Oxygen-dependent subsystem
This part of the microbicidal system relies on reactive oxygen species and enzymes generated during phagocytosis.
- Reactive oxygen species (ROS): Generated during the "respiratory burst" (oxidative stress). These include superoxide radical, hydrogen peroxide, singlet oxygen, and hydroxyl radical, which possess high bactericidal efficacy.
- Myeloperoxidase: Located in the azurophilic granules of neutrophils and the lysosomes of monocytes/macrophages. Its activity increases sharply in the presence of hydrogen peroxide and halides, leading to halide oxidation, chlorination, and destruction of bacterial envelopes.
- Catalase: Reacts with hydrogen peroxide and halides to form potent oxidizing agents. Together with myeloperoxidase, it destroys bacteria, viruses, fungi, and mycoplasmas.
Oxygen-independent subsystem
Includes a range of factors and compounds that function independently of oxygen metabolism:
- Lysozyme (muramidase): Cleaves the muramic acid of peptidoglycans in the cell wall. Gram-positive microorganisms are the most sensitive to it.
- Lactoferrin: Localized in phagosomes. In its unsaturated state, it binds (chelates) iron, depriving bacteria of an essential growth factor and providing a bacteriostatic effect.
- Cationic proteins: Exhibit bactericidal action primarily against Gram-positive microbes within phagolysosomes.
- Acidosis (pH 4.0–6.5): The creation of an acidic environment suppresses the surface charge of bacteria and inhibits membrane processes. An excess of hydrogen ions promotes the formation of nitrites, chloramines, aldehydes, and singlet oxygen, as well as increases the permeability of lysosomal membranes.
Enzymatic digestion
The final stage of destroying engulfed microorganisms involves the activity of intracellular structures.
- Lysosomal hydrolases: More than 30 types of these enzymes are present in primary lysosomes in an inactive state.
- Activation: Increases sharply under conditions of acidosis during phagocytosis.
- Function: Carry out the complete destruction of microbial components down to elementary compounds—amino acids, peptides, fatty acids, and nucleotides.
Briefly: answers to topical questions
Which specific cationic proteins provide oxygen-independent bactericidal activity?
Oxygen-independent bactericidal activity of phagocytes is provided by cationic proteins.
These include:
- Serprocidins — cathepsin G, elastase, azurocidin, and proteinase-3; act together synergistically.
- Lysozyme — destroys the bacterial cell wall.
- Lactoferrin — binds iron ions, creating a deficiency of them for microorganisms; its main action is bacteriostatic.
- BPI proteins (Bacteria permeability inducing) — cationic proteins with bactericidal activity that increase bacterial wall permeability.
Oxygen-dependent mechanisms operate via the "respiratory burst" and enzymes (myeloperoxidase, catalase), while oxygen-independent mechanisms rely on acidosis, lysozyme, lactoferrin, and cationic proteins.
Class and exam questions
What two main subsystems are distinguished in the phagocytic microbicidal system?
Oxygen-dependent and oxygen-independent subsystems are distinguished, each utilizing a unique set of enzymes and chemical factors.
Where is myeloperoxidase localized?
Myeloperoxidase is located in the azurophilic granules of neutrophils and in the lysosomes of monocytes/macrophages.
What is the mechanism of action of lactoferrin?
Lactoferrin in phagosomes is in an iron-unsaturated state and chelates (binds) iron, depriving bacteria of a factor necessary for their growth.
What else to study on this topic
- Age-related characteristics of lysozyme levels
- Sensitivity spectrum of various microorganisms to lysozyme
- Effect of acidosis on the permeability of lysosomal membranes
- Role of halides in the myeloperoxidase reaction
- Dynamics of "respiratory burst" reactions
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