From Big Medical Encyclopedia

The AUTOCATALYSIS (grech, autos itself + a catalysis) — process, in Krom acceleration of this chemical or biochemical change is carried out by one of its products. Example And. hydrolysis of amyl acetate can serve in aqueous solution:

CH 3 COOC 5 H 11 + H 2 O = CH 3 COOH+C 5 H 11 OH.

Reaction product — acetic acid (CH 3 COOH) dissociates:

CH 3 COOH ⇄ CH 3 COO - + H +  ;

the hydrogen ion is the catalyst of this reaction. The initial stage of reaction proceeds slowly, however with increase in quantity of a product catalyst speed of response respectively increases. Thus, autocatalytic reactions tend to proceed with constantly increasing speed. Exhaustion of a reserve of substrate leads to delay of reaction, and then and to its dead stop. Advantage of reactions of this kind is that they provide a possibility of bystry formation of any substances necessary for an organism at some point. So, at receipt of food in a digestive tract there is a need for proteolytic enzymes, and in particular in trypsin — enzyme which catalyzes hydrolytic decomposition of proteins on peptides in a small bowel. Trypsin cosecretes a pancreas in the inactive form known under the name of trypsinogen. Trypsinogen turns into trypsin under the influence of the enzyme which is chipping off peptide (hexapeptide or octapeptide) from trypsinogen. Such enzyme is itself tripsinony Therefore, it is autocatalytic reaction, in a cut reaction product catalyzes education of itself:

Initial formation of trypsin happens under the influence of other enzyme — the enterokinase produced by cells of a mucous membrane of a small bowel; release of enzyme is initiated by receipt of food in went. - kish. path. At once after formation of a small amount of molecules of trypsin reaction begins to accelerate and accelerates more and more until all trypsinogen turns into active enzyme. Thus, thanks to autocatalytic reaction digestive enzymes are put in action just while they are necessary for an organism.

The great value in processes of a metabolism is played by various cycles of reactions among which the important role belongs to a cycle of Tricarboxylic acids (see. Tricarboxylic acids cycle ). In it small amounts lemon, apple and other organic acids can catalyze oxidation of a large number of an active form acetic to - you since at a turn of each cycle the acetyl rest of one molecule atsetil-KOA, the metabolism which is one of intermediate products is oxidized.

A special case of cyclic reactions are reactions with autocatalytic stages in which any reaction product influences in an accelerating way one of the previous stages of a cycle.

Hinshelvud (Page N. Hinshelwood, 1955) were considered cases And., when reactions mutually accelerate each other therefore the maximum speed of response A?V?S is reached at the minimum concentration of substance C (see. Chain reactions ).

The autocatalysis plays an important role in the processes happening in live organisms is normal, and also at pathology. So, autocatalytic reactions take place at the different types of defeat which are followed by destructions of biological structures (B. N. Tarusov, 1954). The role of these reactions at radiation injury of organisms is especially big. Under the influence of ionizing radiation in biological objects there are various reactions, however their importance is not identical to development of radiation injury. Some of them quickly fade, others, in response to one ionization act, are limited only to turning into to some one molecule. And only active centers engendering autocatalytic reactions of chain type (at which thousands of molecules are involved in process and which by the nature shall develop with self-acceleration) have paramount value in radiation injury. In cells of an organism several such reactions proceed; it is shown, in particular, that substrate for them are lipoproteidny structures of membrane formations of cells. Due to similar reactions there is a bystry development of defeat in response to insignificant (in the power relation) the damaging influences.

See also Catalysis .

Bibliography: Biophysics, under the editorship of B. N. Tarusov and O. R. Necklace, M., 1968; Kondratyev V. N. Kinetics of chemical gas changes, M., 1958; Pang-chenkov G. M. and Lebedev V. P. Chemical kinetics and catalysis, M., 1961; Pasynsky A. G. Biophysical chemistry, page 87, M., 1963.

Yu. P. Kozlov.