BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 6. AMINO ACID METABOLISM AND FUNCTIONS. PROTEIN BIOSYNTHESIS

6.5. Ammonia Metabolism. Urea Cycle

6.5.1. Ammonia Binding (Detoxification)

The high rate of AMINO ACID DEAMINATION in Tissues alongside a very low Blood ammonia concentration indicates that Cells actively bind ammonia to form non-toxic compounds that are excreted in the urine. These reactions can be considered mechanisms of ammonia detoxification. Several types of such reactions have been identified in various tissues and Organs.

The primary reaction of ammonia binding, which occurs in all body tissues, is the Synthesis of Glutamine catalyzed by Glutamine Synthetase:

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Glutamine synthetase is localized in the mitochondrial matrix, and its catalytic activity requires Mg²⁺ ions as a cofactor. Glutamine synthetase is a key regulatory enzyme of Amino acid METABOLISM that undergoes allosteric inhibition by AMP, glucose-6-phosphate, as well as Gly, Ala, and His.

Glutamine easily crosses Cell membranes via Facilitated Diffusion (whereas glutamate relies solely on Active Transport) and enters the bloodstream from tissues. The main suppliers of glutamine are skeletal Muscles, the Brain, and the Liver. Through the bloodstream, it is transported to the intestines and Kidneys.

In intestinal cells, the hydrolytic release of amide nitrogen as ammonia is catalyzed by the enzyme glutaminase:

The glutamate produced in this reaction undergoes Transamination with Pyruvate. The α-amino group of glutamic acid is transferred to pyruvate to yield Alanine (Fig. 6.10), a significant amount of which enters the portal vein blood from the intestines and is taken up by the liver. Approximately 5 % of the generated ammonia is excreted with feces, a small fraction reaches the liver via the portal vein, and the remaining ~90 % is eliminated by the kidneys.

Fig. 6.10. Metabolism of glutamine nitrogen in the intestine

The kidneys also carry out glutamine Hydrolysis via glutaminase activity to produce ammonia. This process serves as one of The regulatory mechanisms maintaining the body's acid-base balance and conserving essential cations to preserve osmotic pressure. Renal glutaminase is significantly induced during acidosis. The generated ammonia neutralizes acidic metabolic products and is excreted in the urine as ammonium salts (Fig. 6.11). This reaction protects the body against excessive loss of Na⁺ and K⁺ ions, which might otherwise be utilized and lost for anion excretion. During alkalosis, the level of renal glutaminase decreases. The kidneys produce and excrete approximately 0.5 g of ammonium salts per day.

Fig. 6.11. Metabolism of glutamine amide nitrogen in the kidneys:

A - anions (Cl ,SO₄^2- ); NH₄A - ammonium salts

The high level of glutamine in the Blood and Its ready uptake by cells facilitate its utilization in numerous anabolic pathways. Glutamine is the principal nitrogen donor in the body. The amide nitrogen of glutamine is utilized for the Synthesis of purine and pyrimidine NUCLEOTIDES, as well as asparagine, amino sugars, and Other Compounds (Fig. 6.12).

Fig. 6.12. Pathways of glutamine utilization in the body

Another reaction responsible for ammonia detoxification in tissues is the synthesis of asparagine catalyzed by asparagine synthetase.

There are two isoforms of this enzyme—glutamine-dependent and ammonia-dependent—which utilize different amide group Donors. The former Functions in animal cells, whereas the latter predominates in Bacteria, though it is also present in animals. However, this pathway of ammonia detoxification is rarely used in human cells and entails a higher energy cost (requiring the energy of two high-energy bonds) compared to glutamine synthesis.

The vast majority of ammonia is detoxified in the liver via The Urea Cycle. In the first step of this pathway, ammonia combines with carbon dioxide to form carbamoyl phosphate, consuming two molecules of ATP. The reaction takes place in the Mitochondria of hepatocytes and is catalyzed by carbamoyl phosphate synthetase I. Carbamoyl phosphate synthetase II is localized in the Cytosol of cells across all tissues and is involved in the synthesis of pyrimidine nucleotides. Subsequently, carbamoyl phosphate enters the urea cycle and is utilized for urea synthesis.

In the brain and certain other organs, reductive amination of $\alpha$-ketoglutarate may occur via Glutamate dehydrogenase, which catalyzes the reverse reaction. However, this pathway of ammonia detoxification is rarely utilized in tissues because glutamate dehydrogenase predominantly catalyzes the deamination of glutamate. Nevertheless, taking into account the subsequent formation of glutamine, this reaction is advantageous for cells as it facilitates the binding of two NH₃ molecules simultaneously.

Excess ammonia from muscles and the intestines is removed primarily in the form of alanine. This mechanism is necessary because The activity of

glutamate dehydrogenase in muscles is low, rendering indirect Deamination of Amino acids relatively inefficient. Consequently, an alternative pathway for nitrogen excretion exists in Muscle tissue. The formation of alanine in these organs can be represented by the following scheme:

Amino groups from various Amino Acids are transferred via transamination reactions to pyruvate, the primary source of which is glucose oxidation.

Muscles release particularly large amounts of alanine due to their substantial mass, active glucose utilization during physical exertion, and the fact that they derive part of their energy from Amino Acid Catabolism. The resulting alanine enters the liver, where it undergoes indirect deamination. The released ammonia is detoxified, and the pyruvate is channeled into Gluconeogenesis. Glucose from the liver is transported to peripheral tissues, where it is oxidized back to pyruvate via Glycolysis (Fig. 6.13).

Fig. 6.14. Glucose-alanine cycle.

The synthesis of alanine in muscles, its transport to the liver, and the return of liver-synthesized glucose back to the muscles constitute the glucose-alanine cycle, which operates in coordination with the glucose-lactate cycle.

The overall processes of ammonia Metabolism in the Body are illustrated in Fig. 6.14. The dominant Enzymes involved in Ammonia Metabolism are Glutamate Dehydrogenase and glutamine synthetase.

Fig. 6.14. Ammonia metabolism.

The primary source of ammonia is amino acids. Most of the generated ammonia is detoxified via the Ornithine cycle in The Liver and excreted as urea. The principal pathway for ammonia detoxification in peripheral tissues is the synthesis of glutamine, which is subsequently utilized in anabolic pathways and for xenobiotic detoxification in the liver. The enzymes glutamate dehydrogenase and glutamine synthetase act as key regulatory enzymes determining the rates of Ammonia Production and detoxification.