BIOCHEMISTRY: A TEXTBOOK FOR UNIVERSITIES - E. S. Severin - 2004

CHAPTER 9. AMINO ACID METABOLISM AND FUNCTIONS

IX. Nitrogen-Containing Compounds: Amino Acid Derivatives

Non-peptide nitrogen-containing compounds derived from Amino Acids play a crucial role in The Human Body. These include Adrenal Hormones (norepinephrine, epinephrine), THYROID HORMONES (thyroxine, triiodothyronine), as well as Central Nervous system (CNS) Neurotransmitters (acetylcholine, GABA, etc.), inflammatory mediators (histamine), and Other Compounds.

A. Decarboxylation of amino Acids and Their Derivatives

Certain amino acids and their derivatives can undergo decarboxylation—the removal of the α-carboxyl group. In mammalian tissues, a wide range of amino acids or their derivatives can be decarboxylated: Thr, Tyr, Val, His, Glu, Cys, Arg, Ornithine, SAM, L-DOPA, 5-hydroxytryptophan, etc. The reaction products are CO₂ and amines that exert pronounced biological effects on the Organism (biogenic amines):

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Decarboxylation reactions are irreversible and catalyzed by decarboxylase Enzymes. The prosthetic group of Decarboxylases in animal Cells is Pyridoxal phosphate. Some microbial decarboxylases may contain a Pyruvate residue instead of PLP, such as Histidine decarboxylase in Micrococcus and Lactobacillus, SAM decarboxylase in E. coli, etc. The reaction mechanism resembles Transamination involving pyridoxal phosphate and also proceeds via The formation of a Schiff base between PLP and The amino acid in the initial stage.

Amines formed via the decarboxylation of amino acids frequently function as BIOLOGICALLY ACTIVE SUBSTANCES. They act as neurotransmitters (serotonin, dopamine, GABA, etc.), hormones (norepinephrine, epinephrine), and local regulatory factors (histamine, carnosine, spermine, etc.).

1. Synthesis and Biological Role of Serotonin

Serotonin is a neurotransmitter of Neural Pathways. It is synthesized in the Adrenal Glands and the CNS from the amino acid 5-hydroxytryptophan through the action of aromatic amino acid decarboxylase. This enzyme exhibits broad substrate Specificity and is also capable of decarboxylating Tryptophan and L-DOPA derived from Tyrosine. 5-Hydroxytryptophan is synthesized from tryptophan by phenylalanine hydroxylase with the coenzyme H₄BPreaction (this enzyme is specific to aromatic Amino Acids and also hydroxylates phenylalanine) (see the scheme below).

Serotonin can be converted into the hormone melatonin, which regulates circadian and seasonal metabolic rhythms in the body and participates in The regulation of reproductive function.

Serotonin is a broad-spectrum biologically active substance. It stimulates smooth Muscle contraction, exerts a vasoconstrictive effect, regulates Blood pressure, body Temperature, and Respiration, and possesses antidepressant activity. According to some data, it may also be involved in allergic

reactions, as it is synthesized in small quantities by mast cells.

2. Synthesis and Biological Role of Acetylcholine

Acetylcholine is synthesized in Nervous Tissue and serves as one of the most important excitatory neurotransmitters of the Autonomic nervous system. Its precursor is the amino acid Serine:

3. Synthesis and Biological Role of γ-Aminobutyric Acid

In Nerve Cells, the decarboxylation of glutamate (removal of the α-carboxyl group) leads to the Formation of γ-aminobutyric acid (GABA), which serves as the primary inhibitory neurotransmitter in higher Brain regions (see scheme).

The cycle of GABA transformations in the brain comprises three coupled reactions known as the GABA shunt. The first reaction is catalyzed by glutamate decarboxylase, a pyridoxal-dependent enzyme. This reaction is regulatory and determines The rate of GABA production in brain cells. The product of the reaction is GABA. The subsequent two reactions can be considered GABA Catabolism. GABA aminotransferase, also pyridoxal-dependent, yields succinic semialdehyde, which is then dehydrogenated and converted into succinic acid. Succinate is utilized in The Citric Acid Cycle. GABA inactivation can also occur oxidatively via MAO action.

The concentration of GABA in the brain is dozens of times higher than that of other neurotransmitters. It increases the permeability of postsynaptic membranes to K⁺ ions, thereby inhibiting Nerve Impulse transmission; it enhances the respiratory activity of nervous tissue; and it improves cerebral blood supply.

GABA preparations, marketed as gammalon or aminalon, are used to treat cerebrovascular disorders (atherosclerosis, Hypertension), impaired cerebral Circulation, mental retardation, endogenous depressions, and TRAUMATIC BRAIN INJURIES, as well as CNS disorders associated with acute Cerebral Cortex hyperactivity (e.g., Epilepsy).

4. Other CNS Neurotransmitters: Glycine, Glutamate

Free amino acids play an exceptionally vital role in the brain as precursors of Proteins and biologically active substances such as Neuropeptides, hormones, biogenic amines, etc. Some amino acids can participate in synaptic transmission by functioning as neurotransmitters. A very

crucial role for the brain is also played by the METABOLISM/26.html">Energy Metabolism of amino acids. The content of free amino acids in the brain reaches ~35 µmol/g of tissue, which is significantly higher than in Blood Plasma (~3.5 µmol/L) and CEREBROSPINAL FLUID. Glutamic acid, glutamine, aspartic acid, glycine, GABA, N-acetylaspartate, and others predominate. The amino acids glycine and glutamate are essential neurotransmitters.

Glutamate is found in the brain in very large quantities (up to ~10 µmol/g of tissue) and performs diverse Functions:

✵ it is one of the principal excitatory neurotransmitters in the cortex, hippocampus, striatum, and Hypothalamus;

✵ it participates in the regulation of memory processes;

✵ it is a constituent of A number of small and medium-sized brain regulatory Peptides, such as Glutathione. In the form of pyroglutamate (its cyclic form), it is incorporated into various neuropeptides, including luliberin, thyroliberin, neurotensin, and bombesin;

✵ its energetic role is substantial, as glutamate serves as a supplier of α-ketoglutarate, a component of The Citric Acid cycle;

✵ it participates in ammonia detoxification through the formation of glutamine, which enters Neurons in large quantities across membranes where the enzyme glutaminase is present. Under the action of this enzyme, glutamate is regenerated and subsequently used for GABA synthesis. Given that Biomembranes are less permeable to glutamate than to glutamine, the latter can be regarded as a glial-neuronal transporter of glutamate (and, consequently, of GABA).

Dysfunction of the glutamatergic system occurs in a range of central nervous system disorders, including epilepsy, vestibular system disorders, and ischemia. Glutamate and its analogues are used pharmacologically in the Treatment of chronic Amino acid metabolism deficiency, vegetative-vascular dystonia, and epilepsy (as a precursor of GABA, an inhibitory neurotransmitter).

Another neurotransmitter amino acid is glycine. The concentration of glycine in blood plasma is low, meaning insufficient amounts of this amino acid are delivered to the brain. A significant portion of glycine is synthesized from glucose entering from the bloodstream (the synthetic pathways were discussed above).

Glycine is the most important inhibitory neurotransmitter (after GABA) in the Spinal Cord, Diencephalon, and certain Regions of the brain. High levels of glycine in blood plasma and urine typically indicate impaired brain function.

The degradation of glycine can proceed via three pathways:

✵ conversion of glycine to serine by the action of serine hydroxymethyltransferase;

✵ Cleavage of glycine into ammonia, carbon dioxide, and methylene-H₄-folate;

✵ oxidation catalyzed by amino acid oxidase (see subsection IV above).

Hyperglycinemia develops at an early age and is characterized by episodic vomiting, reduced motor activity, electroencephalographic abnormalities, and frequently a fatal outcome. Hyperglycinemia may result from impaired normal pathways of glycine degradation in nerve cells.

B. Nitrogen-containing compounds — histidine derivatives

The amino acid histidine is subjected to the action of various enzymes in different tissues and enters two distinct metabolic pathways:

✵ catabolism down to end products;

✵ synthesis of histamine (Fig. 9-31).

Fig. 9-31. Schematic representation of Histidine Metabolism in various tissues.

In The Liver and Skin, histidine undergoes deamination catalyzed by the enzyme histidase, yielding urocanic acid. The End products of histidine catabolism are glutamate, NH₃, and H₄-folate derivatives (N₅-formimino-H₄-folate and N₅-formyl-H₄-folate). A hereditary deficiency of histidase leads to the accumulation of histidine and The Development of histidinemia, which manifests as delayed mental and physical development in children. A hereditary defect of Urocanase in the liver can cause urocaninemia, characterized by elevated levels of urocanate in the blood. The symptoms of this pathological condition are largely similar to those of other enzymopathies and present as delayed mental and physical development.

The enzymes histidase and urocanase are hepatospecific; therefore, their assay is clinically used for the Diagnosis of liver damage.

1. Synthesis and biological role of histamine

Histamine is produced by the decarboxylation of histidine in Connective Tissue mast cells (see Scheme A).

Histamine forms a complex with proteins and is stored in the secretory granules of mast cells. It is released into the bloodstream upon tissue injury (such as trauma, Burns, or exposure to endo- and exogenous substances), as well as during immune and allergic reactions. Histamine performs the following functions in the human body:

✵ stimulates the secretion of gastric juice and saliva (thus acting as a digestive hormone);

✵ increases capillary permeability, induces edema, and lowers blood pressure (while increasing intracranial pressure and causing headaches);

✵ contracts the smooth musculature of the Lungs, causing asphyxia;

✵ participates in the development of inflammatory responses by causing vasodilation, skin redness, and tissue Swelling;

✵ triggers allergic reactions;

✵ acts as a neurotransmitter;

✵ serves as a pain mediator.

2. Synthesis and Biological Role of Carnosine and Anserine

The histidine dipeptides carnosine and anserine are synthesized in the Muscles and brain, with particularly high concentrations found in skeletal muscles, reaching levels of about 100–200 mg/100 g of tissue. Carnosine was discovered in 1900 by the Russian biochemist V. S. Gulevich, and anserine was identified somewhat later.

Carnosine is formed from β-Alanine and histidine through the action of carnosine synthetase (see Scheme B).

Subsequently, in the presence of SAM, carnosine undergoes methylation catalyzed by the enzyme N-methyltransferase to yield anserine. The β-alanine required for this synthesis is derived from the Catabolism of pyrimidine NUCLEOTIDES.

Carnosine can be exported from muscles into the bloodstream and taken up by the Kidneys and enterocytes. Human blood and kidneys contain a Zn-dependent enzyme, carnosinase, which is capable of hydrolyzing carnosine into β-alanine and histidine.

The Physiological effects of histidine dipeptides were initially studied by the Russian biochemist S. E. Severin in the 1960s and continue to be investigated by numerous researchers today. Carnosine increases the contraction amplitude of skeletal muscles, activates ion pumps in muscle cells, and stimulates the ATPase activity of Myosin. The concentration of histidine peptides in smooth and cardiac muscle is many times lower than in Skeletal Muscle. They account for up to 40% of the buffer capacity of fast-twitch muscles, enabling them to accumulate high amounts of lactate. In the absence of histidine peptides, excess lactate leads to acidosis and contracture. Furthermore, carnosine and anserine exhibit antioxidant activity, inhibit NO-dependent guanylate cyclase, and slow down human Aging processes by influencing the rate of apoptosis.

C. The Role of Arginine and Ornithine in the Synthesis of Biologically Active Molecules

The metabolism of the amino acid arginine is closely linked to the Reactions of the urea (ornithine) cycle, which can be viewed as the primary pathway for arginine synthesis. Within this cycle, the enzyme arginase catalyzes The breakdown of arginine into ornithine and urea.

Arginine performs several vital functions in the body:

✵ it is utilized in the synthesis of creatine, which, in the form of phosphocreatine, serves as an energy source for muscular work in humans and mammals. In invertebrate muscles, arginine phosphate performs a similar energetic function.

✵ it serves as a source of NO in the body;

✵ it acts as a precursor for ornithine, from which Polyamines are synthesized.

1. Arginine as a Source of NO in the Body

The amino acid arginine serves as the body's source of nitric oxide (NO). The production of NO within cells is catalyzed by a complex, Ca²⁺-dependent enzyme known as NO synthase. The enzyme contains a heme group, requires two Flavin Coenzymes (FAD and FMN), BH₄, and Zn²⁺ ions.

NO production occurs in all Cells and Tissues. Currently, three isoenzymatic forms of NO synthase have been identified in various cells: the neuronal and epithelial forms are constitutive, whereas the inducible form predominates in the liver, muscles, and myocardium.

Nitric oxide is a crucial signaling molecule that activates guanylyl cyclase and stimulates the rapid production of cGMP. This leads to a decrease in myocardial contractility and regulates vascular tone. In addition, the NO radical is involved in regulating the rate of apoptosis, prevents platelet aggregation and thrombosis, modulates the secretion of MEDIATORS AND HORMONES, and exhibits anticarcinogenic activity (Fig. 9-32).

Fig. 9-32. Biosynthesis and biological role of nitric oxide.

2. Synthesis of Spermidine and Spermine, and Their Biological Role

Arginine is converted by arginase into the amino acid ornithine, which is not incorporated into the body's proteins. The polyamines spermidine and spermine are synthesized from ornithine (see Scheme A).

The reaction proceeds via ornithine decarboxylase in the presence of pyridoxal phosphate. Subsequently, aminopropane residues are incorporated through the action of spermidine synthase and spermine synthase. The donor of these groups is a SAM derivative — S-adenosylmethylthiopropylamine (see Scheme B).

Spermidine, spermine, and putrescine are found in The Cell nuclei of all human Organs. Possessing a high positive charge, they readily bind to negatively charged DNA and RNA molecules, become components of Chromatin, participate in DNA Replication, and stimulate Transcription and Translation. Their concentration increases dramatically during intense tissue proliferation.

Ornithine decarboxylase is a regulated enzyme characterized by a very short T_1/2 of only 10 min. Growth Hormone, corticosteroids, and testosterone rapidly increase its levels by 10- to 200-fold.

The catabolism of polyamines into CO₂ and H₂O is mediated by polyamine oxidase in the liver. A portion of them is excreted by The Kidneys in an acetylated form.

Major biogenic amines and their amino acid precursors are listed in Table 9-6.

Table 9-6. Precursors and biological role of certain biogenic amines

Biogenic amines also include catecholamines (dopamine, norepinephrine, and epinephrine). Dopamine, in particular, acts as a neurotransmitter in the Midbrain. Norepinephrine is an excitatory neurotransmitter in the hypothalamus, as well as a neurotransmitter of the sympathetic nervous system and various regions of the brain. Epinephrine is a hormone actively synthesized under stress that regulates basal metabolism and enhances myocardial contraction.

D. Inactivation of Biogenic Amines

Normal biological function in nerve cells requires a specific concentration of biogenic amines. Their excessive accumulation can lead to various pathological disorders. Consequently, the mechanisms responsible for the inactivation of biogenic amines are of great importance.

The inactivation of biogenic amines occurs through two main pathways:

1) Methylation involving SAM via methyltransferases. Various biogenic amines can be inactivated in this manner, but histamine and epinephrine are inactivated most frequently. For example, epinephrine is inactivated by the methylation of the hydroxyl group at the ortho-position (see the scheme below).

Histamine inactivation also occurs predominantly via methylation (see Scheme A).

2) Oxidation by monoamine oxidase (MAO) enzymes using FAD as a coenzyme — this pathway is more typical for the inactivation of dopamine, norepinephrine, serotonin, and GABA. This process involves the Oxidative Deamination of biogenic amines to form aldehydes, followed by the corresponding acids, which are then excreted by the kidneys (see Scheme B).