Transport of Nitrogen to Liver and Kidney

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Amide Group of Asparagine Is Derived from Glutamine
The amide group of asparagine comes from that of glutamine (Figure 11.17), and not from free ammonia, as in the synthesis of glutamine. ATP is needed to activate the receptor a ­carboxyl group. Asparagine is readily synthesized in most cells, but some leukemic cells seem to have lost this ability. A therapeutic approach that has been tried for patients with asparagine synthetase­deficient tumors is treatment with exogenous asparaginase to hydrolyze the blood­borne asparagine on which these cells rely (Figure 11.18). Normal cells synthesize and degrade asparagine.
Figure 11.17 Synthesis of asparagine.
Amino Acid Oxidases Remove Amino Groups
Many amino acids are substrates for L­amino acid oxidase (Figure 11.19). The significance of this reaction in the metabolism of amino acids is uncertain, but appears to be small. The enzyme contains flavin mononucleotide (FMN) and produces hydrogen peroxide. After the hydrogen peroxide is reduced to water, the final products are an a ­keto acid, ammonia, and water, the same products as those of the glutamate dehydrogenase reaction. In the amino acid oxidase reaction, unlike the reaction catalyzed by glutamate dehydrogenase, there is no concomitant production of NADH, and therefore no production of ATP.
Figure 11.18 Reaction catalyzed by asparaginase.
A D­amino acid oxidase also occurs in human cells. Very little of the D­amino acid isomer is found in humans and the role of D­amino acid oxidase may be in degradation of D­amino acids derived from intestinal bacteria.
11.3— Transport of Nitrogen to Liver and Kidney
Protein Is Degraded on a Regular Basis
Whole cells die on a regular and planned basis, and their component molecules are metabolized. This "planned cell death" is called apoptosis. Individual proteins also undergo regular turnover under normal conditions. Even though the reactions involved in intracellular protein degradation have been identified, an understanding of the regulation of protein degradation is in its infancy. The half­life of a protein can be an hour or less, such as for ornithine decarboxylase, phosphokinase C, and insulin, several months for hemoglobin and histones,
Figure 11.19 Reaction of L­amino acid oxidase, a flavoprotein.