BIOCHEMISTRY: A TEXTBOOK FOR HIGHER EDUCATION - E. S. Severin - 2004

SECTION 14. BLOOD BIOCHEMISTRY

II. Specific Features of Phagocytic Cell Metabolism

The ability of certain Blood Cells to undergo phagocytosis is one of the blood's defense mechanisms. Phagocytosis involves Two Types of leukocytes: neutrophils and monocytes. Neutrophils contain a multilobed Nucleus, which is why they are also referred to as polymorphonuclear leukocytes (PMNs). They enter the bloodstream from the Bone Marrow and have a lifespan of approximately 8 days. The interaction of integrin Proteins (see Section 15) with receptors on capillary endothelial cells leads to neutrophil adhesion, followed by their migration into Tissues.

Monocytes can also exit the bloodstream, at which point they are termed macrophages. Both types of phagocytes engulf and destroy Bacteria. In addition, macrophages scavenge old, damaged cells and cellular debris, absorbing approximately 10¹¹ erythrocytes per day. Phagocytosis is a specialized form of endocytosis characterized by The formation of large endocytic vesicles, the size of which is determined by the size of the ingested particles.

Phagosome formation begins with the interaction of specific phagocyte receptors with a bacterium or an antigen-antibody complex. Receptors located in regions of Cell/30.html">The Plasma Membrane where the specialized protein clathrin is localized (see Section 5) recognize Complement components, Oligosaccharides On the surface of microorganisms, or the F_c region of the antigen-antibody complex (see Section 1). Receptor activation transmits a signal into The Cell via the Inositol phosphate system, initiating processes that govern the cell's phagocytic response. This response includes phagosome formation, its fusion with a lysosome, phagolysosome generation, activation of oxygen-dependent microbicidal mechanisms for pathogen destruction and/or The production of microbicidal nitric oxide by the cells, as well as the action of oxygen-independent destruction mechanisms.

Phagosome formation. The Interaction of a microbial cell with the phagocyte surface triggers the extension of membrane projections—pseudopodia—that surround the microbe. The phagosome thus formed, enclosing the ingested bacterium, is internalized into the Cytoplasm of the phagocyte.

Phagolysosome formation. In the Cytosol, phagosomes fuse with primary Lysosomes to form phagolysosomes. Primary lysosomes, produced by the Golgi apparatus, contain a series of granule-encapsulated Hydrolases capable of breaking down organic molecules in the acidic environment of the phagolysosome, including proteinases, Phosphatases, esterases, DNases, and RNases. The low pH inside phagosomes exerts a bactericidal effect and provides an optimal environment for the activation of lysosomal hydrolases. As a result of these enzymatic actions, microbial polymer molecules are degraded into Amino Acids, Monosaccharides, and NUCLEOTIDES, which enter the cytosol and can be utilized by the cell. Most membrane components and indigestible substrates end up in residual bodies, which return to The surface of the phagocyte plasma membrane via exocytosis, although a significant portion of membrane components may also be recycled directly within the membrane (Fig. 14-6).

Class="center">Fig. 14-6. Phagocytosis in neutrophils.

Activation of oxygen-dependent microbicidal mechanisms. The NADPH oxidase enzyme complex in the phagosome membrane reduces O₂ to generate the superoxide anion:

2 O₂ + NАDРН —> 2 O₂^- + NАDР⁺ + Н⁺.

The superoxide anion is converted into hydrogen peroxide, either spontaneously or through the action of the enzyme superoxide dismutase:

O₂^- + O₂^- + 2Н⁺ —> Н₂O₂ + O₂.

Under the action of myeloperoxidase, which enters the phagosome upon its fusion with the lysosome, peroxides in the presence of halides (iodides and chlorides) generate additional toxic oxidants, namely hypoiodite and hypochlorite.

H₂O₂ + Сl^- +Н⁺ —> НОСl + H₂O.

All of these molecules are potent oxidants and exhibit bactericidal activity. The sharp increase in oxygen consumption by a phagocytizing cell is termed the "respiratory burst" (Fig. 14-7).

Fig. 14-7. Production of reactive oxygen species by phagocytic cells during the respiratory burst. Activation of cell membrane-localized NАDРH oxidase triggers the Generation of the superoxide anion. Through membrane invagination, the superoxide, along with the bacterial cell, becomes enclosed within the phagosome. The superoxide anion drives the Formation of other toxic molecules, including Н₂O₂ and ОН^•. Myeloperoxidase, contained within the granules of phagocytic cells, is secreted into the phagosome, where it produces НОСl.

Reactive oxygen species initiate free radical reactions that degrade the Lipids in the cell membranes of engulfed bacteria.

Hereditary NADPH oxidase deficiency, caused by a defect in one of the genes encoding this enzyme complex, leads to chronic granulomatous disease. Due to this enzymatic defect, patients' phagocytes are unable to produce superoxide oxygen radicals and hydrogen peroxide, and therefore fail to rapidly destroy phagocytized bacteria and Fungi. Certain resistant microorganisms remain viable within the phagocytes, and their Antigens provoke a cellular Immune Response and granuloma formation at the site of phagocyte accumulation. The most common form of this disorder is X-linked, associated with a defect in the Gene for one of the complex's polypeptide chains located on the short arm of the X chromosome.

Generation of reactive nitrogen metabolites. Nitric oxide (NO) also plays a microbicidal role in macrophages. In these cells, as in others, nitric oxide is synthesized from Arginine by the enzyme NO synthase (see Section 9). NO synthase activity in macrophages increases significantly during phagocytosis in the presence of y-interferon and tumor necrosis factor. The superoxide anion reacts with NO to yield compounds with even greater microbicidal potency than NO alone:

NO +O₂^- —> ОNOO^- —> ОН^• +NO₂.

Peroxynitrite (ONOO^-), nitric oxide, nitrogen dioxide, and the hydroxyl radical cause oxidative damage to bacterial proteins, Nucleic Acids, and lipids. Nitric oxide can directly interact with iron-sulfur Proteins of the Electron Transport Chain, inhibiting Respiration and ATP Synthesis in bacteria. The interaction of NO with O₂ yields nitrites, which are further converted into nitrates that also exhibit toxicity (see Section 12).

The surge in neutrophil metabolic activity culminates in cell death. Dead neutrophils, macrophages, bacteria, and tissue fluid constitute pus.

Action of oxygen-independent bactericidal mechanisms. Some Gram-positive bacteria are destroyed within neutrophil phagosomes by the lysosomal enzyme Lysozyme, which hydrolyzes the bonds between N-acetylmuramic acid and N-acetyl-D-glucosamine present in The Cell wall, thereby causing its degradation.

Human neutrophils contain cationic Peptides known as defensins, which comprise about 30 amino acid residues and are rich in Cysteine and arginine. They account for 30 to 50% of all granule proteins. Defensins induce the formation of Ion Channels in the microbial cell membrane immediately upon phagolysosome formation, thereby facilitating pathogen destruction. Defensins are also active against enveloped Viruses, such as the Herpes simplex virus.

Thus, the vast diversity of microorganisms attacking human cells has driven the evolution of a wide range of bactericidal mechanisms operating under both aerobic and anaerobic conditions.