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.

Effect of initial muscle glycogen levels on protein catabolism during exercise

Abstract: Serum urea increases with exercise duration suggest prolonged exercise may be analogous to starvation where protein catabolism is known to occur. The purpose of this investigation was to alter muscle glycogen levels and to study the effect on protein catabolism. Six subjects (27-30 yr) pedaled a cycle ergometer for 1 h at 61% VO2max (mean VO2 = 2.33 +/- 0.7 1 . min-1) 1) after CHO loading (CHOL) and 2) after CHO depletion (CHOD). The following urea N measures were made: pre-exercise serum and urine, exercise serum and sweat (15-min serial samples), and serum and urine during 240 recovery min. Results demonstrated that 1) exercise serum urea N increased in CHOD attaining significance (P less than 0.01) at 60 min; 2) serum urea N increases continued into recovery at all measurement points of CHOD (P less than 0.01) and at 240 min of CHOL (P less than 0.05); 3) sweat urea N increased 154.2-fold (CHOD) and 65.6-fold (CHOL) (P less than 0.05). Calculations indicate that CHOD sweat urea N excretion was equivalent to a protein breakdown of 13.7 g . h-1 or 10.4% of the total caloric cost. It was concluded that protein is utilized during exercise to a greater extent than is generally assumed and that under certain conditions protein carbon may contribute significantly to exercise caloric cost.

Evolution of urea synthesis in vertebrates: the piscine connection

Abstract: Elasmobranch fishes, the coelacanth, estivating lungfish, amphibians, and mammals synthesize urea by the ornithine-urea cycle; by comparison, urea synthetic activity is generally insignificant in teleostean fishes. It is reported here that isolated liver cells of two teleost toadfishes, Opsanus beta and Opsansus tau, synthesize urea by the ornithine-urea cycle at substantial rates. Because toadfish excrete ammonia, do not use urea as an osmolyte, and have substantial levels of urease in their digestive systems, urea may serve as a transient nitrogen store, forming the basis of a nitrogen conservation shuttle system between liver and gut as in ruminants and hibernators. Toadfish synthesize urea using enzymes and subcellular distributions similar to those of elasmobranchs: glutamine-dependent carbamoyl phosphate synthethase (CPS III) and mitochondrial arginase. In contrast, mammals have CPS I (ammonia-dependent) and cytosolic arginase. Data on CPS and arginases in other fishes, including lungfishes and the coelacanth, support the hypothesis that the ornithine-urea cycle, a monophyletic trait in the vertebrates, underwent two key changes before the evolution of the extant lungfishes: a switch from CPS III to CPS I and replacement of mitochondrial arginase by a cytosolic equivalent.

The crystal structure of the urea–hydrocarbon complexes

Abstract: The structure of the urea-normal hydrocarbon complex has been determined. The unit cell is hexagonal, a 0 ---8.230, c o ----11.005 /k, space group C6:2-D~, six urea molecules per unit cell. The general features of the structure and the nitrogen positions of the urea were obtained directly from an implication diagram or Patterson-Harker section P(x, y, 16) which, for this space group, is essentially equivalent to an electron-density projection along the c axis. The urea molecules form a hollow channel structure in which the n-hydrocarbon molecules are enclosed. The hydrocarbons are in an extended planar zigzag configuration with their long axis parallel to the c axis. The electron-density projection and the implication diagram indicate that the time average of the positions of the plane of the hydrocarbon molecule are randomly disposed over positions perpendicular to the a axis and at multiples of 60 ° to this position. Diffuse bands observed in most of the complexes are attributed to the hydrocarbon molecules which behave as a system of linear gratings regularly arrayed in the x y plane but with random z coordinates.

Urea-Based Hydrothermally Derived Homogeneous Nanostructured Ce1-xZrxO2 (x = 0−0.8) Solid Solutions: A Strong Correlation between Oxygen Storage Capacity and Lattice Strain

Abstract: Ce1-xZrxO2 (x = 0−0.8) nanoparticulate powders were prepared via a mild urea hydrolysis based hydrothermal method through homogeneous nucleation at 413 K followed by calcination at 773 or 1173 K. X...

Cloning and characterization of the vasopressin-regulated urea transporter

Abstract: UREA is the principal end product of nitrogen metabolism in mammals1. Movement of urea across cell membranes was originally thought to occur by lipid-phase permeation, but recent studies have revealed the existence of specialized transporters with a low affinity for urea (Km > 200 mM)2. Here we report the isolation of a complementary DNA from rabbit renal medulla that encodes a 397-amino-acid membrane glycoprotein, UT2, with the functional characteristics of the vasopressin-sensitive urea transporter previously described in in vitro-perfused inner medullary collecting ducts3,4. UT2 is not homologous to any known protein and displays a unique pattern of hydrophobicity. Because of the central role of this transporter in fluid balance1,3–7 and nitrogen metabolism8, the study of this protein will provide important insights into the urinary concentrating mechanism and nitrogen balance.