthesis was estimated by the rate of conversion of the [15N] guanidino nitrogen of arginine to plasma [15N] ureido citrulline and compared with that based on urinary nitrite (NO2-)/nitrate (NO3-) excretion. Six subjects received on dietary day 7, a 24-hr (12-hr fed/12-hr fasted) primed, constant, intravenous infusion of L-[guanidino-15N2]arginine and [13C]urea. A similar investigation was repeated with three of these subjects, plus an additional subject, in which they received L-[ureido-13C]citrulline, to determine plasma citrulline fluxes. The estimated rates (mean +/- SD) of NO synthesis over a period of 24 hr averaged 0.96 +/- 0.1 mumol .kg-1.hr-1 and 0.95 +/- 0.1 mumol.kg-1.hr-1, for the [15N]citrulline and the nitrite/nitrate methods, respectively. About 15% of the plasma arginine turnover was associated with urea formation and 1.2% with NO formation. De novo arginine synthesis averaged 9.2 +/- 1.4 mumol. kg-1.hr-1, indicating that approximately 11% of the plasma arginine flux originates via conversion of plasma citrulline to arginine. Thus, the fraction of the plasma arginine flux associated with NO and also urea synthesis in healthy humans is small, although the plasma arginine compartment serves as a significant precursor pool (54%) for whole body NO formation. This tracer model should be useful for exploring these metabolic relationships in vivo, under specific pathophysiologic states where the L-arginine-NO pathway might be altered.

Coupling N2 and CO2 in H2O to synthesize urea under ambient conditions

Abstract: The use of nitrogen fertilizers has been estimated to have supported 27% of the world’s population over the past century. Urea (CO(NH2)2) is conventionally synthesized through two consecutive industrial processes, N2 + H2 → NH3 followed by NH3 + CO2 → urea. Both reactions operate under harsh conditions and consume more than 2% of the world’s energy. Urea synthesis consumes approximately 80% of the NH3 produced globally. Here we directly coupled N2 and CO2 in H2O to produce urea under ambient conditions. The process was carried out using an electrocatalyst consisting of PdCu alloy nanoparticles on TiO2 nanosheets. This coupling reaction occurs through the formation of C–N bonds via the thermodynamically spontaneous reaction between *N=N* and CO. Products were identified and quantified using isotope labelling and the mechanism investigated using isotope-labelled operando synchrotron-radiation Fourier transform infrared spectroscopy. A high rate of urea formation of 3.36 mmol g–1 h–1 and corresponding Faradic efficiency of 8.92% were measured at –0.4 V versus reversible hydrogen electrode. Conventionally, urea is synthesized via two consecutive processes, N2 + H2 → NH3 followed by NH3 + CO2. Now, an electrocatalyst consisting of PdCu alloy nanoparticles on TiO2 nanosheets has been shown to directly couple N2 and CO2 in H2O to produce urea under ambient conditions.

Hepatic detoxification of ammonia in the ovine liver: possible consequences for amino acid catabolism

Abstract: The effects of either low (25 μmol/min) or high (235 μmol/min) infusion of NH 4 Cl into the mesenteric vein for 5 d were determined on O 2 consumption plus urea and amino acid transfers across the portal-drained viscera (PDV) and liver of young sheep. Kinetic transfers were followed by use of 15 NH 4 Cl for 10 h on the fifth day with simultaneous infusion of [1- 13 C]leucine to monitor amino acid oxidation. Neither PDV nor liver blood flow were affected by the additional NH 3 loading, although at the higher rate there was a trend for increased liver O 2 consumption. NH 3 -N extraction by the liver accounted for 64-70% of urea-N synthesis and at the lower infusion rate the additional N required could be more than accounted for by hepatic removal of free amino acids. At the higher rate of NH 3 administration additional sources of N were apparently required to account fully for urea synthesis. Protein synthesis rates in the PDV and liver were unaffected by NH 3 infusion but both whole-body (P<0.05) and splanchnic tissue leucine oxidation were elevated at the higher rate of administration. Substantial synthesis of [ 15 N]glutamine occurred across the liver, particularly with the greater NH 3 supply, and enrichments exceeded considerably those of glutamate. The [ 15 N]urea synthesized was predominantly as the single labelled, i.e. [ 14 N 15 N], species. These various kinetic data are compatible with the action of ovine hepatic glutamate dehydrogenase (EC 1.4.1.2) in periportal hepatocytes in the direction favouring glutamate deamination. Glutamate synthesis and uptake is probably confined to the perivenous cells which do not synthesize urea. The implications of NH 3 detoxification to the energy and N metabolism of the ruminant are discussed