Principles of Biochemistry Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Oxidative Degradation of Amino Acids. Urea Cycle
Ammonia is transported from muscles to the liver in the form of alanine

Alanine also plays a special role in transporting ammonia to the Liver in a nontoxic form. In Muscles, as in other Tissues, Ammonia is produced during amino acid degradation. In addition, during strenuous exercise in Skeletal Muscle, the deamination of adenylate (AMP)—which also serves as a source of ammonia—becomes a vital process. Ammonia from these two sources is transported from the muscles to the liver as The amino acid alanine via the glucose-alanine cycle (Fig. 19-14). In this cycle, ammonia is converted into the amino group of glutamate in a reaction catalyzed by Glutamate dehydrogenase

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Образовавшийся глутамат переносит затем свою а-аминогруппу на Пируват, всегда имеющийся в достаточном количестве, поскольку это продукт протекающего в мышцах гликолиза. Реакция переноса катализируется Аланин-транс-аминазой

Alanine (a neutral amino acid carrying no net charge at pH values close to 7) leaves the Cells into the bloodstream and is transported by the Blood to the liver. Here, under the action of alanine transaminase, it transfers its amino group to $\alpha$-ketoglutarate, resulting in The formation of glutamate. Next, this glutamate is deaminated in a reaction catalyzed by glutamate dehydrogenase, yielding $\alpha$-ketoglutarate and ammonia, which is converted into urea in the liver.

Fig. 19-14. The glucose-alanine cycle. This cycle serves two Functions: 1) it transports amino groups from skeletal muscles to the liver, where they are converted into urea, and 2) it supplies working muscles with glucose delivered via the bloodstream from the liver, where the carbon Skeleton of alanine is utilized for its synthesis.

The Selection of a compound such as alanine for transporting ammonia from vigorously working skeletal muscles to the liver is yet another striking example of THE PRINCIPLE OF economy operating in living organisms. During intense exertion, contracting skeletal muscles produce not only ammonia but also large amounts of Pyruvate, which is a product of Glycolysis. Both of these products must be delivered to the liver, where ammonia is converted into urea and excreted in this form, while glucose is resynthesized from pyruvate and returned via the blood to the muscles. Animals have evolved a pathway in which a single cycle solves both problems: in this cycle, ammonia combines with pyruvate to form alanine—a non-toxic, neutral amino acid that is transported via the blood to the liver, where it undergoes further transformations (Fig. 19-14).