Trowell, O. Acta , 25 , Bach, S. Schoenheimer, R. Google Scholar. Download references. You can also search for this author in PubMed Google Scholar. Reprints and Permissions. BACH, S. Nature , — Download citation. Issue Date : 14 November You can find it in urine, sweat, blood and milk in mammals. In its most concentrated form, it is urine. Urea is a crystalline compound, and the nitrogen content is always at least 46 percent when dry.
About one million pounds of urea is manufactured in the United States each year, most of it for fertilizers due to its high nitrogen content, which makes it water soluble. Urea is also in animal feedstock, certain plastics and glues, explosive components and commercial products. The urea cycle starts with deamination, when the liver breaks down amino acids to make ammonia.
Ammonia is highly toxic and would be deadly if it accumulated in the body. Fortunately, carrier molecules and enzymes in the liver quickly convert it to urea. The urea cycle absorbs two molecules of ammonia and one molecule of carbon dioxide, creates one molecule of urea and regenerates one molecule of ornithine for the cycle to begin again.
For example, urea is one of the precursors to the various barbiturates , which are widely used as sedatives and sleeping pills. Urea is also used in the production of urethanes , which are then polymerised to form polyurethane foams. Another important application is in the manufacture of resins and polymers. Urea can react with formaldehyde to make the urea-formaldehyde resins, which are highly important in moulded plastics.
Another product is, melamine , which is formed by the dehydration of urea, and is used primarily in the production of melamine-formaldehyde resins which have much greater hardness and stain resistance than urea-formaldehyde resins. Both of these types of polymer resin have very varied uses including adhesives, laminates, moulding compounds, coatings and textile finishes.
As well as its uses as a chemical precursor, urea has properties which make it useful in its own right. For example, because of its high nitrogen content, urea is often used as a component of agricultural fertilizers. The urea cycle consists of 4 reactions. The first reaction occurs in the matrix of the mitochondria.
The subsequent reactions occur in the cytosol. This is a pathway that spans two cellular compartments.
The first reaction is catalyzed by ornithine transcarbamoylase that transfers a carbamoyl group from carbamoyl phosphate to ornithine to form citrulline. The second reaction is catalyzed by argininosuccinate synthetase. This intermediate is attacked by the amino group of an aspartate residue to form argininosuccinate.
The third step is catalyzed by argininosuccinate lyase that cleaves argininosuccinate into fumarate and arginine.
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