Protein Chemistry - Part 2 - Selected Chapters of Special Protein Chemistry - Ashmarin I. P. 1968

Plasma proteins
The role of plasma proteins in immune responses

A pivotal role in protecting humans and animals against Bacteria, Viruses, toxins, and foreign Proteins is played by a special group of Plasma Proteins known as IMMUNOGLOBULINS, or Antibodies. While various types of immunoglobulins may differ in composition and physicochemical properties, they all share a common ability to selectively interact exclusively with those compounds or microorganisms (Antigens) whose presence triggers The production of that specific antibody type.

Antigens interact with antibodies in various ways. When microbial or other foreign Cells bind with antibodies, they are either destroyed by them (lysis reaction) or 'agglutinated' into large aggregates that are subsequently engulfed by Cells of the reticuloendothelial system (agglutination reaction). Soluble antigens (such as proteins and toxins), upon interacting with antibodies, lose their activity either through the blockage of biologically active sites (neutralization reaction) or through The formation of an insoluble antigen-antibody complex, which is cleared from the bloodstream by cells of the reticuloendothelial system (precipitation reaction).

Class="center">Table 6 Main characteristics of immunoglobulins

Using Physical and Chemical Methods, immunoglobulins can be separated into three main groups that differ in electrophoretic mobility, molecular weight, and composition. The primary data characterizing each of these groups are presented in Table 6.

As can be seen from the table, the bulk of antibodies consists of y₂G-globulins—proteins with the electrophoretic mobility of y₂-globulin, a Molecular Weight of 160,000, and a sedimentation constant of about 7 Svedberg units. The second group of immunologically active proteins comprises y₁A-globulins, which have a similar or slightly higher molecular weight, but exhibit greater electrophoretic mobility and contain up to 10–11% CARBOHYDRATES. High-molecular-weight antibodies (approximately 800,000–1,000,000) with a sedimentation constant of 19S—y₁M-globulins—constitute the third group of immunoglobulins.

All of these proteins are synthesized by specialized cells known as the immunocompetent cells of the lymphoreticular system, with each such Cell typically producing antibodies against a single specific antigen.

The process of antibody formation consists of several stages. The First stage, the inductive phase, proceeds at a relatively high rate and culminates in the production of high-molecular-weight y₁M-globulins. In some cases, antibody production stops at this stage, but typically the first stage is followed by the second, productive phase, during which the bulk of y₂G- and y₁A-immunoglobulins is synthesized. It should be noted that there are significant differences in the rates of Biosynthesis AND DEGRADATION between high- and low-molecular-weight gamma globulins. For instance, in humans, the half-life of y₁M-globulins is 8 days, whereas for y₂G- and y₁A-immunoglobulins it is approximately 20 days.

Since different immunoglobulins possess distinct serological Specificity—meaning they are capable of selectively interacting only with specific antigens—researchers naturally faced the question: what structural and chemical features account for this remarkable selectivity of immunoglobulins? Investigations into the Chemical Composition and PHYSICOCHEMICAL PROPERTIES OF antibodies failed to provide an answer, because The amino acid and carbohydrate composition, as well as the principal physicochemical characteristics (such as sedimentation constant and electrophoretic mobility) of antibodies with different specificities, are in many cases completely identical.

Significant breakthroughs in studying this problem have only been achieved in recent years through the application of Special Methods involving Enzymatic Hydrolysis and reductive Acetylation of immunoglobulins. Both methods make it possible to cleave the complex globulin molecule into simpler fragments and study The Structure of each. It was established that the Antigen-Antibody Reaction involves not the entire immunoglobulin molecule, but only a small region known as the combining or reactive center, which comprises only about 1% of the Amino acids contained in the molecule. Employing the aforementioned techniques, Porter and Edelman demonstrated that the y₂G-globulin molecule consists of four polypeptide chains: two light chains with a molecular weight of about 20,000 (L-chains) and two heavy chains with a molecular weight of 60,000 (H-chains) (Fig. 13). The light chains of various antibodies are quite similar in structure, whereas the H-chains differ noticeably in their Amino Acid and carbohydrate content.

The Structural Features of the heavy chains determine the selectivity of antibody-antigen interactions; their belonging to a specific immunoglobulin class (y₂G, y₁A, or y₁M); species and allotypic (individual) differences among gamma globulins; and the ability of antibodies to cross the Placenta, thereby providing immune protection for the fetus during embryonic development.

Fig. 13. Schematic diagram of a gamma-globulin molecule (Porter, 1962).

AC — active center; TRP — region of the molecule responsible for transplacental permeability; CFP — Complement-fixing region.

1 — bond cleaved by Papain; 2 — bond cleaved by Pepsin; 3 — C-terminal region of the molecules with carbohydrate groups; 4 — N-terminal region of the molecule.

The reductive acetylation method also proved highly effective in investigating the structure of y₁M-macroglobulins. Upon the reduction of Disulfide Bonds, the macroglobulin molecule dissociates into 5–6 subunits, each of which is identical in molecular weight, immunological specificity, and electrophoretic mobility to a y₂G (7Sy)-globulin molecule. Thus, immune macroglobulins can be viewed as a unique 'polymeric form' of 7Sy-globulins.

The structural differences between y₂G- and y₁M-globulins stem from the fact that each is synthesized by different cell types. High-molecular-weight immunoglobulins are produced by lymphoid cells, whereas y₂G-globulins are generated in so-called plasma cells.

Studying the structure, Functions, and biosynthesis characteristics of immunoglobulins is not only of theoretical significance but is also of great interest to physiology and medicine. The identification of the gamma-globulin nature of antibodies and The Development of industrial Methods for the preparative isolation of gamma globulins have opened up opportunities for their widespread use in the Treatment and Prevention of various infectious diseases, such as measles, pertussis, Viral Hepatitis, and many others.

The suppression of gamma-globulin synthesis leads to a sharp decline in the body's immunological resistance and to the onset of bacterial and viral infections. Equally severe consequences result from the uncontrolled synthesis of immunologically inert gamma-globulins that lack The ability to specifically interact with antigens. Such proteins—paraproteins—appear in conditions such as rheumatism, multiple myeloma, and Waldenström's macroglobulinemia, where the levels of y₂G- or y₁M-globulins exceed normal values by 5 to 7 times, yet the body's immune resistance is drastically reduced. Finally, in recent years, disorders have been identified that are accompanied by uncoordinated biosynthesis of immunoglobulin light and heavy chains. An excess of one of The polypeptide chains may occur in the bloodstream, leading to conditions known as Franklin's disease (heavy-chain disease) and Bence-Jones proteinuria (light-chain disease).

Along with antibodies, proteinaceous substances contained in plasma—namely complement and properdin—play a crucial role in protecting the body against bacteria and viruses.

Complement is a vital component of many immunological reactions. It plays a particularly important role in the lysis of Bacterial cells and sensitized erythrocytes (erythrocytes bearing adsorbed antigens on their surface). Using dialysis and Ion-exchange Chromatography, complement can be separated into four components. The total complement content in human Blood Plasma is 20–50 mg%, of which about ³/₄ is accounted for by the so-called first component, C'1; its molecular weight is 100,000, and its isoelectric point is 5.6. This component participates in the initial stage of cell lysis. Complement is thermolabile and is completely inactivated by heating to 56°C for 30 minutes. Its chemical nature and properties remain poorly understood. Presumably, some complement components, C'2 and C'4, belong to mucoproteins. Upon Electrophoresis, complement components are found in the ß-globulin fraction.

In 1954, Pillemer, while studying The properties of complement, discovered a new protein in human blood, which he named properdin (from the Latin perdere, to destroy). This is a high-molecular-weight protein with a molecular weight of approximately 1,000,000, an isoelectric point of 5.5–5.8, and the electrophoretic mobility of gamma globulin. The properdin concentration in human blood plasma is roughly 2 mg%, though its levels can fluctuate noticeably in certain diseases, such as radiation sickness.

In the presence of magnesium ions and all four complement components, properdin destroys numerous bacteria, participates in virus neutralization reactions, and possesses the ability to hemolyze erythrocytes. The specifics of properdin's interaction with microbes or viruses lead to the Conclusion that properdin is not an antibody; rather, its function is to protect the Organism against pathogenic factors for which no specific antibodies exist.

Properdin is thermolabile, losing a significant portion of its activity even at 50°C, and is completely inactivated at 56°C within 30 minutes.

One of the distinctive features of this protein is its ability to form insoluble complexes with zymosan, a polysaccharide isolated from Yeast cells. This property of properdin formed The basis of the initial methods for its preparative isolation and analysis. Studies of properdin using electrophoresis and ultracentrifugation, alongside the application of reductive acetylation methods, have demonstrated that the properdin molecule shares certain properties with macroglobulins, consisting of several subunits with a sedimentation constant of 6, linked together by disulfide bridges. Presumably, properdin, much like high-molecular-weight immune globulins, is produced by cells of lymphoid tissue.