Waste nitrogen urea cycle

Glutamate and Aspartate as Carriers of Waste Nitrogen Urea Cycle Ammonia... 

Metabolism of nitrogen in a patient with a deficiency in the urea cycle enzyme carbamoyl phosphate synthetase I. Treatment with phenylbutyrate converts nitrogenous waste to a form that can be excreted. 

Because of the importance of the urea cycle, the capacity to convert ornithine into arginine is obvious. Complete loss of the ability to produce ornithine (a catalyst or carrier in the urea cycle) would limit the organism s control over production of its nitrogen waste product. 

Urea cycle This pathway converts ammonia, a toxic nitrogen-containing waste product of protein metabolism, into another, less toxic molecule called urea, which can be eliminated from the body as urine. 

Figure 8,12 illustrates the role of the urea cycle in accepting waste nitrogen from a variety of sources and in packaging it in the form of urea. 

FIGURE 8.20 Elimination of ammonium ions via the urea cycle or via direct excretion. With protein catabolism, the excretion of waste nitrogen via the urea cycle results in net production of acid in the body however, excretion of ammonium ions by the kidney into the urine does not result in this production of acid in the body. 

Several standard and nonstandard amino acids act as metabolic intermediates. For example, arginine, citrulline, and ornithine (Figure 5.6) are components of the urea cycle (Chapter 15). The synthesis of urea, a molecule formed in vertebrate livers, is the principal mechanism for the disposal of nitrogenous waste. 

Some patients have an inborn error of metabolism that involves a defective enzyme, or enzymes of the urea cycle. This results in reduced efficiency to excrete their waste nitrogen. The patients are prescribed sodium phenylbutyrate (buphenyl) ( 30 g per day). What is the action of sodimn phenylbutyrate ... 

Primary urea cycle defects are caused by a deficiency of any of the six urea cycle enzymes (Chap. 15) and result in insufficient disposal of waste nitrogen. As a result, nitrogen accumulates in the form of ammonia and as its precursors, as glutamine and glycine. Primary defects in an enzyme of the urea cycle typically result in higher ammonia levels than secondary impairments of the urea cycle, although exceptions occur. 

The primary function of the urea cycle is to rid the body of waste nitrogen. Deficiency in the activity of any of the six enzymes in the urea cycle may result in the accumulation of ammonia, often to toxic concentrations. Treatment involves restricting protein, preventing catabolism, supplementing amino acids that are normally produced by the urea cycle, and promoting the excretion of nitrogen via alternative pathways. Outcomes are guarded and appear to be better for patients identified by NBS compared to patients identified clinically. Liver transplantation is a treatment option, especially for patients with a severe form of the disorder. 

Fig. 4. Compartmental model describing the cycling of nitrogen in a planktonic community in the mixed layer of a water column. Flow pathways are represented by arrows and numbers which correspond to mathematical expressions described in Table 2. The nitrogen pool represents all abiotic nitrogen (nitrate, ammonia and urea), and other compartments represent bacteria, zooflagellates, larger protozoa, and micro-mesozooplankton, giving off waste products (F+U). Arrows (13) and (14) depict sedimentation of zooplankton faeces and phytoplankton cells, respectively (After Moloney et al., 1985).

Ammonia is found in the environment as the result of natural and industrial processes. It is released into the environment by the breakdown of organic wastes, and it is a constituent of the soil, the atmosphere, and bodies of water. Ammonia is also a key intermediate in the nitrogen cycle and is a product of amino acid metabolism (WHO 1986). Anhydrous ammonia is used in the production of nitric acid, explosives, synthetic fibers, and fertilizers (Budavari 1989). It is used as a refrigerant as a corrosion inhibitor in the purification of water supplies in steel production as a catalyst for polymers as a preservative for latex and in the production of nitrocellulose, urea formaldehyde, sulfite cooking liquors, and nitroparaffins (ACGIH 1991 Lewis 1993). Ammonium hydroxide (10-35% ammonia) is a major constituent of many cleaning solutions. Ammonia... 

Animals, such as fish, that live in an aquatic environment excrete nitrogen as ammonia they are protected from the toxic effects of high concentrations of ammonia not only by the removal of ammonia from their bodies but also by rapid dilution of the excreted ammonia by the water in the environment. The principal waste product of nitrogen metabolism in terrestrial animals is urea (a water-soluble compound) its reactions provide some interesting comparisons with the citric acid cycle. Birds excrete nitrogen in the form of uric acid, which is insoluble in water. They do not have to carry the excess weight of water, which could hamper flight, to rid themselves of waste products. 

Box 15.9 deals with the nitrogen cycle and the removal of nitrates and nitrites from waste water, (a) Urea is used to reduce [N02] to N2. Write a balanced equation for the reaction of HNO2 with urea, (b) Sulfamic acid is also used to reduce [N02] to N2 during water treatment. Give an equation for this reaction, (c) Nitrites can be removed using H2O2, [OCl] or HOCl as oxidants. How does the reduction potential for the following process depend on pH ... 

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