Biological Chemistry

BIOLOGICAL CHEMISTRY (biochemistry) studies the chemical processes occurring in living organisms. Every living organism represents a dynamic system that is separated from the surrounding environment but communicates with it; the normal functioning (constant regime, regime permanent) of which is ensured by the fact that a series of substances passes through the organism from the environment, undergoing certain chemical transformations during this passage. A peculiarity of the living organism is that some of its constituent parts, compared with the constituent parts of inanimate nature, possess a large reserve of energy that is easily transferable from a potential state to a kinetic one. This accounts for the instability (lability) of the living organism. In order to maintain a constant regime and, during all the processes continuously occurring within it, to remain equal to itself, a living organism must possess the ability to reproduce its parts as they wear out. But, besides this ability to preserve its individual existence, a living organism also possesses the ability to reproduce organisms similar to itself from itself, i.e., to transfer the processes occurring within it beyond the limits of individual existence, and thereby ensure the preservation of generic existence. These basic properties of a living organism, reflecting the totality of processes occurring within it, determine the logical approach to its study, which to a significant extent coincided with the factual one. If the substance of a living organism differs from the substance of the surrounding inanimate nature, the question naturally arises: where does it come from and how is it formed? Everyday experience teaches that an animal organism can exist only at the expense of substances which it extracts directly or indirectly from plants. The latter, however, can exist quite independently, i.e., they possess the ability to somehow convert substances of inanimate nature into the state that characterizes the substances of a living organism. The first steps of Biological Chemistry as an experimental science were directed precisely in this direction. To clarify the question of where a living plant gets the substance necessary for building its body, Van Helmont (1577–1644) set up the following experiment, which must be considered the first quantitative experiment in the history of Biological Chemistry. He weighed 200 pounds of dried earth, planted a willow branch in it, and watered it with rainwater for 5 years. After this time, he removed the grown tree from the earth and weighed it; he dried the earth and weighed it as well. It turned out that the loss in the weight of the earth did not exceed 1/4 pound, whereas the branch had gained 159 pounds during this time. From this, Van Helmont concluded that the substance of plants is formed from water. Despite all the imperfection of this experiment, the conclusion was partly correct: it is known that the H and O of organic substances formed in living plants do indeed originate from water. But Van Helmont knew nothing about the C of these substances. Only more than 100 years later was the significance that atmospheric CO₂ has for the nutrition of plants and for the formation of organic substances in them clarified. Priestley, who discovered oxygen, already knew that an animal placed in a closed space makes its air unsuitable for breathing. He also noticed that if a green plant is placed under a bell jar containing such spoiled air, then after some time the latter again becomes suitable for breathing; but Priestley did not understand the meaning and significance of this experiment. Ingenhousz (1779) then showed that the air is purified only by the green parts of plants and only in the light, and Senebier at the same time clarified that during this process CO₂ is absorbed from the air, O is released in a free state, and C is assimilated by plants. Thus, one of the most important processes of biological chemistry was discovered—the assimilation of CO₂ by plants—which lies at the basis of the formation of organic substances in a living organism. In close logical connection with it stands the reverse process—the destruction of organic substances in a living organism. Everyday experience teaches that to maintain its existence, an organism must feed, and specifically on substances that include proteins, carbohydrates, and fats. These food substances, breaking down in the organism, maintain its heat. Since they are all easily combustible, the mind involuntarily establishes an analogy between their destruction in the organism and combustion in the open air or in a furnace. This analogy is deepened by the fact that just as draft and an influx of air are necessary for burning combustible substances in a furnace, so the drawing in of air, respiration, is necessary for the destruction of food substances in the organism. It is highly interesting to note that scientific intuition, by a simple comparison of facts, arrived at an almost correct understanding of the process of respiration in an era when nothing was known about oxygen and when the setup of experiments was of the most primitive character. In 1669, therefore, more than a hundred years before the discovery of O, the English physician Mayow arrived at the thought that the surrounding air is not a homogeneous body but contains a gas which is absorbed by a burning substance during combustion and by the blood of animals during respiration. Almost at the same time as Van Helmont, he performed the following experiment, remarkable for that time. Placing a lit candle under a bell jar, he noted the time that passed before the candle went out for lack of air. He performed an analogous experiment with a mouse. Placing the animal and the burning candle simultaneously under the same bell jar, he found that the mouse suffocated and the candle went out in a time half as short as in the first experiment. His conclusion reads: "It must be assumed that both animals and fire extract the same particles from the air." Another English physician, the famous anatomist Willis, at approximately the same time definitely indicated that blood is warmed as a result of the fact that it burns in the process of respiration; suffocation, on the other hand, occurs because the particles of air necessary for combustion cease to enter the lungs, where they act on the blood. These correct concepts did not find sympathy among subsequent naturalists. Only a century later (in 1775) did the founder of scientific chemistry, Lavoisier, proceeding from the discovery of O, establish the meaning and significance of respiration with complete clarity. He showed that in this process, the absorbed O is converted into an equal volume of CO₂. On the basis of this, Lavoisier stated the proposition that respiration is nothing other than the slow combustion of C and H, in all respects similar to the combustion occurring in a lamp or candle; in this respect, breathing animals are true combustible bodies that are slowly destroyed by combustion. To the middle of the 17th century also belongs the beginning of the study of another question that occupies a very large place in Biological Chemistry—the question of fermentation (see). The already mentioned Van Helmont recognized that fermentation (it is a matter of the alcoholic fermentation of liquids containing sugar) is caused by a special agent, or ferment (fermentum). Willis even tried to explain the action of the ferment by the assumption that the latter communicates its own internal movement to other bodies capable of decomposition. Mac Bride proved that the gas released during the alcoholic fermentation of sugar is identical to CO₂. In this question, Lavoisier was the first to clarify the basic reactions of fermentation. He subjected a weighed amount of sugar to fermentation, determined the amount of released CO₂ and formed alcohol, and came to the conclusion that sugar is completely converted into CO₂ and alcohol in this process. The analytical methods of that time were very crude, and if Lavoisier arrived at a correct conclusion, it was only because the errors of the experiment happened to compensate for each other. But it is quite possible that the correctness of his conclusion was facilitated by the correctness of the fundamental formulation of the question. Lavoisier, who discovered the law of conservation of matter, proceeded in every experiment from the position that there is complete equality between the body subjected to analysis and the substances that are extracted from it. If sugar yields CO₂ and alcohol, then one can say that sugar = CO₂ + alcohol. We now know that the fermentation of glucose indeed proceeds according to the equation C₆H₁₂O₆ = 2CO₂ + 2C₂H₅OH. Thus, by the beginning of the 19th century, the scientific achievements of Biological Chemistry were expressed in the establishment of three facts: 1) plants, with their green parts, decompose atmospheric CO₂ under the action of light, build organic substances from its C, and release O in a free state; 2) the respiration of animals is the slow combustion of the organic substances of their body at the expense of atmospheric O, whereby the C of these substances is exhaled in the form of CO₂; 3) alcoholic fermentation is the complete breakdown of sugar into alcohol and CO₂ caused by a ferment. In the history of humanity, the 19th century was a turning point in that, during this period, science and technology, continuously influencing each other, reached such a high level that is completely incommensurate with their former state. Together with other disciplines, biological chemistry also moved forward significantly.

It is necessary to note here briefly the most important moments of its further development, in the process of which the three indicated directions became closely intertwined. The successes of analytical chemistry made it possible to approach more closely the study of the composition of the plant organism, on the one hand, and the composition of the environment surrounding it, on the other. It was clarified that, besides C, which the plant takes from the CO₂ of the air, it requires a number of mineral elements, which it extracts from the soil. Research in the field of plant nutrition led to a radical change in one of the oldest branches of production—agriculture, and the needs of rational agriculture, in turn, prompted a number of highly important studies both in the field of plant physiology and in the field of chemistry. Research in the field of respiration led to the conclusion that, contrary to the long-held opinion, plants breathe just as animals do, i.e., they absorb O and release CO₂. Under the action of solar rays, along with respiration, a reverse process occurs (assimilation of CO₂ with the release of O), and depending on external conditions, the latter process can completely cover the former. The chemistry of respiration has been largely clarified thanks to the successes of the doctrine of slow combustion, or spontaneous oxidation (see Autooxidation), on the one hand, and the doctrine of enzymes (see) on the other. The development of the doctrine of enzymes began with the study of alcoholic fermentation and for a long time revolved around it. The successes achieved in the construction of the microscope made it possible to study such bodies which are invisible to the naked or poorly equipped eye. It turned out that the fermentum of Van Helmont is a collection of living unicellular organisms which feed on sugar as a source of C, live, multiply, and die. Without the participation of living cells there is no fermentation, since fermentation is a manifestation of the vital activity of the cell. Thus arose the vitalistic theory of fermentation, which found in the person of the great French chemist Pasteur its most vivid exponent. The correctness of the vitalistic theory was disputed by the well-known German chemist Liebig, who opposed it with a chemical theory of fermentation. According to this theory, an enzyme is a chemical body which, by decomposing itself, thereby causes the decomposition of other bodies capable of fermentation. In constructing his theory, Liebig relied on the fact that by the middle of the 19th century, a number of so-called soluble enzymes were already known, which could be extracted from animal and plant organisms with water: pepsin, which digests protein; diastase, which converts insoluble starch into soluble sugar; invertin, which converts cane sugar into a mixture of glucose and levulose, etc. If there is an enzyme in cells that converts starch into sugar, then why can there not be an enzyme in them that converts sugar into alcohol and carbon dioxide? Liebig, unfortunately, could not confirm his views directly, whereas the experimental data of Pasteur were unshakable. Only in 1897 did Buchner succeed in extracting from yeast by high pressure a juice which did not contain living cells, but nevertheless rapidly fermented sugar. By this it was proven that the yeast cell ferments sugar not because it is itself an enzyme, but because it contains an enzyme—a chemical body producing a specific chemical action. A monistic understanding of enzymatic processes became possible in Biological Chemistry only after it was established, thanks to the successes of physical chemistry, that there is no fundamental difference between enzymatic and catalytic processes. As a catalyst, an enzyme accelerates chemical reactions occurring in the organism. The significance of this acceleration is immense, since the tempo of life is conditioned by it. Without the assistance of digestive enzymes, the conversion of starch into sugar, and protein into peptone under the influence of water, would require not tens of minutes as it does now, but tens of months; without respiratory enzymes, the organism would suffocate in an atmosphere of O. The study of enzymes and their actions, which has made enormous progress over the last 30 years, led in a short time to a partial clarification of the mechanism of metabolism. As one of the most striking examples, one can point to the question of the abnormal course of carbohydrate metabolism, known by the name of sugar disease. In this disease, sugar does not burn rapidly enough in the organism and, circulating in the blood, produces a number of disorders. Recently it was established that an aqueous extract from certain sections of the pancreas, known by the name of the islets of Langerhans, possesses an almost miraculous property of lowering the amount of sugar in the blood in the shortest possible time. The substance contained in this extract, under the name of insulin, has received wide application in medicine. In a normal state, the gland secretes insulin into the blood in an amount sufficient for the complete regulation of carbohydrate metabolism. Upon the disorder of this gland, sugar disease occurs. Similar normal secretions—internal secretions—have been found for other glands as well. The adrenal gland secretes adrenaline, the thyroid—thyroxine, etc. The latter two substances are now prepared artificially and are widely used in therapy. No less important are the successes achieved by Biological Chemistry in the field of normal nutrition of the animal organism. It has been established that carbohydrates, proteins, and fats in themselves are still insufficient for maintaining the organism in a normal state. A whole series of additional substances is also needed, which are found in fresh fruits, milk, etc. These substances have received the name of vitamins (see). The great importance which Biological Chemistry has acquired among other disciplines brings to the forefront the question of the methods of teaching it. For a long time, in connection with the theory of vitalism (see), it was considered that the organic components of living beings represent special substances which are created only by living organisms and cannot be artificially reproduced. Biological Chemistry therefore set as its main task the study of the chemical composition of organisms; in accordance with this, in the teaching of Biological Chemistry, the main attention was devoted to the description of organic substances; chemical processes occurring in the living organism were assigned a secondary place. But the enormous successes achieved in organic chemistry in the second half of the 19th century radically changed views on this question. After it became possible to reproduce by purely chemical means sugars, peptones, and other substances considered until then the exclusive property of living beings, the study of the nature of the organic components of the cell began to be relegated to the field of organic chemistry, and Biological Chemistry occupied itself with the clarification of the interaction of these components, i.e., the processes occurring in the living organism. Since the majority of these processes proceed catalytically, the doctrine of enzymes and their actions is now the central point in the teaching of Biological Chemistry. Some instructors divide the course of Biological Chemistry into two parts: descriptive Biological Chemistry and dynamic, whereby the first part serves as a preparatory course for the second. In the majority of foreign universities, until the very last time, there existed no separate chairs of Biological Chemistry; its teaching was conducted in parallel with the course of physiology. Only in the last years has the separation of these two disciplines begun, and special chairs and courses have been established. In Russia, however, Biological Chemistry has long been separated into a separate subject of university teaching in medical faculties: its course was originally called "medical chemistry"; recently, in accordance with the changed content and tasks of the discipline, the name biological or physiological chemistry has been adopted; in other faculties (physico-mathematical, pedagogical), special courses of Biological Chemistry are being introduced only in the very last time. Research in Biological Chemistry is conducted both abroad and here, partly in biochemical laboratories of higher educational institutions, partly in special research institutes. Of particular renown are the Biochemical Institute of C. Neuberg, which is part of the Kaiser Wilhelm Institut in Berlin, the institute of Hopkins in Cambridge (England), and some others. In the USSR, there are only two special research institutes for Biological Chemistry—the Biochemical Institute of the People's Commissariat of Health in Moscow and the Biochemical Institute in Kharkov. As a very young science, numbering only a few decades of independent existence, Biological Chemistry has special societies uniting workers in this field in only a few countries. Of such societies, one can point to: the "Biochemical Society" (England) and the "Société de Chimie biologique" (France). For the most part, however, biochemists join older and more powerful physiological, chemical, and natural history societies.