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All the physical and chemical processes by which living, organized substance is produced and maintained and the transformations by which energy is made available for use by an organism.
In defining metabolism, it is customary to distinguish between energy metabolism and intermediary metabolism, although the two are, in fact, inseparable. Energy metabolism is primarily concerned with overall heat production in an organism, while intermediary metabolism deals with chemical reactions within cells and tissues. In general, the term metabolism is interpreted to mean intermediary metabolism. See Energy metabolism
Metabolism thus includes all biochemical processes within cells and tissues which are concerned with their building up, breaking down, and functioning. The synthesis and maintenance of tissue structure generally involves the union of smaller into larger molecules. This part of metabolism, the building of tissues, is termed anabolism. The process of breaking down tissue, of splitting larger protoplasmic molecules into smaller ones, is termed catabolism. Growth or weight gain occurs when anabolism exceeds catabolism. On the other hand, weight loss results if catabolism proceeds more rapidly than anabolism, as in periods of starvation, serious injury, or disease. When the two processes are balanced, tissue mass remains the same.
The metabolism of the three major foodstuffs, carbohydrates, fats, and proteins, is intimately interrelated, so any clearcut division of the three is arbitrary and inaccurate. Thus the metabolism of protoplasm is concerned with all three of these foodstuffs. The metabolic pathways of carbohydrates, fats, and proteins cross at many points; thus certain pathways of metabolism are shared in common by fragments of these different classes of foodstuffs.
Some of the metabolic processes of the protoplasm of both plant and animal cells occur along common pathways; carbohydrate metabolism in plants is similar in many details to carbohydrate metabolism in animals. Therefore the study of metabolism in any organism is, in a sense, the study of metabolism in all protoplasm. See Carbohydrate metabolism, Lipid metabolism, Protein metabolism
the sum of chemical reactions occurring in living cells that provide the organism with matter and energy for its vital activities, growth, and reproduction.
In its most widely used meaning, the word “metabolism” refers to the exchange of matter and energy; in its more precise and narrower sense, it refers to the chemical conversions, in the cells, of matter, from entry until formation of the end products (intermediate metabolism). In this sense, the word is used with reference to a particular substance or class of compounds (for example, protein metabolism, glucose metabolism). After entering the cell, a nutrient is metabolized—that is, it undergoes a series of chemical changes catalyzed by enzymes. The particular sequence of these changes is called the metabolic pathway; the intermediate products formed are called metabolites.
Two aspects of metabolism are distinguished—anabolism and catabolism. Anabolic reactions are directed at the formation and renewal of the structural elements of cells and tissues and involve the synthesis of complex molecules from simpler ones. These reactions, chiefly reductive, are accompanied by the expenditure of free chemical energy (endergonic). Catabolic conversions are processes by which complex molecules, both those entering with food and those forming part of the cell, are broken down into simple components. These reactions, usually oxidative, are accompanied by the release of free chemical energy (exergonic). Both aspects of metabolism are closely connected in time and space. Elucidation of the individual steps of metabolism in different classes of plants, animals, and microorganisms has revealed that the pathways of biochemical conversions in living nature are fundamentally alike.
REFERENCESMahler, H., and E. Cordes. Osnovy biologicheskoi khimii. Moscow, 1970. (Translated from English.)
Daugley, S., and D. Nicholson. Metabolicheskie puti. Moscow, 1973. (Translated from English.)
Bing, F. C. “The History of the Word ‘Metabolism.’ “Journal of the History of Medicine and Allied Sciences, 1971, vol. 26, no. 2.
the sum total of chemical reactions that occur in the body. Metabolism is the organized sequence of chemical and energetic transformations that ensures the preservation and self-perpetuation of living systems. In his definition of life, F. Engels remarked that life’s most important property is metabolism, the constant exchange of matter with the external environment; life ceases with the discontinuance of this exchange. Thus, metabolism is an essential and indispensable characteristic of life.
Every organ and tissue in all living beings is in a state of continuous chemical interaction with other organs and tissues and with the environment. It has been established with the use of isotope tracers that intensive metabolism occurs in every living cell (seeISOTOPE TRACERS).
With the consumption of food, a variety of substances enters the body. These substances then undergo changes; that is, they become metabolized. The result of metabolism is the partial incorporation of these substances into the body. This process of incorporation is called assimilation. The reverse process, dissimilation, is closely related to assimilation. In a living organism, substances do not remain unchanged but are more or less quickly broken down, with the resultant release of energy. They are replaced by newly assimilated compounds as the decomposition products are eliminated from the body.
The chemical processes that take place in living cells are highly ordered. Catabolic and anabolic reactions are spatially and temporally organized and coordinated to form an integral, delicately regulated system that has evolved over a long period of time. The close relationship between assimilation and dissimilation is manifested by the fact that the latter is both the source of the energy and the source of the starting materials for anabolic reactions.
The sequence of events in metabolism relies on the coordination of the rates of separate chemical reactions. This coordination is dependent, in turn, on the catalytic action of specific proteins, called enzymes. Almost every substance that participates in metabolism must react with an enzyme and consequently be rapidly and definitely altered by this enzymatic reaction. Every enzymatic reaction is a separate link in the chains of transformations—the metabolic pathways—that together constitute metabolism. Enzymes have a very wide range of catalytic activity. They are controlled by a complex, delicate regulatory system that establishes optimum conditions under which the life processes of the body can continue while the external environment changes. Thus, the regular sequence of chemical transformations depends on the composition and activity of the enzyme system, which is structured according to the needs of the organism.
To learn about metabolism, it is essential to study both the sequence of the individual chemical transformations and the immediate factors that determine the sequence. Metabolism developed at the very beginning of life on earth and is therefore based on a biochemical pattern that is common to all the organisms of our planet. However, over the course of the evolution of living matter, different changes and improvements in metabolism have occurred in various animals and plants. Thus, the metabolisms of organisms belonging to different taxonomic groups and that exist at different levels of evolution exhibit significant and characteristic differences, as well as a fundamental similarity in the main sequence of chemical transformations. The evolution of life has been effected by changes in the structures and properties of the biopolymers and in the energy mechanisms of the systems that regulate and coordinate metabolism.
Assimilation. The most substantial differences between the metabolisms of groups of organisms are found in the initial stages of assimilation. The primordial organisms are believed to have fed on organic matter that was formed abiogenetically. In the course of evolution, some living organisms acquired the ability to synthesize organic matter from mineral substances. All organisms can be classified as either heterotrophs or autotrophs (seeAUTOTROPHIC ORGANISMS and HETEROTROPHIC ORGANISMS). In heterotrophs, which include all animals, all fungi, and many bacteria, metabolism is based on the consumption of metabolizable organic matter. Indeed, heterotrophs are able to assimilate a relatively small quantity of CO2 in order to synthesize more complex organic compounds, but this process is completed only by the utilization of the energy that is contained in the chemical bonds of the organic substances that are found in foods. Autotrophs, which comprise green plants and some bacteria, do not rely on metabolizable organic matter; rather, such organisms synthesize organic matter from primary constituent elements. In order to carry out this synthesis, some autotrophs—sulfur bacteria, iron bacteria, and nitrifying bacteria—utilize the energy obtained from the oxidation of inorganic matter. Green plants use the energy of sunlight to synthesize organic matter in the course of the photosynthetic process. This process is the main source of organic matter on earth.
BIOSYNTHESIS OF CARBOHYDRATES. Green plants assimilate CO2 and form carbohydrates during photosynthesis. Photosynthesis constitutes a chain of successive oxidation-reduction reactions in which chlorophyll, a green pigment capable of capturing solar energy, takes part. The photochemical decomposition of water is achieved with the energy of light; oxygen is released into the atmosphere, while hydrogen is used to reduce CO2. Phosphoglyceric acid, which is formed in the relatively early stages of photosynthesis, is reduced to yield three-carbon sugars, or trioses. Two trioses—glyceraldehyde phosphate and dihydroxyacetone phosphate—are condensed by the enzyme aldolase to form a hexose, fructose diphosphate, which is converted, in turn, into other hexoses—glucose, mannose, and galactose. The condensation of dihydroxyacetone phosphate with certain aldehydes results in the formation of pentoses.
The hexoses that are formed in plants are the starting material for the synthesis of complex carbohydrates, including sucrose, starch, inulin, and cellulose. Pentoses give rise to pentosans, which participate in the construction of supportive tissues in plants. In many plants, hexoses can be converted to polyphenols, phenolcarboxylic acids, and other aromatic compounds, which, in turn, can form by polymerization and condensation such complex compounds as tannins, anthocyanins, and flavonoids.
Animals and other heterotrophs obtain metabolizable carbohydrates in food—mostly as disaccharides, such as sucrose, and polysaccharides, such as starch. Carbohydrates are broken down in the alimentary canal by enzymes into monosaccharides, which are absorbed and transported by the blood to all the tissues. Glycogen, the storage polysaccharide in animals, is synthesized in the tissues from monosaccharides.
BIOSYNTHESIS OF LIPIDS. The starting materials for the synthesis of lipids—fats and other fatlike substances—are the primary products of photosynthesis, the carbohydrates formed from these primary products, and the carbohydrates that are obtained in food. For example, the accumulation of synthesized fats in maturing seeds of oil-bearing plants occurs at the expense of sugars. The dry weight of some microorganisms, for example, Torulopsis lipofera, consists of about 11 percent fat after five hours of growth in glucose solutions. Glycerin, a substance needed for the synthesis of fats, is formed by the reduction of phosphoglyceraldehyde. High-molecular fatty acids, for example, palmitic acid, stearic acid, and oleinic acid, react with glycerin to form fats. These acids are synthesized from acetic acid, a product of photosynthesis or of the oxidation of substances that are formed as a result of the breakdown of carbohydrates. Animals also obtain fats from food. These fats are first broken down in the alimentary canal by lipases into glycerin and fatty acids and are then assimilated by the organism (see LIPOMFTABOLISM).
BIOSYNTHESIS OF PROTEINS. In autotrophic organisms, protein synthesis starts with the uptake of inorganic nitrogen (N) and with the synthesis of amino acids. Nitrogen fixation is the process by which some microorganisms take up molecular nitrogen from the air and convert it into ammonia (NH3). Higher plants and chemosynthesizing microorganisms consume nitrogen in the form of ammonium salts and nitrates. The latter are first enzymatically reduced to NH3. Subsequently, NH3 is combined with keto or hydroxy acids under the influence of the corresponding enzymes to form amino acids. For example, pyruvic acid and NH3 produce alanine, one of the most important amino acids. The amino acids that are thus formed may then undergo transamination and other changes to yield all the other amino acids of which proteins are made.
Heterotrophic organisms are able to synthesize amino acids from ammonium salts and carbohydrates. However, animals and man obtain most of their amino acids from protein contained in food. Heterotrophic organisms cannot synthesize certain amino acids. Consequently, they must obtain these amino acids with food proteins.
Amino acids combine with one another under the influence of the corresponding enzymes to form a variety of proteins (seePROTEINS). All enzymes are proteins. Some structural and contractile proteins also possess catalytic activity. For example, the muscle protein myosin can hydrolyze adenosine triphosphate (ATP), a process which supplies the energy needed for muscles to contract. Simple proteins react with other substances to form complex proteins. For example, proteins form glycoproteins, lipoproteins, and nucleoproteins by combining with carbohydrates, lipids, and nucleic acids, respectively. Lipoproteins are the main structural element in biological membranes. Nucleoproteins are incorporated in the chromatin of the cell nucleus; they also form the protein-synthesizing particles caller ribosomes.
Dissimilation. The energy source needed to maintain life and vital activities, such as growth, reproduction, mobility, and excitability, is the oxidation of some of the cleavage products that are used by the cells to synthesize structural elements. The oldest process, and therefore the process most common to all organisms, is the anaerobic—without the participation of oxygen— cleavage of organic matter (seeFERMENTATION and GLYCOLYSIS). Later this primitive mechanism by which living cells obtained energy was supplemented by the oxidation of the intermediate products of fermentation and glycolysis by atmospheric oxygen, which was appearing in the earth’s atmosphere as a result of photosynthesis. This was the origin of intracellular, or tissue, respiration.
DISSIMILATION OF CARBOHYDRATES. Carbohydrates are the main source of the energy that is stored in the chemical bonds of most organisms. The cleavage of polysaccharides begins with enzymatic hydrolysis. For example, the starch present in plant seeds is hydrolyzed by amylase when the seeds germinate. In animals, the starch absorbed with food is hydrolyzed in the saliva and in the pancreas by amylase to form maltose. Maltose is then hydrolyzed to form glucose. Glucose is also formed in animals as a result of glycogenolysis. Glucose undergoes further conversions during fermentation or glycolysis, resulting in the formation of pyruvic acid.
Depending on the type of metabolism that developed in the particular organism over the course of evolution, pyruvic acid may undergo further conversions. It can be anaerobically transformed in muscles during glycolysis and different kinds of fermentation. Under aerobic conditions—during respiration—pyruvic acid can be oxidatively decarboxylated to form acetic acid. It may also serve as a source for the formation of other organic acids—oxaloacetic, citric, os-aconitic, isocitric, oxalosuccinic, ketoglutaric, succinic, fumaric, and malic. The mutual enzymatic conversion of these acids that results in the complete oxidation of pyruvic acid to CO2 and H2O is called the tricarboxylic acid, or Krebs, cycle.
DISSIMILATION OF FATS. The dissimilation of fats also begins with hydrolysis by lipases to yield free fatty acids and glycerin. These substances can then be easily oxidized to yield, eventually, CO2 and H2O. Fatty acids are oxidized mainly by beta oxidation—a process in which two carbon atoms separate from the fatty acid molecule to form an acetic acid radical. The resultant new fatty acid can then undergo further beta oxidation. The acetic acid radicals are either oxidized to CO2 and H2O, or they are used to synthesize a variety of compounds, for example, aromatic compounds and isoprenes.
DISSIMILATION OF PROTEINS. The first step in the dissimilation of proteins involves hydrolysis by proteolytic enzymes and the resultant formation of low-molecular peptides and free amino acids. This secondary type of amino-acid formation can occur rapidly, for example, during the germination of seeds, when the proteins present in the endosperm or cotyledons are hydrolyzed to form free amino acids, which are partly used to build the tissues of the developing plant and partly subjected to oxidative degradation. The oxidative degradation of amino acids during dissimilation is accomplished by deamination and results in the formation of the corresponding keto or hydroxy acids. The latter are either further oxidized to CO2 and H2O, or they are used to synthesize a variety of compounds, including new amino acids. In man and animals, the liver is the site of particularly intensive amino-acid degradation.
The free NH3 formed by the deamination of amino acids is toxic. It can be neutralized by being bound to acids or by conversion to urea, uric acid, asparagine, or glutamine. Animals excrete ammonium salts, urea, and uric acid, while plants use asparagine, glutamine, and urea as nitrogen reserves. Thus, one of the major biochemical differences between plants and animals is the complete absence of nitrogen wastes in plants. Urea is formed during the oxidative dissimilation of amino acids mainly by means of the ornithine cycle, which is closely associated with other transformations of proteins and amino acids. Amino acids can also be dissimilated by decarboxylation, resulting in the formation of CO2 and any amine or new amino acid. For example, histamine, a physiologically active substance, is formed during the decarboxylation of histidine, and a new amino acid— alpha- or beta-alanine—is formed during the decarboxylation of aspartic acid. Amines may undergo methylation to form betaines and such important compounds as choline. Plants combine amines and certain amino acids in the biosynthesis of alkaloids.
Relationship between the metabolisms of carbohydrates, lipids, proteins, and other compounds. All the biochemical processes that occur in the body are interrelated. The relationship between proteins and oxidation-reduction processes is realized in various ways. Some biochemical reactions involved in respiration occur because of the catalytic action of the corresponding enzymes, which are proteins. Moreover, the products of protein degradation themselves—amino acids—can undergo such various oxidation-reduction transformations as decarboxylation and deamination. For example, the products of the deamination of aspartic and glutamic acids—oxaloacetic acid and alpha-ketoglutaric acid—are at the same time major links in the chain of oxidative transformations of carbohydrates that occur during respiration.
Pyruvic acid, the most important intermediate product formed during fermentation and respiration, is also closely related to protein metabolism: in reacting with NH3 it yields the important amino acid alpha-alanine. The very close association of fermentation and respiration with lipid metabolism is evidenced by the fact that phosphoglyceraldehyde, which is formed in the initial stages of carbohydrate dissimilation, is the starting material for the synthesis of glycerin. The oxidation of pyruvic acid, on the other hand, produces acetic acid radicals, from which high-molecular fatty acids and a variety of isoprenes— terpenes, carotenoids, and steroids—are synthesized. Thus, fermentation and respiration result in the formation of compounds that are essential for the synthesis of fats and other substances.
Function of vitamins and minerals in metabolism. Vitamins, water, and various mineral compounds play an important part in the transformation of substances in the body. Vitamins participate in numerous enzymatic reactions as constituents of coenzymes. For example, a vitamin B1 derivative, thiamine pyrophosphate, serves as a coenzyme in the oxidative decarboxylation of alpha-keto acids, including pyruvic acid. A phosphate ester of vitamin B6, pyridoxal phosphate, is essential for catalytic transamination, decarboxylation, and other reactions of aminoacid metabolism. A certain vitamin A derivative is a constituent of a visual pigment. The functions of a number of vitamins, for example, ascorbic acid, have not been conclusively determined. Various species of organisms differ both in the ability to synthesize vitamins and in the requirements for various vitamins. Vitamins are essential to a normal metabolism.
Sodium, potassium, calcium, phosphorus, trace elements, and other inorganic substances play an important role in mineral metabolism. Sodium and potassium participate in the cellular and tissue bioelectric and osmotic phenomena that underlie the mechanisms of membrane permeability. Calcium and phosphorus are the principal constituents of bones and teeth. Iron is a structural element of several enzymes as well as of the respiratory pigments, hemoglobin and myoglobin. Other trace elements—copper, manganese, molybdenum, and zinc—are also essential to enzymatic activity.
The esters of phosphoric acid, especially adenosinephosphoric acids, play a decisive role in the energy mechanisms of metabolism. These substances take up and store the energy that is released during glycolysis, oxidation, and photosynthesis. Like certain other high-energy compounds, they supply the energy that is stored in their chemical bonds for mechanical, osmotic, and other kinds of work. High-energy compounds also provide the energy for energy-requiring synthetic reactions.
Regulation of metabolism. The astonishing coordination and harmonious functioning of the metabolic processes in living organisms is achieved by the strict, adaptable coordination of metabolism in cells, tissues, and organs. For a given organism, this coordination defines the nature of the metabolism that has developed over the course of evolution. Metabolic coordination is maintained and directed by heredity and by the interaction of the organism with the environment.
At the cellular level, metabolism is the result of the regulation of the synthesis and activity of enzymes. The synthesis of each enzyme is controlled by the corresponding gene. By acting through various factors on a portion of the DNA molecule that contains information about the synthesis of a given enzyme, various intermediate metabolic products can trigger, intensify, or suppress synthesis of that enzyme. For example, the colon bacillus ceases to synthesize isoleucine in a nutrient medium when this amino acid is present in excess. An excess of isoleucine acts in two ways: (1) it inhibits the activity of the enzyme threonine dehydratase, which catalyzes the first stage of the chain reactions that end in the synthesis of isoleucine, and (2) it suppresses the synthesis of all the enzymes essential to the biosynthesis of isoleucine, including threonine dehydratase. The inhibition of threonine dehydratase is an example of the allosteric inhibition of enzymatic activity.
The theory of genetic regulation that was advanced by the French scientists F. Jacob and J. Monod views repression and induction of enzyme synthesis as two aspects of the same process. The cell cytoplasm contains various repressor proteins, each of which has a specific receptor site that is able to interact with a certain metabolite. This interaction can either induce or repress the synthesis of the enzyme that controls the synthesis of that or some other metabolite. Thus, the polynucleotide chains of DNA in the genes contain instructions for the synthesis of a great variety of enzymes; each of these enzymes can be formed by the action of a signal metabolite—the inducer—on the corresponding repressor.
A major role in the regulation of metabolism and energy in the cell is played by protein-lipid biological membranes, which surround the cytoplasm, nucleus, mitochondria, plastids, and other subcellular structures. The passage of various substances into and out of the cell is regulated by the permeability of the membrane. A large percentage of the cell’s enzymes is mounted within the cellular membranes. As a result of the interaction of a given enzyme with lipids and other constituents of the membrane, the enzyme’s conformation and, consequently, its properties as a catalyst are not the same as if the enzyme were acting in a homogeneous solution. The interaction of the membrane and the enzyme plays a large role in the regulation of enzymatic activity and of metabolism in general.
Hormones are the most important regulators of metabolism in living organisms. For example, in animals, the release of epinephrine is triggered by very low blood sugar levels (epinephrine promotes glycogenolysis and glucose formation). An excess of blood sugar intensifies the secretion of insulin, which inhibits glycogenolysis in the liver, with the result that less glucose enters the blood. Cyclic adenosine monophosphate (CAMP) plays an important role in the reactive mechanism of hormones. In animals and man, hormonal regulation of metabolism is closely related to the coordinating activity of the nervous system (seeHORMONAL REGULATION).
The close interdependence of all the biochemical reactions that constitute metabolism enables the organism to interact with the environment, an essential prerequisite of life. F. Engels wrote: “From the exchange of matter, which takes place through nutrition and excretion … proceed all the other most simple factors of Life” (F. Engels, Anti-Dühring, Moscow, 1954, p. 117). Thus, all the most important life-functions, including ontogeny (the development and growth of organisms), heredity and variability, and excitability and higher nervous activity can be understood and manipulated according to man’s will by ascertaining the hereditarily fixed patterns of metabolism and by evaluating the metabolic changes that occur under the influence of changing environmental conditions (given that these changes are limited by the reactive norms of the organism under study).
REFERENCESEngels, F. Dialektika prirody. In K. Marx and F. Engels. Soch., 2nd ed., vol. 20.
Engels, F. Anti-Dühring. Ibid.
Wagner, R., and G. Mitchell. Genetika i obmen veshchestv. Moscow, 1958. (Translated from English.)
Anfinsen, C. Molekuliarnye osnovy evoliutsii. Moscow, 1962. (Translated from English.)
Jacob, F., and J. Monod. “Biokhimicheskie i geneticheskie mekhanizmy reguliatsii ν bakterial’noi kletke.” (Translated from French.) In Molekuliarnaia biologiia: problemy i perspektivy. Moscow, 1964.
Oparin, A. I. Vozniknovenie i nachal’noe razvitie zhizni. Moscow, 1966.
Skulachev, V. P. Akkumuliatsiia energii ν kletke. Moscow, 1969.
Molekuly i kletki, fascs. 1–5. Moscow, 1966–70. (Translated from English.)
Kretovich, V. L. Osnovy biokhimii rastenii, 5th ed. Moscow, 1971.
Zbarskii, B. I., I. I. Ivanov, and S. R. Mardashev. Biologicheskaia khimiia, 5th ed. Leningrad, 1972.
V. L. KRETOVICH
Metabolic disturbances. All diseases are associated with some type of metabolic disturbance. The most dramatic disturbances arise with diseases that affect the trophic and regulatory functions of the nervous system and the endocrine glands, which are under nervous control. Metabolism is also disturbed by malnutrition, including an excessive or insufficient diet and an unbalanced diet, for example, a deficiency or an overabundance of a certain vitamin in the diet. Changes in the basal metabolic rate (BMR) are a symptom of systemic chemical and energetic metabolic disturbances. These disturbances are related to changes in the rate of the oxidative processes. An increase in the BMR is characteristic of diseases related to intensified thyroid functions. A decreased BMR is characteristic of thyroid insufficiency, pituitary and adrenal dysfunction, and general starvation. Disturbances of protein, fat, carbohydrate, mineral, and water metabolism are distinguished, but the metabolisms of all these substances are so interrelated that such a distinction is arbitrary.
Several conditions are indicative of metabolic disturbances. The accumulation of metabolites can be excessive or insufficient, or the interactive qualities of the metabolites can be altered, or the transformations that the metabolites undergo can change. Intermediate metabolic products can accumulate in the body, and end products of metabolism can be insufficiently or excessively excreted. Sometimes, abnormal metabolic products are formed. For example, diabetes mellitus is a disease in which carbohydrates are not completely assimilated, and the process by which they are converted to fat is disturbed. In obesity, too large an amount of carbohydrate is converted to fat.
Interference with the excretion of uric acid results in gout. Excessive excretion of uric, phosphoric, and oxalic acid salts in the urine may cause these salts to precipitate and give rise to nephrolithiasis. Uremia arises from the inadequate excretion of certain end products of protein metabolism in some kidney diseases. Certain intermediate metabolic products—lactic, pyruvic, and acetoacetic acids—accumulate in the blood and tissues in disturbances of oxidative processes and in nutritional disorders and avitaminoses. The impairment of mineral metabolism can alter the acid-base balance. A disturbance in cholesterol metabolism underlies atherosclerosis and some forms of cholelithiasis.
Other serious metabolic disorders include disturbances of protein assimilation in thyrotoxicosis, chronic suppuration, and certain infections. Water uptake is disturbed in diabetes insipidus, while many bone diseases—including rickets and osteomalacia—are caused by disturbances in the uptake of calcium and phosphorus salts. Addison’s disease is caused by abnormalities in the uptake of sodium salts.
Metabolic disorders are diagnosed by gas-exchange tests (seeGAS EXCHANGE), by measuring the ratio of the quantity of a given substance that enters the body to the quantity that is excreted, and by chemically analyzing the blood, urine, and other excretions. Isotope tracers are used to study metabolic disorders. For example, radioactive iodine, chiefly in the form of 131I, has been useful in studies of thyrotoxicosis. Treatment of metabolic diseases is aimed chiefly at eliminating the primary causes.
S. M. LEITES