Protein Chemistry - Part 2 - Selected Topics in Special Protein Chemistry - Ashmarin I. P. 1968

Plasma Proteins
Regulatory Functions of Plasma Proteins

Plasma Proteins play a significant role in The regulation of many metabolic processes. Their influence on hormone activity and their importance in Lipid METABOLISM were noted above. The biologically active Polypeptides contained in the Blood—kallidin, bradykinin, and vasopressin—exert a pronounced effect on blood pressure and vascular permeability.

Special attention should be given to The Role of plasma proteins in regulating the colloid-Osmotic Pressure of the blood. The osmotic pressure due to the presence of proteins in the blood is relatively small because it depends on the molar concentration of proteins in the plasma; even for albumin, the latter does not exceed 5–6∙10^-5 M, which is hundreds of times lower than the molar concentration of salts in the blood.

Despite its small absolute value, colloid-osmotic pressure plays a vital role in the redistribution of Water between the bloodstream and Tissues, as well as in salt metabolism. Since the protein concentration in plasma is higher than in Lymph and tissue fluids, plasma proteins, which possess high hydrophilicity, bind a large amount of water and retain it within the vascular bed. Albumin is of primary importance in this process, accounting for ³/₄ of the total colloid-osmotic pressure of the plasma. Each gram of albumin in the bloodstream increases plasma volume by 17 ml. All other proteins, whose relative content in the blood is about 40%, account for a smaller share of the colloid-osmotic pressure. A decrease in blood albumin levels, observed in Liver and Kidney diseases, is accompanied by a drop in colloid-osmotic pressure within the bloodstream, resulting in fluid shifting out of the vessels and causing tissue edema. Occasionally, the reduction in colloid-osmotic pressure caused by lowered blood albumin can be partially offset by an increase in a- and y-globulins, though the extent of this compensation is usually small.

The content and fractional composition of plasma proteins determine not only the colloid-osmotic pressure but also blood viscosity. Plasma viscosity is governed by the concentration and Structure OF THE proteins it contains. High-molecular-weight proteins such as fibrinogen, a₂-, and y₁M-macroglobulins exert the greatest influence on viscosity. In diseases accompanied by an elevated level of these proteins, a marked increase in blood viscosity is observed, which adversely affects the Blood supply to Organs and tissues.

As noted above, blood flow velocity, blood pressure, and vascular permeability depend on the polypeptides and low-molecular-weight proteins present in the blood. These include kallidin and bradykinin, which have a vasodilating effect and lower blood pressure; vasopressin, which increases blood pressure; and kallikrein and the so-called factor P, which alter vascular permeability. The concentration of these substances is normally low, but it can change significantly under certain physiological conditions or pathological processes. The Molecular Weight of the aforementioned compounds ranges from 1,300 (bradykinin) to 40,000 (kallikrein). Detailed studies of The structure of bradykinin and kallidin have shown that they are polypeptides consisting of 9 and 10 amino acid residues, respectively, and differ only in that kallidin contains an additional Lysine residue. This minor change in the Amino Acid Composition of the polypeptides significantly affects their functional activity.

When considering the regulatory Functions OF BLOOD proteins, it is necessary to examine the blood Enzymes. Blood Enzymes can be divided into two groups.

1. Enzymes permanently present in the plasma, the so-called endoenzymes—ceruloplasmin, pseudocholinesterase, lipoprotein lipase, as well as the Enzymes and Coenzymes of the Blood Coagulation and anticoagulation systems.

2. Enzymes entering the bloodstream As a result of tissue breakdown or impaired Cell membrane permeability. This group of enzymes is referred to as excretory enzymes or exoenzymes. Their activity depends on the nature, localization, and severity of the pathological process causing the enzymes to enter the bloodstream.

Endoenzymes perform specific regulatory functions in lipid metabolism, blood clotting, and oxidation. Exoenzymes entering the blood generally do not take direct part in the specific enzymatic reactions occurring in the bloodstream; however, their study is of great interest for the Diagnosis of various diseases, as many of these enzymes exhibit a certain tissue Specificity, and their appearance in the blood indicates damage to a specific organ or tissue.

According to S. Ya. Kaplansky and B. Hess, over 40 different excretory enzymes can enter the bloodstream. Currently, the Determination of the activity of transaminases, Phosphatases, aldolase, and certain dehydrogenases (Lactate dehydrogenase, sorbitol dehydrogenase) is of the greatest diagnostic importance.

A very promising and exciting direction in clinical enzymodiagnostics is The Study of Isoenzymes present in the blood. Isoenzymes (isozymes) are enzymes that possess identical catalytic activity but differ in their physicochemical properties (electrophoretic mobility, thermostability, etc.). These differences are determined by the Structural Features of the enzymes synthesized by different tissues. The study of isozymes allows for a more precise identification of the sources from which a particular isoenzyme enters the blood and, consequently, the localization of the lesion. Studies by Wroblewski, Markert, Helm, I. M. Markelov, B. F. Korovkin, and others have shown that isoenzyme analysis Methods, and specifically the determination of lactate dehydrogenase isozymes, are of great importance for diagnosing Diseases of the liver, Heart, and other organs.