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

Plasma proteins
The role of plasma proteins. The history of their study

Plasma is the liquid component of Blood obtained after the sedimentation of erythrocytes, leukocytes, and thrombocytes; it contains Proteins, CARBOHYDRATES, Lipids, Vitamins, Hormones, as well as many Metabolic waste products. Approximately 85% of the dry weight of plasma consists of proteins, which serve as the primary components of intravascular and interstitial fluids.

The protein content in plasma depends on the species and age CHARACTERISTICS OF THE Organism:

The physiological state, as well as Water and dietary regimens, are also of great importance. Under normal conditions, plasma protein levels fluctuate within relatively narrow limits, but various diseases can cause significant changes in both their total content and fractional composition.

Plasma Proteins, along with Lymph proteins that are compositionally similar, participate in numerous vital processes:

— in The transport of metabolic products and Other Compounds entering the bloodstream (the transport function of blood proteins);

— plasma proteins play a central role in protecting the body against Bacteria, Viruses, toxins, or foreign proteins (the protective function);

— plasma contains essential Components of the coagulation and anticoagulation systems, which maintain blood in a fluid state and prevent blood loss during injuries and vascular damage;

— plasma proteins ensure the constancy of circulating blood volume and regulate the redistribution of intra- and extravascular fluids (colloid-osmotic regulation);

— plasma proteins are capable of compensating for amino acid deficiencies during protein starvation or inadequate Nutrition (the reserve function).

Even this Brief Overview of the processes involving blood proteins is sufficient to appreciate their role in the vital activity of humans and animals and to understand the close attention they receive.

The protein nature of substances contained in plasma was established at the end of the 18th century by the French chemist Fourcroy, who demonstrated that blood contains several individual proteins differing in their properties. Fourcroy named these proteins albumin, fibrinogen, and gelatin. The Study of the Chemical composition of Blood proteins began in the 1930s, but only the advancement of preparative Protein Chemistry (in the late 19th and early 20th centuries) made it possible to isolate individual proteins and investigate the physicochemical properties, Structure, and physiological Functions of plasma proteins.

The first effective method of protein fractionation, proposed in 1878 by Méhu and Levites, was based on the differential solubility of plasma proteins in ammonium sulfate solutions. Varying the concentrations of ammonium sulfate and sodium sulfate made it possible to divide plasma proteins into two main fractions: albumin and globulins.

Albumin is soluble in a half-saturated solution of ammonium sulfate, whereas globulins precipitate under these conditions. It was later demonstrated that the globulin fraction itself can be subdivided into two subfractions with distinct physicochemical properties. One of these, soluble in distilled water, was designated as pseudoglobulin, while the other, soluble only in the presence of salts, was termed euglobulin (true globulin). Numerous attempts to improve the protein salting-out method by using other salts and more precisely refining precipitation conditions proved largely ineffective. This necessitated the search for alternative fractionation techniques with higher resolving power.

Significant breakthroughs in this direction were achieved by E. Cohn and his coworkers, who between 1940 and 1945 developed the ethanol method for blood protein Separation, based on the varying Solubility of proteins in water-ethanol mixtures. By altering alcohol concentration, Ionic strength, pH, and Temperature, individual Proteins can be selectively precipitated while preserving their Native State and physiological activity. To prevent Protein Denaturation and reduce the solubility of the precipitates, fractionation is carried out at low temperatures (—2-:-—6°). The Cohn method has gained widespread use in both laboratory and industrial practice, primarily for isolating therapeutic plasma proteins such as albumin, gamma globulin, and fibrinogen. However, alcoholic fractionation is quite complex. The pursuit of new approaches to preparative protein separation led to The Development of such effective Methods as Cellulose and Sephadex Chromatography, molecular sieve methods, and techniques based on the precipitation of individual proteins as sparingly soluble complexes formed through the interaction of proteins with polyphosphates, zinc and aluminum ions, etc., as well as with organic cations such as diaminoacridine and methyl orange.

When reviewing various protein separation pathways, special attention should be given to the Electrophoresis method proposed in 1935 by Tiselius. A protein molecule is a colloidal particle whose migration in an electric field depends on its charge, mass, and shape, thereby enabling the Separation of proteins according to their electrochemical characteristics. Based on this principle, electrophoresis permits both Qualitative and quantitative analysis of complex protein mixtures, as well as the preparative isolation of individual protein fractions in their native state in quantities sufficient for subsequent chemical and biological research. Its application has yielded vital data on the normal levels of albumin and globulins in the blood of humans and animals, as well as the alterations in blood protein composition that occur under pathological conditions.

Electrophoresis confirmed that serum albumin at physiological pH values is a homogenous protein, whereas globulins can be separated into at least five subfractions that differ in their migration velocity in an electric field. Each of these fractions is designated by a letter of the Greek alphabet. The globulin with the highest electrophoretic mobility, positioned on the electropherogram immediately following albumin (Fig. 12), is conventionally termed a1-globulin, while the components following it in order of decreasing mobility are designated as a₂-, ß-, y₁-, and y₂-globulins.

This blood protein nomenclature remains widely used today, although subsequent research has demonstrated that globulin fractions are compositionally heterogeneous and can be resolved into A large number of individual proteins differing in composition, properties, and physiological activity.

As analytical and preparative methods in protein chemistry advanced, our understanding of blood Protein Biochemistry broadened and deepened. Utilizing various isolation and analytical techniques, researchers demonstrated that plasma contains approximately one hundred individual proteins differing in chemical composition, physicochemical properties, Biosynthesis conditions, and biological functions. An appreciation of The complexity of blood protein composition is provided in Table 3, which lists the main chemical and PHYSICOCHEMICAL CHARACTERISTICS OF the most important plasma proteins.

As seen in Table 3, albumin accounts for approximately half of the total plasma proteins, gamma globulins comprise 15—20%, while the Proportions of the remaining proteins range from a few percent (transferrin, fibrinogen) down to fractions of a percent (properdin, ceruloplasmin).

It should be noted that proteins with identical or very similar electrophoretic mobilities may differ markedly in other physicochemical properties and biological activity, whereas proteins with similar functional activities frequently appear in different electrophoretic fractions. For example, transport proteins for carbohydrates, lipids, metals, etc., are constituents of a- and ß-globulins. As Table 3 shows, the molecular weights, sedimentation and diffusion constants, Isoelectric and Isoionic points, as well as The chemical composition of plasma proteins, vary widely. While this facilitates the preparative separation and analytical determination of blood proteins, it complicates their study and systematization. Therefore, it is more practical to classify proteins not by their physicochemical properties, but rather by the functions they perform within the organism.

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Fig. 12. Electropherogram of human Serum proteins obtained using Interference optics.

Table 3 Physical and Chemical characteristics of plasma proteins (after G. V. Troitskii, 1962)

Depending on The Nature of the processes involving a particular protein, plasma proteins can be divided into the following groups (Table 4): 1) Metabolite Carrier Proteins; 2) proteins with regulatory functions; 3) Reserve Proteins; 4) protective (defense) proteins.

Along with proteins constantly present in plasma, the bloodstream may receive proteins whose appearance is caused by tissue damage or altered tissue and cellular permeability. These components are absent or nearly absent under normal conditions, but their concentration can increase sharply in certain pathological states (such as Liver damage, myocardial infarction, etc.). Since they significantly affect the composition of plasma, it is also advisable to examine them when studying the functional biochemistry of blood.