Textbook - BIOLOGICAL CHEMISTRY - Hubsky Y.I. - 2000

Chapter VI. BIOCHEMISTRY OF PHYSIOLOGICAL FUNCTIONS AND SPECIALIZED TISSUES

CHAPTER 28. BIOCHEMISTRY AND PATHOBIOCHEMISTRY OF BLOOD

28.4. BIOCHEMICAL COMPOSITION OF BLOOD IN HEALTH AND DISEASE

The Biochemical Composition of Blood in a healthy human is relatively constant, which is explained by the presence of powerful homeostatic systems in the body (central nervous and endocrine systems) that ensure the Stability of the internal environment through regulatory mechanisms that counteract significant Changes in the concentrations of major metabolites and shifts in the physicochemical parameters of plasma and other Body Fluids. At the same time, the composition of plasma biochemical components at any given moment reflects any changes in the METABOLISM of Cells in individual Organs and the intake of bioorganic compounds from the environment, in particular postprandial nutrients, which immediately triggers the aforementioned regulatory mechanisms.

A striking example of the physiological Regulation of Blood plasma glucose, amino acid, and fatty acid concentrations is the postprandial (after a meal) secretion of Insulin by pancreatic β-cells and the suppression of the secretion of its antagonists—counter-regulatory Hormones such as Glucagon, catecholamines, and Growth Hormone. Insulin increases the permeability of Plasma Membranes, particularly in Muscle and adipose tissue, to glucose and Certain Amino Acids, and stimulates anabolic processes leading to the active intracellular utilization of these Biomolecules: induction of Enzymes for Glycogen synthesis in The Liver and Muscles, triglyceride synthesis in the liver and adipocytes, and Protein Synthesis in the liver, muscles, and other Tissues. The combination of these biochemical transformations leads to the normalization of glucose and other nutrient metabolite levels in Blood Plasma back to the pre-prandial baseline.

Cell/6.html">Chemical Components of blood plasma can be divided into the following groups: Plasma Proteins, non-protein organic plasma components (intermediate and final metabolic products), and inorganic plasma components.

Class="center">Blood Plasma Proteins

Blood plasma contains several dozen different proteins that differ in their physicochemical and functional properties: transport proteins, enzymes, proenzymes, Enzyme Inhibitors, hormones, Antibodies, antitoxins, coagulation factors, and anticoagulants, among others. The total concentration of human blood plasma proteins ranges from 65-85 g/L; this value may decrease (hypoproteinemia) in elderly individuals and under pathological conditions accompanied by suppressed PROTEIN SYNTHESIS AND accelerated breakdown of tissue proteins (starvation, debilitating infectious diseases, post-traumatic or postoperative states, cachexia in malignant neoplasms).

The amount of individual protein fractions detected in blood plasma depends on the Separation method used (paper Electrophoresis, gel electrophoresis, particularly Polyacrylamide gel electrophoresis, Immunoelectrophoresis, etc.). Using paper electrophoresis, which is widespread in clinical practice, plasma proteins are divided into five fractions: albumins (serum albumins), α₁-globulins, α₂-globulins, β-globulins, and γ-globulins. The Biochemical characteristics of the main human blood plasma proteins are presented in Table 28.1.

Table 28.1. Main protein fractions of human blood plasma

Albumins (serum albumins) are a polydisperse fraction of plasma proteins characterized by high electrophoretic mobility and ready solubility in Water and salt solutions. Due to their high hydrophilicity, albumins bind a significant amount of water, doubling the volume of their molecules upon Hydration. The hydration shell formed around serum albumin molecules accounts for 70-80% of the oncotic pressure of blood plasma proteins, which can be utilized clinically when administering albumin solutions to patients with tissue edema. Conversely, a decrease in serum albumin concentration—for example, due to impaired synthesis in hepatocytes during Liver failure—can cause fluid to shift from the vascular bed into tissues, leading to The Development of oncotic edema.

Albumins also perform an important physiological function as carriers of numerous metabolites and other low-molecular-weight compounds. Albumin molecules possess several domains with binding sites for organic Ligand molecules attached via electrostatic and Hydrophobic bonds. Through these interactions, serum albumins can bind and transport Fatty acids, Cholesterol, Bile pigments (such as bilirubin), Vitamins, hormones, certain amino acids, phenol, and other toxic and pharmacological compounds.

Globulins are a heterogeneous fraction of blood proteins that perform transport (α₁-globulins transport Lipids, thyroxine, corticosteroid hormones; α₂-globulins transport lipids, copper ions; β-globulins transport lipids, free and heme iron) and protective Functions (participation of β-globulins in immune responses as antitoxins; γ-globulins acting as the immunoglobulin fraction).

In clinical practice, the ratio between the concentrations of albumins and globulins in blood plasma (the so-called "protein coefficient") is used, which averages 1.5-2.0.

Blood IMMUNOGLOBULINS. Immunoglobulins (IgA, IgG, IgE, IgM) are proteins of the blood plasma γ-globulin fraction that function as antibodies, the primary effectors of humoral Immunity. The Structure AND FUNCTIONS of immunoglobulins will be discussed in detail in Chapter 30.

Of significant clinical interest are certain glycoprotein blood plasma proteins, notably Components of the body's nonspecific resistance system—C-reactive protein, cryoglobulin, serum proteinase inhibitors, Fibronectin, Complement system proteins—as well as transport proteins such as haptoglobin, transferrin, and ceruloplasmin.

C-reactive protein (CRP) is a protein named for its ability to react with the C-polysaccharide of pneumococcus, forming precipitates in the process. Chemically, it is a glycoprotein.

C-reactive protein is absent in the blood serum of healthy individuals and appears during pathological conditions accompanied by inflammation and tissue necrosis; notably, it was first discovered in lobar Pneumonia. The presence of CRP is characteristic of the acute phase of diseases, earning it the designation "acute-phase protein." The determination of CRP is of diagnostic importance in the acute phase of rheumatic fever, myocardial infarction, and pneumococcal, streptococcal, and Staphylococcal infections.

Cryoglobulin is a protein of the γ-globulin fraction that, similar to C-reactive protein, is absent in the blood plasma of healthy people and appears in conditions such as leukemias, lymphosarcoma, myeloma, rheumatism, liver cirrhosis, and nephrosis. A characteristic physicochemical feature of cryoglobulins is their solubility at normal body Temperature (37 °C) and their ability to form gel-like precipitates when blood plasma is cooled to 4 °C.

α₂-Macroglobulin is a protein of the α₂-globulin fraction, acting as a universal serum proteinase inhibitor whose blood concentration is the highest among all proteinase inhibitors, averaging 2.5 g/L. α₂-Macroglobulin is a glycoprotein with a Molecular Weight of 725 kDa.

The inhibitory activity of α₂-macroglobulin is manifested against most natural proteinases across all four catalytic classes: Serine, thiol, carboxy, and metalloproteinases. In complex with this inhibitor, proteinases lose their catalytic activity toward high-molecular-weight proteins while retaining The ability to hydrolyze low-molecular-weight Peptides. The Biological Role of α₂-macroglobulin involves The regulation of tissue proteolysis systems, which play a crucial role in such physiological and pathological processes as blood clotting, Fibrinolysis, immune responses, complement system functioning, inflammatory reactions, and vascular tone regulation (kinin and renin-angiotensin systems).

α₁-Antitrypsin (α₁-proteinase inhibitor) is a glycoprotein with a molecular weight of 55 kDa and a plasma concentration of 2-3 g/L. The primary biological property of this inhibitor is its ability to form complexes with proteinases, thereby suppressing the proteolytic activity of enzymes such as Trypsin, Chymotrypsin, plasmin, Thrombin, and proteases released during the destruction of leukocytes or foreign cells in inflammatory foci.

During an inflammatory process, the blood content of α₁-antitrypsin increases significantly due to The stimulation of its synthesis in hepatocytes. The inhibitory activity of α₁-antitrypsin is also of great importance in pancreatic necrosis during acute pancreatitis, which is accompanied by the release of active pancreatic proteinases into tissues and biological fluids. Congenital deficiency of α₁-antitrypsin leads to the development of pulmonary emphysema at a young age due to the tissue trypsin-mediated degradation of interalveolar septa.

Fibronectin is a plasma glycoprotein synthesized and secreted into the extracellular space by many types of cells. It is present on cell surfaces, in basement membranes, Connective Tissue, and blood. Acting as an "adhesive" protein, fibronectin binds to the carbohydrate groups of sialoglycolipids (gangliosides) On the surface of plasma membranes, thereby playing an integrative role in intercellular interactions. Furthermore, by forming complexes with Collagen fibrils, fibronectin significantly contributes to the Organization OF THE pericellular matrix.

Haptoglobin is a protein belonging to the α₂-globulin fraction of blood plasma. It has the ability to bind free Hemoglobin, forming a complex that migrates with the β-globulin electrophoretic fraction. The normal plasma concentration is 0.10–0.35 g/L.

Within the haptoglobin-hemoglobin complex, hemoglobin is taken up by Cells of the reticuloendothelial system, particularly in the liver, and degraded into bile pigments. This function of haptoglobin helps conserve iron ions derived from hemoglobin during both physiological and pathological erythrocyte breakdown. Hemoglobin entry into the urine (Hematuria) is observed only in cases of severe Hemorrhage.

Transferrin (also known as siderophilin) is a glycoprotein of the β-globulin fraction with a molecular weight of 80 kD. Transferrin binds iron ions (Fe³⁺) in blood plasma. Its surface features two iron-binding sites, where iron binds to transferrin along with a bicarbonate anion.

Transferrin serves as the transport form of iron, delivering it to storage and utilization sites. Specifically, it accepts Fe³⁺ ions entering the blood following intestinal absorption and transfers the iron to tissue ferritin, in which iron is stored in the liver, Spleen, Bone Marrow, and other organs. The plasma concentration of transferrin is approximately 4 g/L.

Ceruloplasmin is an α₂-globulin fraction glycoprotein that binds copper ions in blood plasma. Its molecule contains 8 Cu⁺ ions and 8 Cu²⁺ ions, with a molecular weight of about 150 kD.

Ceruloplasmin accounts for up to 3% of the body's total copper content and over 90% of plasma copper. It functions as a copper-containing ferroxidase, oxidizing iron from the ferrous (Fe²⁺) to the ferric (Fe³⁺) state. This reaction is essential for converting iron into an ionic form that can be bound by ferritin and subsequently utilized for the synthesis of iron-containing proteins (such as hemoglobin and Cytochromes). A reduced plasma level of ceruloplasmin (characteristic of Wilson's disease) leads to the leakage of copper ions from the vascular bed and their accumulation by connective tissue Proteoglycans, resulting in pathological alterations in the liver, Brain (hepatolenticular degeneration), and cornea, among other tissues.

Plasma Enzymes

Blood contains a considerable amount of enzymatic proteins, which are classified into intrinsic blood enzymes and those that enter the plasma from cells of other organs, tissues, or biological secretions.

Intrinsic plasma enzymes include various proteases, Phosphatases, and esterases, notably components of the Blood Coagulation and anticoagulation systems, enzymes involved in immune responses, complement system activation, and other mechanisms of non-specific resistance. The second group comprises enzymes that enter the blood via the absorption of Pancreatic juice and saliva (trypsin, amylase, lipase) or leak into the bloodstream due to increased permeability of plasma membranes in organs such as the liver, myocardium, Kidneys, and Skeletal Muscle. This group, known as indicator enzymes, holds significant clinical and diagnostic value as markers of pathological damage to hepatocyte membranes (Alanine aminotransferase) or myocardial cells (aspartate aminotransferase, creatine phosphokinase).

Kallikrein-Kinin System

Kins are low-molecular-weight peptides found in blood, other biological fluids, and tissues, where they participate in the regulation of vascular tone (vasodilation), microcirculatory processes, inflammation, and allergic reactions.

The primary blood kinins are the nonapeptide bradykinin and the decapeptide kallidin:

Arg-Pro-Pro-Gly-Phe-Ser-Pro-Phe-Arg

Bradykinin

Lys-Arg-Pro-Pro-Gly-Phe-Ser-Pro-Phe-Arg

Kallidin (Lysyl-bradykinin)

Kinins are synthesized from precursor proteins called kininogens through the action of Proteolytic Enzymes known as kallikreins, which cleave specific nona- or decapeptide fragments from the kininogen molecules. Plasma kallikrein itself exists in an inactive precursor form, prekallikrein, which is converted into the active enzyme via the serine protease factor XII of the blood coagulation cascade.

The half-life of plasma kinins is very short (20–30 seconds). Kinins are degraded by kininases, enzymes that cleave peptide bonds within kinin molecules, thereby abolishing their biological activity.

The pathway of plasma kinin generation and degradation is illustrated in Fig. 28.2:

Fig. 28.2. The plasma kinin system.

Kinins relax the smooth muscle of Blood Vessels, causing a drop in blood pressure as well as vasodilation of the microvasculature in inflamed areas. Bradykinin is the most potent endogenous vasodilator in the body. Furthermore, the local generation of kinins within sites of inflammation increases vascular permeability and triggers pain sensations.

Given the prominent role of kinins in the Pathogenesis of inflammatory conditions, clinical practice makes widespread use of pharmacologic agents that act as inhibitors of kinin formation (such as Contrykal and Gordox).

Non-Protein Organic compounds of Blood Plasma

Nitrogen-containing compounds

Organic nitrogen in biochemical compounds that can be measured in the supernatant following the precipitation of plasma or Serum proteins is referred to in clinical biochemistry as residual nitrogen, or non-protein nitrogen (NPN). This non-protein nitrogen consists of the nitrogen derived from End products of PROTEIN AND NUCLEIC acid Catabolism, such as urea, amino acids, free NUCLEOTIDES, uric acid, creatine, creatinine, and several Other Compounds. The plasma concentrations of the principal non-protein nitrogenous compounds are presented in Table 28.2.

Table 28.2. Non-protein nitrogenous components of blood plasma

The total concentration of residual (non-protein) nitrogen in the blood plasma of healthy individuals ranges from 20-40 mg % (0,2-0,4 g/L). A sharp increase in this parameter—known as hyperazotemia, driven primarily by elevated urea nitrogen—is most frequently observed in impaired renal nitrogen excretion, characteristic of acute or chronic renal failure.

Nitrogen-free compounds

The nitrogen-free chemical components of blood plasma include CARBOHYDRATES, lipids, and organic acids that serve as metabolic intermediates (lactic, pyruvic, and acetoacetic acids, as well as Tricarboxylic Acid Cycle metabolites).

Blood plasma carbohydrates. Blood plasma predominantly contains Monosaccharides, primarily glucose (at a fasting concentration of 65-119 mg %, or 3,58-6,05 mmol/L), alongside fructose, galactose, and certain pentoses (ribose, deoxyribose). The main Glycolysis products are present in blood plasma at the following concentrations: lactic acid — 8-17 mg %, pyruvic acid — 0,4-2,5 mg %.

Blood plasma lipids. The total lipid concentration in human blood plasma varies depending on dietary patterns, food quality, and the individual's constitutional characteristics (age, sex), averaging 5-7 g/L. Under physiological conditions, total blood lipid levels may rise to 10-15 g/L following a fat-rich meal (alimentary hyperlipemia).

Among plasma lipids, the highest proportions are accounted for by compounds of the following classes (A.N. Klimov, 1980):

- triglycerides — 0,5-1,9 g/L;

- Phospholipids — 1,1-2,75 g/L;

- total cholesterol — 1,5-2,6 g/L;

- esterified cholesterol — 1,0-2,1 g/L;

- fatty acids (unesterified) — 0,08-0,2 g/L.

As hydrophobic substances, lipids cannot exist in a free (soluble) state in blood plasma, which, from a physicochemical standpoint, is an aqueous salt solution. Plasma lipids are stabilized by specialized proteins (apoproteins, or apolipoproteins) that facilitate The formation of lipoprotein micelles, allowing various lipid classes to be transported through the bloodstream. There are five classes of apoproteins (A, B, C, D, E), which are incorporated into various Lipoproteins in specific quantitative ratios (Chapter 16).

The following transport forms of plasma lipoproteins are distinguished:

- chylomicrons (1-2,5 g/L) — the primary transport vehicle for triacylglycerols;

- very low-density lipoproteins (VLDL), or pre-β-lipoproteins (1,3-2,0 g/L) — contain substantial amounts of triacylglycerols, as well as phospholipids and cholesterol;

- low-density lipoproteins (LDL), or β-lipoproteins (2,1-4,0 g/L) — the major transport form of cholesterol; elevated blood concentrations of LDL promote cholesterol infiltration into the endothelium and the formation of atherosclerotic plaques, which is a risk factor for the development of atherosclerosis;

- high-density lipoproteins (HDL), or α-lipoproteins (0,2-0,25 g/L) — contain significant quantities of phospholipids, along with cholesterol and triacylglycerols; HDL are considered "antiatherogenic" lipoproteins that promote the efflux of cholesterol from the vascular wall.

Elevated plasma concentrations of various lipoprotein classes (hyperlipoproteinemias) indicate severe disturbances in Lipid Metabolism, most commonly associated with specific Genetic Disorders. Five primary types of hyperlipoproteinemias are recognized, depending on defects in the enzyme systems responsible for the degradation or metabolism of specific lipoproteins (types I, II, III, IV, V). These can manifest as the Early Development of atherosclerosis, obesity, xanthomatosis, liver or Kidney disease, and decreased glucose tolerance.

Inorganic plasma components

The primary Inorganic Components of blood plasma include electrolyte cations—calcium (2,5 mmol/L), sodium (140 mmol/L), potassium (5 mmol/L), and other mineral elements (predominantly Iron and copper) along with Trace Elements—as well as bicarbonate, chloride, phosphate, sulfate, and iodide anions.