Waste nitrogen

Many of the reactions described so far point to the concept of waste nitrogen. What happens to the ammonium ions discharged during catabolism of the various amino acids What happens to the amino groups transferred from various amino adds to oxaloacetic acid, yielding aspartate The ammonium ions and excess amino groups, carried in the form of aspartate, are handled in the following marmer. They are incorporated into a small, water-soluble molecule known as 

FIGURE 8.10 Positions of entry of amino acid carbon skeletons into the Krebs cycle. Amino acids that are broken down to three-carbon skeletons may enter the Krebs cycle at the point of pyruvate. Glycine, after conversion to serine, can enter the Krebs cycle as pyruvate. Alternatively, glycine can be broken down by the glycine cleavage system (see Folate section). The products of this reaction are carbon dioxide and an ammonium ion. Five-carbon skeletons enter the Krebs cycle at the point of a-ketoglutarate, whereas four-carbon skeletons enter at the points of succinyl-CoA and oxaloacetic acid. 

The first step in the breakdown of a variety of amino acids results in transfer of the a-amino group to a-ketoglutarate, yielding glutamic acid. What happens to 

FIGURE 8.11 The urea cycle. The urea cycle is based on the ornithine molecule. Ornithine acts as a chemical scaffold, on which a moiety of urea is built. The finished moiety of urea is discharged by hydrolysis, producing free urea and regenerating the original molecule of ornithine. 

The urea cycle may be considered to be a mitochondrial pathway, as carbamyl phosphate synthase and ornithine transcarbamylase are mitochondrial enzymes however, the enzymes catalyzing subsequent steps of the pathway are cytosolic. The steps leading to conversion of citrulline to ornithine occur in the cytosol. Hence, the pathway is shared by the mitochondrial and cytosolic compartments. The fumarate produced by the urea cycle is converted to malate by a cytoplasmic form of fumarase. Mitochondrial fumarase is part of the Krebs cycle. Cytoplasmic malate can enter the mitochondrion by means of a transport system, such as the malate/phosphate exchanger or the malate/a-ketoglutarate exchanger. These transport systems are membrane-bound proteins. 

Ammonium ions are produced by the catabolism of a number of amino acids. Glutamate dehydrogenase is the major source of ammonium ions in the body. Ammonium ions are also produced from the catabolic pathways of serine, histidine, tryptophan, glycine, glutamine, and asparagine. L-Amino acid oxidase and 

FIGURE 8,12 Overall direction of flux of waste nitrogens derived from amino acid catabo lism, The waste nitrogerts are funneled, via free ammonium iona and aspartate, to produce gtea 

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Fig. 8. Transient NO emissions at synthetic wastes nitrogen contents of (D), 1% ( ), 3% and (0)> 10%, where the % of O2 is 2ero. Aniline is used as a nitrogen source and toluene is used to keep the total hydrocarbon weight fixed at 35 g (29).

Fig. 9. Waste nitrogen conversion to NO as a function of the nitrogen content of the synthetic soHd waste where (D) represents aniline ( ), pyridine (0)>...

As was the case with terpenes, the function of alkaloids in plants is not known. It has been proposed that they are merely nitrogen-containing waste products of plants, like urea in animals. However, most plants reutilize nitrogen, rather than wasting it. Furthermore, it is difficult to imagine why such complex structures would be needed to store waste nitrogen. Like terpenes, alkaloids have been proposed to serve as protection from herbivores and insects. However, only a few examples of such protection can be demonstrated. Whatever the role of alkaloids is, some 70% to 80% of plants manage to do quite nicely without them. 

Figure 18-2. Nitrogen ( ) incorporation into benzoate and phenylacetate as phenylacetylglutamine (PAG) and hippuric acid (HA) provide an alternate path to urea for waste nitrogen excretion.

Nutrients and Eutrophication Nitrogen and phosphorus cause significant problems in the environment ana require special attention in industrial wastes. Nitrogen, phosphorus, or both may cause aquatic... 

FIGURE 4,40 LThie of alanine for carrying waste nitrogen from rnuscic to the liver. 

Clulamalc and Aspartate as Carriers of Waste Nitrogen Uma C yde Ammonia... 

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. 

Toxicity and Packaging of Waste Nitrogen Illustrated by Experimental Nutrition... 

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