Glycogen

GLYCOGEN, or animal starch, is a polysaccharide in the form of which carbohydrate reserves are deposited in the human body and other animals. G. belongs to the group of colloidal polysaccharides, the particles of which are built from several particles of simple carbohydrates-monosaccharides (in this case hexoses). Since it is not yet known from how many hexoses the particle of G. is built, their number is denoted by the coefficient x and the composition of G. is expressed by the formula (C₆H₁₀O₅)_x. Colloidal solutions of G. opalesce and are incapable of diffusion through animal membranes. Solutions of G. are precipitated by alcohol. With iodine, G. gives an intense reddish-brown coloration, with the so-called Best's carmine-red. When boiled with weak mineral acids, G. eventually breaks down into glucose, giving as intermediate products of such hydrolytic cleavage various dextrins (among which the same polyamyloses were found as are formed during the hydrolysis of starch), as well as the disaccharide-maltose. When G. is cleaved by diastatic enzymes, dextrins are formed first, and then the disaccharide-maltose. Views on the structure of G. are similar to views on the structure of starch. Pringsheim considers that G. is built in the same way as the amylopectin of starch, i.e. that the particle of G., devoid of phosphoric acid, consists of several particles linked together in a ring-triamyloses (C₆H₁₀O₅)₃ and is therefore a β-polyamylose. Carrer considers G. as a polymerized anhydride of maltose, Irvine-as an anhydrottrisaccharide. G. is deposited in the human and animal body mainly in reserve in the liver and in the muscles. The content of G. in the liver is subject to large fluctuations, reaching even 20%. With good nutrition and in the absence of heavy work, there is much G. in the liver (in humans up to 150 g), while during fasting, on the contrary, there is little or none at all. The content of G. in the muscles also fluctuates greatly, sharply decreasing during fasting and during intense physical work. In other organs of the body (lungs, kidneys, etc.) G. is present in small amounts. G. has also been found in plants. G. is formed in the liver and muscles under the influence of special enzymes from glucose brought to the liver (or muscles) by blood and entering the latter from the digestive organs. G. deposited in the muscles is consumed during their work: it is thereby again broken down into glucose molecules, and the latter undergoes a series of further transformations. The glucose necessary for the synthesis of new quantities of G. enters the muscles from the blood. If its content in the blood is not replenished by the intake of glucose from the digestive organs, then G. deposited in the liver will, under the influence of the diastatic enzyme of the liver, break down into glucose molecules, which will pass into the blood; thus all reserves of G. in the liver can be used up. G. can be synthesized in the liver not only from glucose, but also from fructose, D-mannose and D-galactose; all these are true formers of G. The ability of the liver to accumulate carbohydrate reserves, converting them into G., is not unlimited and depends on the amount of carbohydrates being absorbed (if a large amount is absorbed at once, the liver cannot retain it all and alimentary hyperglycemia and glycosuria occur) and on the nature of the carbohydrates (100 g of glucose eaten at one time do not cause glycosuria; but with respect to galactose, for example, the assimilative capacity of the liver is much less). In diseases of the liver, its assimilative capacity with respect to sugar may decrease, and therefore the test of the liver's ability to assimilate carbohydrates (convert them into glycogen) is used for functional diagnosis of the liver. A sugar injection (injection into the medulla oblongata) causes enhanced conversion of liver G. into glucose. The injection of adrenaline also causes the disappearance of G. from the liver as a result of its rapid conversion into glucose. Insulin, on the contrary, stimulates the synthesis of G. in the liver from glucose. Therefore, in diabetes, associated with a weakening of the internal secretory functions of the pancreas, the synthesis of G. in the liver and muscles is weakened, and the breakdown of glycogen is enhanced.

A. Palladin.

For the detection of G. in tissues, it is recommended to fix the material with absolute alcohol (aqueous fixatives dissolve glycogen) and as soon as possible after death, since from the moment of death and even in agony, the gradual disappearance of G. from the cells begins, partly due to dissolution in tissue fluids, but mainly due to enzymatic conversion into sugar. In an unstained state, cellular glycogen is distinguished by its strong luster and complete lack of structure; as for its form, in organs taken from a corpse and processed appropriately, it usually appears as grains or, more often, as larger, unevenly sized drops and clumps located in greater or lesser quantities in the protoplasm, and under pathological conditions often also in the cell nuclei [see color plates (pp. 91-92), Fig. 5]. However, drops and clumps apparently arise postmortem, since when examining completely fresh material, the same staining reactions (with iodine, Best's carmine, etc.) show that G. is distributed in the cell either diffusely or in the form of very small grains. In this connection, for many cells (leukocytes, liver cells, cartilage cells, muscle fibers, and some others), it has been established that G. is bound in them with one or another pre-existing structural protein elements (plasmosomes, mitochondria, etc.), which thus serve as so-called "glycogen carriers" (Glykogentrager). The bond of G. with its carriers may be more or less close, according to which the following are distinguished: 1) "stable" G., which is firmly incorporated into the cell protoplasm, does not undergo significant quantitative fluctuations, and is often detected with difficulty (or not detected at all) by microchemical reactions, and 2) "labile" or "consumable" G., temporarily deposited in the cell, easily detached from it as the body's need for combustible material arises, subject to sharp quantitative fluctuations and clearly detectable microchemically. The first type is apparently present in all organs of the human body except the nervous system, mammary gland, and bones under physiological conditions, however, in many of them its quantity is so small, and the combination with cell protoplasm is so close, that it is detected with great difficulty and not always under the microscope. It is characteristic of its constant presence in those tissue elements that are at some distance from the bloodstream, namely in cartilage cells and in various types of stratified epithelium. Here it is detected without difficulty and in more noticeable quantities. Similarly, the tissues of the embryo are quite rich in G., which under normal conditions is found everywhere except in the nervous system. Its increased content in the embryo is by most authors connected with the particularly lively metabolism in growing cells. Sometimes the presence and quantity of G. are clearly dependent on one or another functional state of the organ. Thus, the epithelium of the normal uterine mucosa immediately after the menstrual period shows the least content of G. or even its complete absence. During the interval, its quantity usually increases, reaching a maximum in the premenstrual phase, and then falling again during menstruation. The onset of pregnancy (both uterine and extrauterine) leads to an increase in G. content, which is always detected here not only in the covering and glandular epithelium but also in the decidual cells. The main depots of labile G. are the liver and skeletal muscle. In this connection, in the liver, its disappearance in a starving animal always begins from the periphery of the lobule (perhaps due to the arrival of diastatic enzyme with the blood from the portal vein), gradually spreading toward the center. Its accumulation, however, when feeding is resumed, occurs in the reverse order, i.e., from cells located near the central veins to the periphery. Sometimes G. can also be found outside cells (in the interstitial substance, lymph spaces, etc.). This is apparently most often the result of its postmortem washing out of the cell protoplasm, less frequently the result of antemortem damage or death of cells, m. skvorcov. Microscopic technique for the determination of G. To avoid dissolution of G.-fixation of as fresh material as possible with strong alcoholic fixatives (96-100° alcohol, Carnoy's fluid). According to some data, G. is also preserved when fixed according to Zenker. 1. Staining with iodine (Langhans). Paraffin sections are mounted by the so-called dry method, for which they are firmly pressed to slides rubbed with a glycerin-protein mixture and slightly melted by careful heating on a weak flame. The sections are passed through toluene and absolute alcohol and immersed for 5-10 minutes in Lugol's solution, and from there into a mixture: 4 parts absolute alcohol + 1 part T-hae Jodi. Mounting in Canada balsam. Result: G. and amyloid are red-brown in color. However, if the preparation is previously treated for 12 hours with ptyalin (for which it is sufficient to use filtered saliva), then G., unlike amyloid, will dissolve and not stain. The method is very reliable. 2. Staining with carmine according to Best (see Carmine). Fixation-with alcohol or according to Carnoy. Embedding in celloidin or celloidin-paraffin. Sections are mounted in 60° alcohol. Preliminary short staining with some alum hematoxylin, washing in 70° alcohol. Staining for 5-60 minutes in a mixture: 20.0 of Best's basic solution + 30.0 of 25% Liquor ammonii caustici + 30.0 of absolute alcohol. Differentiate for 1-5 minutes in the following mixture: 40.0 of methyl alcohol + 80.0 of absolute alcohol + 100.0 of distilled water. Mount in Canada balsam. Results: G.-bright red, nuclei-blue. Mucus, fibrin, and some cell granules also stain red. For differential diagnosis in doubtful cases, preliminary treatment with ptyalin is necessary. Both methods are also suitable for total objects, e.g., when examining Protozoa in smears.

G. Epstein.