Textbook - BIOLOGICAL CHEMISTRY - Hubsky Y.I. - 2000

Section III. METABOLISM OF THE MAIN CLASSES OF BIOMOLECULES

CHAPTER 17. AMINO ACID METABOLISM I. GENERAL PATHWAYS OF CONVERSION

17.5. AMMONIA METABOLISM. UREA BIOSYNTHESIS

Class="center">Pathways of ammonia production in The Human Body

1. Quantitatively, the main source of ammonia accumulation in the human body is the Oxidative Deamination of Amino Acids, i.e., Protein Catabolism: urea nitrogen — the ultimate nitrogenous product of protein degradation — accounts for approximately 90 % of all excreted nitrogen. Additional sources of endogenous ammonia include deamination reactions of Biogenic Amines and nitrogenous bases formed during nucleotide catabolism. A significant amount of free ammonia is absorbed into the Blood from the PORTAL VEIN SYSTEM (v. porta) As a result of its production during the catabolism of nitrogenous bioorganic compounds (mainly dietary Proteins) by intestinal Bacteria.

2. Production of ammonia in the Brain

The primary source of Ammonia Production in brain tissue is the hydrolytic deamination of AMP to inosine monophosphate (IMP), catalyzed by the enzyme adenosine deaminase:

The released ammonia is detoxified via the Glutamine Synthetase reaction, converting L-glutamate into glutamine, which is then removed from the brain (see below).

Toxicity of ammonia

Ammonia is a toxic substance, particularly hazardous to the brain; the normal concentration of free ammonia in Blood Plasma (in the form of the ammonium ion NH₄⁺) is negligible, ranging from 25-40 µmol/L (0.4-0.7 mg/L). Excessive accumulation of ammonia in the body occurs in Disorders of the urea-synthesizing function of the Liver (viral and toxic hepatitis, liver cirrhosis), the nitrogen-excreting function of the Kidneys (acute or chronic renal failure), and hereditary (congenital) hyperammonemias caused by Genetic Defects in urea synthesis Enzymes. Clinically, hyperammonemia is characterized by profound Central Nervous system dysfunctions, up to The Development of a comatose state.

The toxicity of ammonia is attributed to its ability to disrupt the tricarboxylic acid (TCA) cycle in the Cell/35.html">Mitochondria of brain Neurons by depleting α-ketoglutarate from the TCA cycle:

This reaction (reductive amination of α-ketoglutarate), which is an NADPH-dependent Reversal of the Glutamate dehydrogenase reaction, removes α-ketoglutarate from the pool of TCA cycle metabolites, ultimately reducing The rate of AEROBIC GLUCOSE OXIDATION — the primary energy source for the brain. An alternative theory of ammonia neurotoxicity links its adverse effects to the damaging impact on neurons caused by high concentrations of glutamine, which is produced in excess from ammonia and L-glutamate (see below).

Mechanisms of ammonia detoxification

The high toxicity of ammonia has led to the evolution of specialized biochemical mechanisms for its detoxification in animal organisms. Depending on the molecular form in which the End products of nitrogenous (amino) catabolism are excreted, animal organisms are classified into three types:

1) ammonotelic organisms — those that excrete amino nitrogen in the form of the soluble ammonium ion (this includes most aquatic vertebrates);

2) uricotelic organisms — those that excrete amino nitrogen in the form of uric acid (birds, terrestrial reptiles);

3) ureotelic organisms — those in which urea is the primary product of ammonia detoxification and excretion (most terrestrial vertebrates, including mammals, notably humans).

Urea Biosynthesis occurs exclusively in the liver. An additional mechanism for ammonia detoxification at the sites of its production is its binding in the form of glutamine, mediated by glutamine synthetase.

Urea biosynthesis

According to the studies of H. Krebs and K. Henseleit (1932), urea is synthesized from ammonia and carbonic acid through a cyclic process in which the amino acids Arginine, Ornithine, and Citrulline play a catalytic role (the Krebs-Henseleit ornithine cycle):

As shown below, the sources of the two amino groups used to form a urea molecule are ammonia released during the oxidative deamination of L-glutamate, and the amino group of The amino acid L-aspartate.

ENZYMATIC REACTIONS OF urea synthesis

1. Formation of carbamoyl phosphate from ammonia and carbon dioxide in an ATP-dependent reaction:

The reaction is catalyzed by carbamoyl phosphate synthetase. The source of the amino group (in the form of an ammonia molecule) is the glutamate dehydrogenase reaction.

2. Transfer of the carbamoyl group to ornithine to yield citrulline (enzyme: ornithine transcarbamylase):

3. Acceptance of the second amino group via the interaction of citrulline with L-aspartate (enzyme: argininosuccinate synthetase):

4. Cleavage of argininosuccinate by argininosuccinate lyase; the reaction products are arginine, the direct precursor of urea, and fumarate:

5. Hydrolysis of arginine by arginase to produce urea and regenerate ornithine (completion of the metabolic cycle):

Fumarate produced in the argininosuccinate lyase reaction (4) is a substrate of The Tricarboxylic Acid Cycle and can be converted to malate and oxaloacetate; oxaloacetate, in turn, can undergo Transamination to form aspartate, the donor of the second amino group in the urea molecule:

Metabolic cycle of urea synthesis.

Genetic defects of Urea Cycle enzymes

There are hereditary enzymopathies caused by complete or partial deficiencies in the hepatic synthesis of individual urea cycle enzymes. The most severe clinical manifestations are associated with impaired synthesis of carbamoyl phosphate synthetase and ornithine transcarbamylase. Children with these genetic defects suffer from severe encephalopathy, the symptoms of which are somewhat alleviated by the complete elimination of dietary protein.

Ammonia Transport to the liver

The molecular forms for transporting ammonia from Organs and Tissues where it is produced (Muscles, brain, intestines, etc.) are glutamine (the amide of glutamic acid) and Alanine. The concentrations of glutamine and alanine in human blood plasma significantly exceed those of Other Amino Acids.

Glutamine is synthesized from L-glutamate in an ATP-dependent reaction catalyzed by glutamine synthetase:

Glutamine synthesis occurs predominantly in brain Cells (where the glutamine synthetase reaction serves as the primary mechanism for ammonia detoxification) and Skeletal Muscle. From these and other organs, the amide is transported via the bloodstream to the intestines, liver, and kidneys:

(1) intestinal cells take up the largest amount of blood-borne glutamine; within this organ, glutamine is converted (via transamination reactions) into alanine, which is subsequently released into the bloodstream and taken up by hepatocytes; in liver cells, the carbon Skeleton of alanine is utilized in Gluconeogenesis (the glucose-alanine cycle), while its amino group is used in urea synthesis;

(2) in the liver, glutamine is cleaved into L-glutamate and ammonia by the enzyme glutaminase:

The released ammonia is incorporated into urea synthesis pathways.

(3) in the kidneys, the glutaminase reaction serves as the primary source of ammonium ion (NH₄⁺) generation, with approximately 0.5 g excreted in the urine daily. Glutaminase synthesis in renal tubular epithelial cells is stimulated during acidosis, acting as a key mechanism for neutralizing and excreting excess acid equivalents from the body:

Alanine is predominantly synthesized in muscles via transamination reactions with other Amino Acids and plays a vital role in its transport to the liver; hepatic cells take up the largest share of blood alanine, utilizing this amino acid for gluconeogenesis and urea synthesis.