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The transformation and fate of food proteins from their ingestion to the elimination of their excretion products. Proteins are of exceptional importance to organisms because they are the chief constituents, aside from water, of all the soft tissue of the body. Special proteins have unique roles as structural and functional elements of cells and tissues. Examples are keratin of skin, collagen of tendons, actin and myosin of muscle, the blood proteins, enzymes in all tissues, and protein hormones of the hypophysis. See Blood, Enzyme, Hormone, Muscle
Isotopic labeling experiments have established that body proteins are in a dynamic state, constantly being broken down and replaced. This is a rapid process in organs active in metabolism, such as liver, kidney, intestinal mucosa, and pancreas, much slower in skeletal muscle, and extremely slow in connective tissue elements and skin.
Protein is digested to amino acids in the gastrointestinal tract. These are absorbed and distributed among the different tissues, where they form a series of amino acid pools that are kept equilibrated with each other through the medium of the circulating blood. The needs for protein synthesis of the different organs are supplied from these pools. Excess amino acids in the tissue pools lose their nitrogen by a combination of transamination and deamination. The nitrogen is largely converted to urea and excreted in the urine. The residual carbon products are then further metabolized by pathways common to the other major foodstuffs—carbohydrates and fats. See Carbohydrate, Lipid
Ingestion of protein is needed primarily to supply amino acids for the formation of new and depleted body protein and as a source of various other body constituents derived from the amino acids. The amino acids of proteins fall into two nutritional categories: essential or indispensable, and nonessential or dispensable. For a number of amino acids, the category to which they belong changes between the periods of body growth and adulthood and changes also in different animal species. Eight essential amino acids are needed for maintenance of nitrogen equilibrium in healthy young men. The remaining amino acids can be formed in the body from other materials. See Amino acids
Protein digestion occurs to a limited extent in the stomach and is completed in the duodenum of the small intestine. The main proteolytic enzyme of the stomach is pepsin, which is secreted in an inactive form, pepsinogen. Its transformation to the active pepsin, initiated by the acidity of the gastric juice, involves liberation of a portion of the pepsinogen molecule as a peptide. Pepsin preferentially hydrolyzes peptide bonds containing an aromatic amino acid, and it requires an acid medium to function. See Digestive system
The acid chyme is discharged from the stomach, containing partially degraded proteins, into a slightly alkaline fluid in the small intestine. This fluid is composed of pancreatic juice and succus entericus, the intestinal secretion. The pancreas secretes three known proteinases, trypsin, chymotrypsin, and carboxypeptidase. All three are secreted as inactive zymogens. Activation starts with the transformation of the inactive trypsinogen into the active trypsin. Trypsin, in turn, activates chymotrypsin and carboxypeptidase. See Peptide
Trypsin and chymotrypsin are endopeptidases; that is, they cleave internal peptide bonds. The so-called peptidases are exopeptidases; they cleave terminal peptide bonds. Trypsin has a predilection for those containing the basic amino acid residues of lysine and arginine. These two proteinases perform the major share in hydrolyzing proteins to small peptides. Digestion to amino acids is completed by the exopeptidases. Carboxypeptidase acts on peptides from the free carboxyl end; aminopeptidases from the free amino end. Other peptidases act on di- or tripeptides, or peptides containing such special amino acids as proline.
The amino acid digestion products of the proteins are absorbed by the small intestine as rapidly as they are liberated. The absorbed amino acids are carried by the portal blood system to the liver, from which they are distributed to the rest of the body. Small amounts of the peptides formed during digestion escape further hydrolysis and may also enter the circulation from the intestine. This is shown by a rise in the peptide nitrogen in the blood.
The unabsorbed food residue in the small intestine is passed into the cecum, then the colon, and finally is eliminated as feces.
The absorbed amino acids that escape decomposition become part of the amino acid pools of the body. From these amino acids, new tissue proteins are synthesized to meet body needs. The rate of tissue replacement varies greatly for different tissues. In humans, it has been estimated that the average half-life of the total body protein is 80 days; that of lung, brain, bone, skin, and most muscle combined is 158 days; while that of liver and serum proteins combined is only 10 days.
The major organ of plasma protein synthesis is the liver. It forms all of the plasma albumin and fibrinogen and a considerable proportion of the globulins. (A portion of the total plasma globulin is synthesized in other tissues containing reticuloendothelial cells. The hormones and enzymes present in blood plasma are derived in the main from nonhepatic sources.) See Albumin, Fibrinogen, Liver
The plasma proteins have numerous important physiological functions. The albumin is the major factor in the regulation of the blood volume through its osmotic action, which counteracts the fluid expulsion effect of the hydrostatic pressure resulting from the contractions of the heart. Fibrinogen is one component of a sequential process essential for coagulation of the blood. Other plasma components include the blood platelets and prothrombin. The globulins include fractions that are carriers of phospholipids and sterols and certain essential metal ions, iron, and copper. Other fractions, chiefly γ-globulin, contain the antibodies that are the defenses against numerous diseases.
Synthesis and utilization of the plasma proteins is a rapid process. There is a complete turnover of the major plasma proteins in a period of a few days. The difference from normal in the turnover times in a variety of diseases provides an insight into the nature of the disease processes.
the sum of the processes of the transformation of proteins and the products of their decomposition—amino acids—in organisms. Protein metabolism is an essential part of metabolism. Since amino-acid metabolism is closely connected with the metabolism of other nitrogen compounds, protein metabolism is often included in the more general concept of nitrogen metabolism. In autotrophic organisms—that is, plants (except fungi) and chemo-synthesizing bacteria—protein metabolism begins with the assimilation of inorganic nitrogen and synthesis of amino acids and amides. In man and animals, only a portion of the amino acids—the so-called nonessential ones—can be synthesized in the organism from simpler organic compounds. The other portion—the essential amino acids—must be obtained from food, usually as protein. Proteins contained in various foods are broken down by cleavage under the action of such proteolytic enzymes as pepsin, trypsin, and chymotrypsin into amino acids, which are absorbed into the blood and carried to organs and tissues.
Plant tissues also contain proteolytic enzymes that hydrolytically break up proteins. The succeeding processes of protein metabolism in plants and animals are essentially amino-acid metabolism. A considerable portion of amino acids are used in the formation and completion of various proteins in the body, including functionally active proteins (enzymes, hormones, antibodies, and so forth), plastic proteins, structural proteins, and others. At the same time, the body’s proteins undergo constant breakdown and renewal, replenishing the reserve of free amino acids. The other portion of the amino acids is used in the formation of a number of low-molecular hormones, biologically active peptides, amines, pigments, and other substances necessary for the maintenance of life. For example, the amino acid glycine is used to form purine bases, and aspartic acid is used to synthesize pyrimidine bases. Glycine is the principal source for the formation of the pigmented grouping of hemoglobin. The hormones of the thyroid gland (thyroxin and its derivatives) and of the adrenal glands (epinephrine and norepinephrine) are formed from the amino acid tyrosine. Tryptophan serves as the source for the formation of biogenic amines and also (in part) of nicotinic acid and its derivatives. A number of other nitrogenous substances of the animal organism, such as glutathione, carnosine, anserine, and creatine, are products of the union or transformation of amino acids. Alkaloids in plants are also formed from amino acids.
The mutual transformation of amino acids is, in significant measure, produced by a process that is widespread in all organisms—the enzyme process, involving the transfer of amino groups. This process, called transamination was discovered by the Soviet scientists A. E. Braunshtein and M. G. Kritsman. Excess amino acids undergo enzyme processes of decomposition. The most common initial reaction of amino-acid decomposition is deamination, primarily oxidative deamination, after which the nitrogen-free remainder of the amino-acid molecule degrades to the end products—carbon dioxide, water, and nitrogen that splits off in the form of ammonia.
Ammonia is rendered harmless in animals through the synthesis of urea (which in man, mammals, and several other animals forms in the liver and is discharged with urine) or uric acid (in birds, reptiles, and insects) and is partially given off in the form of ammonium salts. In plants and some bacteria, inorganic ammonium nitrogen may be reutilized, that is, used again in the synthesis of amino acids and amides and then of proteins. In these processes the amides of aspartic and glutamic acids play an important role, being the most important reserve compounds of nitrogen in plants. These compounds play an important role in animal organisms as well. Urea is also found in a number of plants; its essential role in rendering ammonia harmless in fungi, bacteria, and higher plants has been established. In contrast to processes in animals, urea in plants may be used again in the processes of protein synthesis when a sufficient quantity of carbohydrates is formed. Thus, the principal difference between protein metabolism in animals and plants is that plants synthesize protein, first forming amino acids and amides from inorganic substances, and the ammonia that is formed in the deamination of amino acids is again used (through glutamine, asparagine, and urea) in the resynthesis of protein. Animals and man synthesize proteins from amino acids that are obtained from food and that are partially formed as a result of transamination; the cleavage products of amino acids are discharged by the body. Intermediate stages of protein metabolism in plants and animals have much in common.
The ratio of the total quantity of nitrogen entering the human or animal organism to the quantity of nitrogen given off is called the nitrogen balance. The nitrogen balance depends not only on the quantity of protein consumed and the species, age, and physiological condition of the organism but also on the amino-acid composition of the food proteins. If the body is provided with the essential amino acids in the necessary proportions, nitrogen equilibrium may be established with a minimal intake of protein in food. Protein metabolism in man and animals is regulated with the aid of the nervous system (there are data on the presence of a protein-metabolism center in the hypothalamus) and by means of changes in the normal secretions of the thyroid and other endocrine glands.
The problems of protein metabolism are of great practical significance in medicine (standards of protein nutrition, disturbances of protein metabolism in various diseases and their treatment) and in agriculture (fattening beef cattle, conditions fostering increased protein in cereal grains, and so forth).
REFERENCESBraunshtein, A. E. Biokhimiia aminokislotnogo obmena. Moscow, 1949.
Meister, A. Biokhimiia aminokislot. Moscow, 1961. (Translated from English.)
Kretovich, V. L. Osnovy biokhimii rastenii, 4th ed. Moscow, 1964. Chapter 13.
Haurowitz, F. Khimiia i funktsii belkov [2nd ed.]. Moscow, 1965. (Translated from English.)
Ferdman, D. L. Biokhimiia, 3rd ed. Moscow, 1966. Chapter 17.
I. B. ZBARSKII