Creatine

CREATINE, alpha-methyl-guanidinoacetic acid, one of the end products of nitrogenous metabolism, discovered by Chevreul in meat extract. When boiled with acids, C. easily gives up water and passes into its anhydride-creatinine. NH₂-C(:NH)-NH₂ → N-CH₂-C(:NH)-NH₂CH₂-COOH creatine

In the human body and other animals, C. is contained in the largest quantities in muscles, where it is apparently mainly formed (as a product of the metabolism of muscle tissue protein). Creatinine was discovered by Liebig in urine, for which it is a normal constituent. -Formation of creatine. Several hypotheses have been expressed regarding the formation of C.: according to the opinion of Riesser, C. is formed from choline and urea; there was talk of the formation of C. from histidine, of the connection of C. with uric acid. The most experimentally substantiated is the hypothesis of the formation of C. from arginine (see) of the protein molecule. This hypothesis fully corresponds to our knowledge of the mechanism of the transformation of amino acids in the animal organism; according to it, arginine, undergoing deamination, is converted into guanidinobutyric acid, which, oxidizing according to the Knopp rule, by oxidation in the beta-position, is converted into guanidinoacetic acid; the latter, undergoing methylation (the possibility of which has been experimentally proven by A. Palladin), is converted into methyl-guanidinoacetic acid, i.e., into creatine. The fact of the absence in muscles of the enzyme arginase, under the influence of which arginine could split into ornithine and urea, also speaks in favor of the transformation of arginine into C. The comparative-biochemical data also speak in favor of this hypothesis: in crustaceans (animals with slowed metabolic processes), the breakdown of proteins in muscles does not reach the formation of creatine, but stops at arginine, which is contained in the muscles of the crab in fairly large quantities. In accordance with this, while in the muscles of mammals creatine-phosphoric acid (see below) is contained, in the muscles of crustaceans there is arginine-phosphoric acid (Meyerhof).

Connection between C. and urinary creatinine. Under normal conditions, C. is formed in muscles and from there enters the blood. Undergoing anhydriding, creatine is converted into creatinine, which is excreted in the urine. The excretion of this endogenous creatinine is determined by the intensity of tissue protein breakdown and is not in direct proportion to the amount of all nitrogenous substances in the urine, which depends on the amount of nitrogenous substances in food. If the daily amount of creatinine nitrogen in the urine is calculated per kilogram of body weight, i.e., the creatinine coefficient is calculated, it turns out that in animals there is a direct relationship between the percentage content of C. in their musculature (limb muscles) and the creatinine coefficient: the richer the muscles of an animal in C., the higher its creatinine coefficient (this confirms the connection of urinary creatinine with muscle C.). For example, the data are: Animal Rabbit ... White rat Human ... Dog ... Percentage of C. content in muscles Creatinine coefficient 0.52 14.3 13.5 9.0 8.4 Creatinuria. The ability to anhydride C. in the animal organism is limited to certain limits; therefore, when the formation of C. in muscles increases, its anhydriding does not increase to the same extent, and then C. begins to be excreted in the urine (creatinuria sets in) along with somewhat increased excretion of creatinine. -The formation of C. in muscles, and therefore its content in them, can increase under the influence of a number of causes, for example, under the influence of causes that increase the breakdown of muscle protein. Increased formation of C. in muscles and creatinuria are observed during fasting, during scurvy avitaminosis, during cooling of the animal, causing a decrease in its body temperature. Furthermore, creatinuria occurs at the end of pregnancy and after childbirth, during tetanus, various forms of tetany, various febrile infectious diseases, during phosphorus poisoning, etc., as well as in various forms of diabetes and during carbohydrate starvation. In small children, unlike adults, creatinuria is a physiological phenomenon. In all these cases, creatinuria is endogenous; however, exogenous C. as well as creatinine can be excreted in the urine. This occurs when consuming food rich in C. (meat); in this case, exogenous C. is anhydrided to some extent and excreted in the form of creatinine, but partially excreted as C. The excretion of exogenous C. and creatinine ceases as soon as C. is eliminated from food.

C. in muscles. C. is contained in both striated and smooth muscles; in the latter-in much smaller quantities. The content of C. in the same muscles or in similar groups of muscles in different individuals of the same species of animals, for example, in different rabbits, differs by great constancy; so, for example, various researchers, taking for analysis the entire musculature of the hind limbs of rabbits, always found in it the same amount of C., namely-0.52%. At the same time, the content of C. in the muscles of various animals, as well as in different muscles of the same animal, varies: white muscles (capable of faster contractions) are always richer in C. than red (working more slowly) muscles; so, for example, in rabbits, white muscles contain 0.526%, red muscles-0.28% C.; in roosters-white 0.42%, red-0.26%. -The physiological role of C. in muscles has not yet been finally clarified; the theory of the connection between the processes of creatine metabolism and tonic muscle contractions is currently rejected by a number of authors. Undoubtedly, however, C. plays some role in muscle activity: this is indicated at least by the fact of the different content of C. in functionally different muscles. It is also necessary to point out here that with "muscle training, associated with an increase in their performance, the content of creatine in them increases (A. Palladin and Ferdman). C. of the brain. Second place after muscles in terms of C. content is occupied by the brain, with the cerebellum being richer in C. than the cerebral hemispheres; so, in the human hemispheres there is 0.132%, and in the cerebellum-0.204%. The content of C. in the brain increases in acute forms of polyneuritis; conversely, in its chronic forms, as well as in scurvy, it remains within normal limits. -C. and creatinine are also contained in the blood-in very small quantities (about 0.001%). Creatine-phosphoric acid. C. is contained in muscles not only in free form-a significant part of it is in combination with phosphoric acid (Eggleton, Fiske, Subbarow) in the form of creatine-phosphoric acid. Eggleton called this compound phosphagen, and Fiske-phosphocreatine. The structure of creatine-phosphoric acid is as follows: ___ /on

- HN=C(OH)-C(=NH)-N-CH₃ CH₂ COOH. Muscles, different in their function, contain different amounts of creatine phosphate acid: white muscles are richer in it, red muscles are poorer. In white muscles (e.g., in the t. biceps of a rabbit or guinea pig) about 28-30% of all C. is bound with phosphoric acid in the form of creatine phosphate acid; in red muscles there is less bound C.: in rabbits 20-30%, in guinea pigs 20%. Creatine phosphate acid plays a certain, not yet fully elucidated role in the chemistry of muscle activity. During muscle contraction, it breaks down, and this breakdown proceeds exothermically and is associated with the release of a significant amount of energy (for each gram of cleaving phosphoric acid, 110-120 calories are released). Creatine phosphate acid thus plays an energy role. During muscle rest, the reverse synthesis of creatine phosphate acid occurs, i.e., its reserves are restored. During muscle training, the content of creatine phosphate acid in them increases and so significantly that a large part of C. is found bound with phosphoric acid (despite the fact that the total amount of creatine also increases). There are data indicating a connection between the muscle's ability to be excited and its reserves of creatine phosphate acid. Creatine phosphate acid is also contained in the brain-both in the cerebral hemispheres and in the cerebellum. Methods for determining C. Creatinine gives a yellowish-red coloring with picric acid in an alkaline medium; this reaction, called the Jaffe reaction, is used as a qualitative reaction for creatinine. The same reaction is also the basis of the quantitative colorimetric method for determining creatinine and C. proposed by Folin. The determination is carried out by adding to a certain amount of urine (e.g., 1 cm³) poured into a measuring flask, 0.5 cm³ of a 10% solution of caustic soda, 1.5 cm³ of a saturated solution of picric acid, and distilled water up to 50 cm³. At the same time, in another identical flask, a standard solution is prepared [10 cm³ of a creatinine solution containing a certain amount of it (e.g., 3 mg%), plus 2.5 cm³ of 10% NaOH plus 7.5 cm³ of picric acid]. The intensity of the color of both solutions is compared in a colorimeter (see Colorimetry, colorimeters), which makes it possible, knowing the content of creatinine in the standard solution, to calculate its content in the portion of urine taken for analysis. The Jaffe reaction does not occur with C. Therefore, to determine the content of C. by Folin's method, it must first be converted to creatinine; this is achieved by boiling the C. solution with some acid (preferably in an autoclave). When examining urine, it is most convenient to boil a portion of urine (e.g., 2-5 cm³) for 1 hour with 10 cm³ of a saturated solution of picric acid. After this, NaOH is added and the total amount of creatinine (both previously present in the urine and formed from C.) is determined colorimetrically. By subtracting this total amount (sum of C. and creatinine) from the amount previously found by examining another portion of the same urine (preformed creatinine), the amount of creatinine formed from C. is found, i.e., the content of C. in the urine is determined. The same method is used to determine the content of C. in muscles (Risser-Palladin method); here it is necessary to first remove all muscle proteins and obtain C. in solution. For this, a sample of muscles is ground in a mortar with quartz sand and then boiled in a slightly acidified with acetic acid physiological NaCl solution. The solution is then filtered, and the precipitate is again ground and boiled with NaCl solution. This last procedure is repeated three times, after which all filtrates are collected, evaporated to a certain volume, and in this solution the content of C. is determined by the above method. On methods of determination in blood-see Blood. For the quantitative determination of creatine phosphate acid, methods by Fiske and Subbarov (2 methods) and Ferdman have been proposed.