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Physiological Functions of Amino AcidsAmino acids are linked together by peptide bonds to form peptides and proteins. Amino acids, peptides, and proteins are the basic components of the tissues and cells of organic living organisms and play a crucial role in life activities. Certain amino acids, in addition to forming proteins, participate in some special metabolic reactions and exhibit important characteristics.
(1) Lysine
Lysine is an essential alkaline amino acid. Due to its very low content in cereal foods and its susceptibility to destruction during processing, it is known as the first limiting amino acid. Lysine can regulate the metabolic balance of the human body. It provides structural components for the synthesis of carnitine, which promotes the synthesis of fatty acids in cells. Adding a small amount of lysine to food can stimulate the secretion of pepsin and gastric acid, enhancing the efficacy of gastric juice secretion, thereby increasing appetite and promoting growth and development in children. Lysine can also increase calcium absorption and accumulation in the body, accelerating bone growth. A deficiency in lysine can lead to insufficient gastric juice secretion, resulting in anorexia and nutritional anemia, as well as impaired central nervous system function and poor development. In medicine, lysine can also serve as an auxiliary drug for diuretics to treat lead poisoning caused by a decrease in blood chloride levels. It can also form salts with acidic drugs (such as salicylic acid) to reduce adverse reactions and, when combined with methionine, can inhibit severe hypertension. Simplexvirus is a factor or cause of cold sores, fever blisters, and genital herpes, while its close relative, varicella-zoster virus, is the causative agent of chickenpox, shingles, and infectious mononucleosis. Research published by the Indianapolis Lilly Laboratory in 1979 showed that supplementing with lysine can accelerate the recovery from herpes infections and inhibit their recurrence. Long-term use of lysine can antagonize another amino acid—arginine, which promotes the growth of herpes viruses.
(2) Methionine
Methionine is a sulfur-containing essential amino acid closely related to the metabolism of various sulfur-containing compounds in living organisms. A deficiency in methionine can lead to symptoms such as reduced appetite, slowed growth or no weight gain, enlarged kidneys, and liver iron accumulation, ultimately resulting in liver necrosis or fibrosis. Methionine can also utilize its methyl group to methylate toxic substances or drugs, thereby acting as a detoxifying agent. Therefore, methionine is used to prevent and treat chronic or acute hepatitis, liver cirrhosis, and other liver diseases, as well as to alleviate the toxicity of harmful substances such as arsenic, chloroform, carbon tetrachloride, benzene, pyridine, and quinine.
(3) Tryptophan
Tryptophan can be converted into 5-hydroxytryptamine (serotonin), an important neurotransmitter in the human brain. Serotonin neutralizes norepinephrine and epinephrine and improves sleep duration. When the content of serotonin in the animal brain decreases, abnormal behavior, hallucinations, and insomnia may occur. Additionally, serotonin has a strong vasoconstrictive effect and is present in many tissues, including platelets and intestinal mucosal cells. The injured body releases serotonin to stop bleeding. In medicine, tryptophan is often used as an antidepressant, antispasmodic, gastric secretion regulator, gastric mucosal protector, and potent anticonvulsant.
(4) Valine, Leucine, Isoleucine, and Threonine
Valine, leucine, and isoleucine are all branched-chain amino acids and are essential amino acids. When valine is insufficient, the central nervous system function of rats becomes disordered, leading to ataxia and tremors in the limbs. Examination of brain tissue by histological sections reveals degeneration of red nucleus cells. Advanced liver cirrhosis patients, due to liver damage, often develop hyperinsulinemia, resulting in a decrease in blood branched-chain amino acids. The ratio of branched-chain amino acids to aromatic amino acids drops from 3.0–3.5 in healthy individuals to 1.0–1.5, so intravenous injections of valine and other branched-chain amino acids are commonly used to treat liver failure and other diseases. Additionally, it can also be used as a treatment to accelerate wound healing. Leucine is used to diagnose and treat sudden hyperglycemia in children and can also be used as a treatment for dizziness and a nutritional supplement. Isoleucine can treat neurological disorders, anorexia, and anemia and is also very important in muscle protein metabolism. Threonine is one of the essential amino acids and participates in fat metabolism. A deficiency in threonine can lead to hepatic fatosis.
(5) Aspartic Acid and Asparagine
Aspartic acid is converted into oxaloacetate through deamination, promoting the tricarboxylic acid cycle, making it an important component of the cycle. Aspartic acid is also closely related to the urea cycle, responsible for converting ammonia in the blood into urea for excretion. At the same time, aspartic acid is also a raw material for the synthesis of precursors such as orotic acid and other nucleic acids. Aspartic acid is usually used in the form of salts with calcium, magnesium, potassium, or iron. Because these metals can enter cells through active transport after binding to aspartic acid, they can exert their effects inside the cells. The mixture of potassium aspartate and magnesium salt is primarily used to eliminate fatigue and is clinically used to treat heart disease, liver disease, diabetes, and other diseases. Potassium aspartate can be used to treat hypokalemia, and iron salt can treat anemia. Many types of cancer cells require large amounts of a specific amino acid for proliferation. Finding analogs of this amino acid—metabolic antagonists—is considered an effective means of cancer treatment. Asparaginase can inhibit the proliferation of cancer cells that require asparagine (e.g., leukemia). The analog of asparagine, S-carboxymethylcysteine, has shown significant effects against leukemia in animal experiments. More than 10 types of amino acid-based anticancer drugs have been developed, such as N-acetyl-L-phenylalanine and N-acetyl-L-valine, some of which can inhibit cancer cells by up to 95% or more.
(6) Cystine and Cysteine
Cystine and cysteine are sulfur-containing non-essential amino acids that can reduce the body's need for methionine. Cystine is an essential component for the formation of the skin and can accelerate the healing of burn wounds and provide chemical protection against radiation damage, stimulating the increase of red and white blood cells. The thiol group (–SH) in cysteine has many physiological functions and can alleviate the toxicity of harmful substances or drugs (phenol, benzene, naphthalene, cyanide ions) and has preventive effects against radiation. The derivative of cysteine, N-acetylcysteine, due to the thiol group, has the effect of reducing viscosity and can be used as a mucus dissolving agent to treat difficulties in coughing up phlegm, such as bronchitis. Additionally, cysteine can promote hair growth and is used to treat baldness. Other derivatives, such as L-cysteine methyl ester hydrochloride, can be used to treat bronchitis and nasal mucosal exudative inflammation.
(7) Glycine
Glycine is the simplest amino acid, which can be generated by the loss of a carbon atom from serine. Glycine participates in the synthesis of purines, porphyrins, creatine, and glyoxylate. Glyoxylate, when oxidized, produces oxalic acid, which can lead to the genetic disease oxaluria. Additionally, glycine can bind to a wide variety of substances, allowing them to be excreted through bile or urine. Additionally, glycine can provide a nitrogen source for non-essential amino acids, improving the tolerance of amino acid injections in the body. When used together with glutamic acid and alanine, glycine can be effective in preventing and treating complications of benign prostatic hyperplasia, urinary obstruction, frequent urination, and residual urine.
(8) Histidine
Histidine is non-essential for adults but essential for infants. Adding a small amount of histidine to the diet of patients with chronic uremia increases the rate at which amino acids bind to hemoglobin, alleviating renal anemia, so histidine is also an essential amino acid for uremic patients. The imidazole group of histidine can form coordination compounds with Fe2+ or other metal ions, promoting iron absorption, and thus can be used to prevent and treat anemia. Histidine can lower gastric acid secretion, relieve pain from gastrointestinal surgery, reduce nausea during pregnancy and heartburn, inhibit gastrointestinal ulcers caused by autonomic nervous tension, and is effective in allergic diseases such as asthma. Additionally, histidine can dilate blood vessels and lower blood pressure, and is clinically used to treat angina pectoris and heart failure. Patients with rheumatoid arthritis have significantly reduced histidine levels in their blood, and after taking histidine, their grip strength, walking ability, and erythrocyte sedimentation rate have improved. Under the action of histidine decarboxylase, histidine is decarboxylated to form histamine. Histamine has a strong vasodilatory effect and is related to various allergic reactions and inflammation. Additionally, histamine stimulates the secretion of pepsin and gastric acid.
(9) Glutamic Acid
Glutamic acid and aspartic acid have excitatory neurotransmitter effects and are the most abundant amino acids in the central nervous system of mammals. Their excitatory effects are limited to the central nervous system. When the content of glutamic acid reaches 9%, increasing 10–15 mol of glutamic acid can have an excitatory effect on cortical neurons. Therefore, glutamic acid is essential for improving and maintaining brain function. Glutamic acid is converted into γ-aminobutyric acid (GABA) through the decarboxylation of glutamic acid decarboxylase. GABA is a substance in the brain that has inhibitory effects on central nervous system excitation. When the content of GABA decreases, it affects cellular metabolism and function. Various derivatives of glutamic acid, such as ethyl acetylagmatine, are clinically used to treat motor disorders, memory disorders, and encephalitis caused by cerebral vascular disorders. GABA is effective in memory disorders, speech disorders, paralysis, and hypertension. γ-Aminobutyric acid β-hydroxybutyric acid is effective in local paralysis, memory disorders, speech disorders, instinctive renal hypertension, epilepsy, and mental retardation. Glutamic acid, like aspartic acid, is closely related to the tricarboxylic acid cycle and can be used to treat hepatic coma. The amide derivative of glutamic acid—glutamine—has a significant effect on gastric ulcers. The reason is that the amino group of glutamine transfers to glucose, forming glucosamine, a component of glycoproteins in the epithelial tissues of the digestive tract.
(10) Serine, Alanine, and Proline
Serine is a precursor for the synthesis of purines, thymine, and choline. Alanine plays an important role in the process of protein synthesis in the body. During its metabolism in the body, it is deaminated to form ketones, which are then metabolized through the glucose metabolic pathway to produce sugar. The pyrrole ring in the proline molecule is structurally related to hemoglobin. Hydroxyproline is one of the components of collagen. An imbalance in the concentration of proline and hydroxyproline in the body can weaken the flexibility of cartilage and ligament tissues in teeth and bones. Proline derivatives, when combined with diuretics, have antihypertensive effects.
Taurine
Taurine is a component of bile. It is widely present in animal milk, brain, and heart, with the highest content in muscle, existing in a free form and not participating in protein metabolism. It is found only in algae in plants and has not been discovered in higher plants. The taurine in the body is derived from the metabolism of cysteine. A deficiency in taurine can affect growth, vision, and the normal development of the heart and brain. Patients infected with bacteria may develop taurine deficiency due to the large-scale proliferation of bacteria consuming taurine in the body, leading to changes in the electroretinogram of the retina. Supplementation with taurine can improve the lesions of the retina. Since humans can only synthesize taurine to a limited extent, dietary taurine is very important. The content of taurine in dairy products is very low. In poultry, the content of taurine in dark meat is higher than in white meat. Compared to poultry and livestock, seafood has the highest content of taurine, such as oysters, clams, and mussels, which can contain up to 400 mg/100g or more, and heating does not affect its taurine content. Daily foods, including grains, fruits, and vegetables, do not contain taurine.
Arginine
(1) Arginine is a component of the urea cycle and has extremely important physiological functions. Consuming more arginine can increase the activity of arginine enzymes in the liver, helping to convert ammonia in the blood into urea for excretion. Therefore, arginine is effective in treating hyperammonemia and liver dysfunction. Arginine is a dibasic amino acid, and although it is not essential for adults, the body cannot maintain normal nitrogen balance and physiological functions if it is deficient in arginine under certain conditions, such as immature development or severe stress. If patients lack arginine, it can lead to hyperammonemia and even coma. Infants born with certain enzyme deficiencies in the urea cycle also require arginine, as it is essential for maintaining normal growth and development. The important metabolic function of arginine is to promote wound healing. It can promote the synthesis of collagen tissue, thus repairing wounds. Increased arginine enzyme activity can be observed in wound secretions, indicating a significant increase in the demand for arginine near the wound. Arginine can promote microcirculation around the wound, accelerating its healing. The immunomodulatory function of arginine can prevent the atrophy of the thymus (especially after injury), and supplementing with arginine can increase the weight of the thymus and promote the growth of lymphocytes in it. Supplementing with arginine can also reduce the size of tumors in animals, lower the metastasis rate of tumors, and increase the survival time and survival rate of animals. In the immune system, in addition to lymphocytes, the activity of phagocytes is also related to arginine. Adding arginine can activate its enzyme system, making it more effective at killing target cells such as tumor cells or bacteria.
Dr. Jianxian Zheng, Professor at South China University of Technology
Amino Acids and Human Health
Amino acids are the most basic substances that constitute biological proteins and are related to life activities. They are the basic units that form protein molecules in living organisms and are closely related to the life activities of organisms. They have special physiological functions in the body and are one of the indispensable nutritional components.
I. Basic Substances for the Human Body, the Material Basis of Life