Investigation of enzymes and isoenzymes

Enzymes are specific proteins that fulfill the role of biological catalysts in the body. Enzymes are contained in all cells of the body, where their concentration is much higher than in the blood plasma. The serum

of blood, the enzyme composition of which is relatively constant, is most often used as an object for research. Normal activity of enzymes in the serum reflects the relationship between biosynthesis and the release of enzymes( with the usual cell renewal), as well as their clearance from the bloodstream. An increase in the rate of renewal of enzymes, damage to cells usually lead to an increase in the activity of enzymes in the serum. In the serum of blood, three groups of enzymes are distinguished: cellular, secretory and excretory.

Cell enzymes are divided into two groups depending on the localization in tissues:

■ non-specific enzymes that catalyze exchange reactions common to all tissues and are found in most organs and tissues;

■ organ-specific or indicator enzymes specific for a specific tissue type only.

Serum activity of cellular enzymes is low or nonexistent. In pathological processes, the activity of the enzymes of this group in the serum depends on the rate of release from the cells, which in turn is determined by the rate and degree of their damage.

Secretory enzymes( ceruloplasmin, pseudocholinesterase, lipoprotein lipase) go directly to the blood plasma and perform specific functions in it. These enzymes are synthesized in the liver and are constantly released into the plasma. Their activity in the serum is higher than in cells or tissues. In clinical practice, they are of interest when their activity in the blood serum becomes lower than normal due to impaired liver function.

Excretory enzymes are formed by the digestive system( pancreas, intestinal mucosa, liver, endothelium of bile ducts).These include a-amylase, lipase, alkaline phosphatase, etc. Normally their serum activity is low and constant. However, in pathology, when any of the usual pathways of excretion is blocked, the activity of these enzymes in serum significantly increases.

The measured activity of enzymes can be due to the presence of very close in properties but slightly different molecular forms of enzymes. These various forms are called isoenzymes. Investigation of isoenzymes in clinical practice is of interest when individual isoenzymes are formed in different tissues( for example, in the heart and liver, various LDH isoenzymes predominate).

For the quantification of enzyme activity, the Commission on Enzymes of the International Biochemical Union recommended a standard international unit( IU).The unit of activity of any enzyme is taken as its amount, which under optimal conditions catalyses the conversion of 1 μmol of substrate per minute( μmol / min).

The activity of the enzyme is judged by the rate of the catalyzed reaction at a certain temperature and pH of the medium, the concentration of the substrate. Therefore, when determining the activity of enzymes, the same conditions must be strictly observed.

The enzymatic reaction is sensitive to temperature changes. It is usually done at a temperature within the

range of 25-40 ° C, but at different temperatures, the optimum pH, buffer, substrate and other parameters are different. The maximum activity of most enzymes in the human body is observed at a temperature of about 37 ° C.Therefore, for the purpose of international standardization, enzyme activity measurements are carried out at a given temperature. Below the normal values ​​of the enzyme activity are given for a temperature of 37 ° C.

Investigation of enzymes is used in clinical practice to solve various problems: diagnosis, differential diagnosis, assessment of the course of the disease, determining the effectiveness of therapy, the degree of recovery and the prognosis of the disease. Three types of enzyme changes are known in the case of pathology: hyperferment-mia - increased enzyme activity compared to the norm, hypo-fermentation - its decrease, dysfermentation - the appearance of enzymes in the blood, normally not detectable.