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lysosome

Eukaryotic organelles

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This entry is made by Compilation and application of scientific encyclopedia entries of "Science Popularization China" to examine.

Lysosomes are decomposed protein 、 nucleic acid 、 Polysaccharide And other biological macromolecules. The lysosome has a single membrane with various shapes, which is 0.025-0.8 μ m bubble structure, containing many hydrolase 。 The function of lysosome in the cell is to decompose the substances entering the cell from the outside, and also to digest the local cytoplasm or organelles of the cell itself. When the cell is aging, its lysosome breaks, releases hydrolytic enzymes, digests the whole cell and causes its death.^[1]

Lysosomes are generally Eukaryotic cell One of Organelle ； It is a single-layer film coated cystic structure, with a diameter of about 0.025-0.8 μ m (mostly spherical, but with olives); Includes a variety of hydrolase , specially designed to decompose various exogenous and endogenous Macromolecular substances 。 In 1955, it was found in rat hepatocytes by Belgian scholar Cristian de Duve (1917-2013) and others.

Chinese name: lysosome
Foreign name: lysosomes
membrane structure: Monolayer

Diameter: 0.025~0.8 μ m
Role: Maintain pH value and decompose
Classification: Primary, secondary and tertiary lysosomes
Features: Mature higher plant cells have no lysosomes

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Lysosomal enzyme

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More than 60 acid hydrolases have been found in lysosomes (up to 2006), including protease 、 Nuclease 、 phosphatase 、 Glycosidase 、 lipase 、 Phosphatase And sulfatase, etc. These enzymes control a variety of endogenous and exogenous Macromolecular substances Digestion. Therefore, lysosomes have dissolution Or digestive function, which is intracellular Digestive organ 。

Lysosome under electron microscope (blue)^[2]

In rat liver, from mitochondrion The particle partition containing hydrolase is obtained at the place with lighter partition, and is named lysosome; lss in the sense of some that can be hydrolyzed. The enzyme in lysosome is acid phosphatase Ribonuclease 、 Deoxyribonuclease 、 cathepsin , aryl sulfate acetate enzyme, B-glucosidase, acetyltransferase, etc., are the hydrolytic enzyme groups with the most appropriate pH in acidic areas. according to electron microscope It is observed that lysosomes are particles or vesicles with a diameter of 0.4 μ m to several μ m surrounded by 6-8 nm thick monolayers. Due to its extremely diversified forms acid phosphatase The substance with positive activity was identified as lysosome.

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The lysosomal enzyme has three characteristics:

(1) The lysosome surface is highly glycosylated, which helps to protect itself from enzymatic hydrolysis. Most membrane proteins are glycoprotein The internal surface of the lysosome membrane is negatively charged, which helps the enzymes in the lysosome remain free. It is of great significance to exercise normal functions and prevent cells from being digested;

(2) All hydrolases have the best activity when the pH value is about 5, but the pH value in the surrounding cytoplasm is 7.2. Lysosome membrane contains a special transport protein, which can be used ATP hydrolysis The energy of + (hydrogen ion) pumped into lysosome to maintain its pH=5;

lysosome

(3) Only when the hydrolyzed substance enters the lysosome can the enzymes in the lysosome perform their decomposition. Once lysosome membrane is broken, hydrolase escapes, which will lead to cell autolysis.

Structure of lysosome

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Lysosomes are round or ovoid, with different sizes. Most of them are 0.2~0.8 μ m in diameter. The small ones are only 0.05 μ m, and the large ones can reach several microns. It is surrounded by a unit membrane with a thickness of 7~10nm, and contains more than 60 kinds of acid hydrolases, including protease, nuclease, glycosidase, lipase, phosphatase and sulfatase. However, it is usually impossible to find all enzymes in the same lysosome. The types and quantities of enzymes in different types of cell lysosomes are also different. The optimal pH of lysosomal hydrolase is 3.5~5.5, and the acid environment in lysosomal body is maintained by the special transport protein (H pump) on the membrane^[3] 。

Lysosomal structure^[4]

Classification Overview

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Traditional classification

According to different contents and formation stages, lysosomes can be divided into two categories. Granular lysosomes with homogeneous matrix are called primary lysosomes, and vacuolar lysosomes with complex myelin like structure are called secondary lysosomes. The lysosome belongs to the primary lysosome and has Hepatoparenchymal cells （ Hepatocyte ）High electron density particles, etc. Although this lysosome contains hydrolase, it is a lysosome without digestion. The secondary lysosome (digestive vesicle) is composed of primary lysosome and cell Phagocytosis The generated phagosomes are fused with each other and are lysosomes that have supplied hydrolases. The secondary lysosomes contain ingested substances and digest them. The undigested matter left after digestion is called residual corpuscle. It is generally believed that residual corpuscles are excluded from cells in amoeba and other cells, but in other cells, they remain in cells for a long time and become Cell senescence Reason for.

New formulation

There are new expressions about the type and naming of lysosomes. Some research data show that lysosomes are named as endosomes and lysosomes according to the formation process and function of lysosomes. The late intracellular body fuses with the unpacked transport vesicles containing lysosomal enzymes to form the pre lysosome, which is transferred from the Golgi apparatus or the transport vesicles on the cell membrane (if it is from the cell membrane through pinocytosis Clathrin The lysosomal enzyme recovered under the mediation forms an intracellular body after removing the coating) receives the newly synthesized hydrolase and lysosomal membrane protein, and the pH is maintained at a low level under the action of proton pump. Subsequently Glycosylation And Phosphorylation The lysosomal enzyme of was separated from the M6P receptor on the membrane. Next, M6P is separated from lysosomal enzymes and lysosomal enzymes are dephosphorylated. When the pH further decreases, it becomes lysosome. The M6P receptor can be further recovered to Golgi TGN or cell membrane.

Phagosomes fuse with prolysosomes or lysosomes to form phagolysosomes; Autophagy lysosomes are formed by the fusion of autophagosomes and pre lysosomes or lysosomes. Phagolysosomes and autophagic lysosomes hydrolyze substances into small molecules, which are absorbed by cells, and some substances that are not digested and absorbed remain are called residues. The cells are discharged by exocytosis, but most of the residues remain in the cells, such as lipofuscin and senile plaque, which are the deposits of this pigment.

Classification by functional stage

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Digestive function of lysosomes

Lysosome was first discovered in 1955. It is a capsule surrounded by a single membrane and containing a variety of acid hydrolases Vesicular cell Organ, its main function is to carry out intracellular digestion.

It is heterogeneous, and its shape, size and type of hydrolase may be very different, Marker enzyme It is acid phosphatase. It can be divided into primary lysosomes according to different stages of completing their physiological functions（ primary lysosome）， Secondary lysosome and Remains （residual body）。

Primary lysosome

The diameter is about 0.2~0.5um, the film thickness is 7.5nm, the content is uniform, no obvious particles Golgi apparatus Secreted. It contains a variety of hydrolases, but has no activity. Only when the lysosome breaks or other substances enter, can it have enzyme activity. Its hydrolases include protease, nuclease, lipase, phosphatase, sulfatase, phospholipase, and more than 60 kinds are known. These enzymes belong to acid hydrolases, and the optimal pH value for reaction is about 5. Although the thickness of lysosomal membrane is similar to that of plasma membrane, the composition is different. The main differences are: ① The membrane has proton pump , pump H+into lysosome to reduce its pH value. ② The membrane protein is highly glycosylated, which may help prevent the degradation of its own membrane protein.

Secondary lysosome

These are digestive vesicles, lysosomes that are undergoing or completing digestion, containing hydrolases and corresponding substrates, can be divided into heterophage lysosomes and autophagy lysosomes. The materials digested by the former come from external sources, while the materials digested by the latter come from various components of the cell itself.

According to the source of lysosomal agents, the secondary lysosomes are divided into:

(1) Heterologous lysosome (het - erolysosome), which means that it cannot penetrate plasma membrane Of Macromolecular solution Or viruses, bacteria, etc., the former forms pinocytosis vesicles (or endosomes) through pinocytosis (including receptor mediated endocytosis), and the latter forms phagocytosis vesicles through phagocytosis, which fuse with primary lysosomes (or endosomes) to form secondary lysosomes (or lysosomes) respectively.

(2) Autolysosome or autophagy lysosome refers to the secondary lysosome formed by the fusion of autophagosome and primary lysosome (or endolysosome) that surrounds some damaged or aging organelles (mitochondria, endoplasmic reticulum fragments, etc.). The substances digested are endogenous. The secondary lysosomes containing undigested residues are called residual bodies. Some residual substances can be discharged, and some can be stored in cells for a long time without being discharged.

Remains

Residues, also known as post lysosomes, have lost enzyme activity, leaving only undigested residues, hence the name. Residues can be discharged from cells through efflux, or may be left in cells increasing year by year, such as Lipofuscin 。

Function

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The lysosome has two functions: one is associated with Food bubble Fusion, digestion of large particles such as food or pathogenic bacteria swallowed by cells into Biomacromolecule , the residue expels cells through exocytosis; Second, in cell differentiation In the process, some senescent organelles and biological macromolecules fall into lysosomes and are digested, which is the need for the body to renew its own tissues.

The main function of lysosome is digestion, which is the intracellular digestive organ. Cell autolysis, defense and utilization of certain substances are all related to the digestion of lysosome.

Intracellular digestion For higher animals, cell nutrients mainly come from macromolecular substances in the blood, and some macromolecular substances enter the cell through endocytosis, such as endocytosis of low-density lipoprotein to obtain cholesterol. For some single cell eukaryotes, the digestion of lysosomes is more important.

Apoptosis : Ontogenesis often involves the transformation or reconstruction of tissues or organs, such as insects and frogs Metamorphosis wait. This process is realized under gene control, called programmed cell death. The cells destined to be eliminated form apoptotic bodies in the form of budding, and are macrophage Engulf and digest.

Autophagy : Remove useless biomacromolecules and aging organelles from cells. For example, the half-life of many biomacromolecules is only a few hours to several days, and the average life of mitochondria in liver cells is about 10 days.

Defense function: if phagocytes can swallow pathogens, they can kill and degrade pathogens in lysosomes.

Participate in the regulation of secretion process, such as Thyroglobulin Degraded into active thyroxine 。

formation sperm Top body of: Acrosome It is equivalent to a chemical drill, which can be dissolved and penetrated egg The cortex that allows sperm to enter the egg.

Selectively packaged into primary lysosomes.

All white blood cells contain lysosomal particles, which can eliminate invading microorganisms. However, some pathogenic bacteria (such as Leprosarium leprae, Mycobacterium tuberculosis, etc.) can tolerate the action of lysosomal enzymes and thus survive in macrophages. Lysosomes also play an important role in the pathological process. because Pulmonary macrophage Phagocytosis of inhaled silicon or asbestos dust can cause lysosome breakdown and release of hydrolase, stimulate the increase of connective tissue fibers, and lead to the occurrence of silicosis. Tissue hypoxia (such as myocardial infarction) can also cause the acute release of lysosomes, which can rapidly increase the concentration of related enzymes in the blood.

Formation process

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Lysosome formation process^[5]

The primary lysosome is formed in the form of budding on the trans surface of Golgi apparatus, and its formation process is as follows:

Endoplasmic reticulum ribosome Synthesis of lysosomal protein → entering the endoplasmic reticulum cavity for N-linked glycosylation modification, lysosomal enzyme protein first takes 3 glucose, 9 mannose and 2 N-acetylglucosamine After that, three molecules of glucose and one molecule of mannose were removed → into the Cis mask capsule of Golgi body → N- Acetylglucosamine Phosphotransferase recognition of lysosomal hydrolase Signal spot → N-acetylglucosamine phosphate is transferred onto 1-2 mannose residues → N-acetylglucosamine is cut off by N-acetylglucosidase in the intermediate membrane capsule to form M6P ligand → it binds to the receptor on the trans membrane capsule → it is selectively packaged into the primary lysosome.

lysosome and disease

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silicosis

Silica dust particles (silica [x ī] dust) are swallowed by macrophages after inhalation into the alveoli. Phagosomes containing silica dust merge with lysosomes to form secondary lysosomes. silicon dioxide The hydroxyl group of the lysosomal membrane forms a hydrogen bond with phospholipids or proteins of lysosomal membrane, resulting in Phagocyte Lysosomes disintegrate, cells themselves are destroyed, silica dust is released, and then it is swallowed by other macrophages, and so on repeatedly. Damaged or destroyed macrophages release "fibrogenic factors" and activate fibroblasts, leading to collagen fiber deposition and pulmonary fibrosis.

pulmonary tuberculosis

Mycobacterium tuberculosis It does not produce internal and external toxins, capsule and invasive enzymes. However, cerebroside sulfate, a bacterial component, can resist the intracellular bacteriolytic killing effect, so that tuberculous bacteria can grow and multiply in large numbers in the alveoli, leading to the lysis of macrophages, and the released tuberculous bacteria can be swallowed again to repeat the above process, eventually causing calcification and fibrosis of lung tissue.

Various lysosomal storages

lysosome Mannosidosis Lysomesome Storage Diseases (LSDs for short) is caused by genetic defects. Due to the variation of lysosomal enzymes and loss of function, a large amount of substrates are stored in lysosomes, thus affecting cell function. Common storage diseases mainly include the following:

Tay Sachs syndrome: Amaurotic familial dementia The lysosome lacks hexosaminidase A (β - N-hexosaminidase), leading to the accumulation of GM2 in ganglia (Figure 6-30), affecting cell function, causing mental dementia and death at the age of 2 to 6. The patients showed progressive blindness, stupor and paralysis, which mainly occurred in the Jewish community.

Type II Glycogen accumulation disease (Pompe disease or Pompeii disease): lysosome deficiency α - 1,4- Glucosidase Glycogen accumulates in lysosomes, leading to enlargement of heart, liver, tongue and muscle weakness. genus Autosome Defective genetic diseases, mostly children, often die before the age of two.

Gaucher disease Cerebrosidosis It is caused by the lack of β - glucosidase in lysosomes of macrophages and brain nerve cells. A large amount of glucocerebroside is deposited in these lysosomes, macrophages become Goetsche cells, and the patient's liver, spleen, lymph nodes, etc. swell, central nervous system Degenerative changes occur, often death within 1 year old.

Inclusion cell disease (I-cell disease): a more serious storage disease caused by single gene mutation of N-acetylglucosamine phosphotransferase. Due to gene mutation, M6P sorting signal cannot be formed on the pre lysosomal enzyme processed in Golgi apparatus, and the enzyme is transported out of the cell (default path). Such patients Fibroblast There is no hydrolase in the lysosome of, resulting in a large amount of substrate storage in the lysosome, forming the so-called "inclusion body". In addition, there are normal lysosomes in the liver cells of such patients, indicating that the formation of lysosomes also has a pathway other than M6P.

Hereditary disease

The synthesis of acid hydrolase in lysosome, like the biosynthesis process of other proteins, is determined by genes. When gene mutation causes the obstruction of enzyme protein synthesis, it can cause the lack of lysosomal enzymes. The body may lack some hydrolase in the lysosome due to gene defect, so that the corresponding agent cannot be degraded and accumulated in the lysosome, resulting in cell metabolic barrier and lysosomal storage disease. Its main pathological manifestation is the overload of lysosomes in the relevant organs (liver, kidney, heart muscle, skeletal muscle), that is, the cells ingest too much or indigestible substances, or because the activity of lysosomal enzymes decreases, as well as the aging of the body, a large number of lysosomes accumulate in the cells, resulting in overload. At present, more than 40 kinds of such diseases are known, and more than 30 kinds can be detected in China (see entry: lysosomal storage disease). among Glycogen storage disease Type II was the first discovered. Due to a gene defect on the hepatocyte autosome, the lysozyme lacks α - glucosidase, which leads to the inability to degrade glycogen into glucose, and causes glycogen to accumulate in the liver and muscle. This disease usually occurs in infants. The clinical manifestations are muscle weakness, cardiac enlargement, progressive heart failure, and death more than two years old, so this disease is also called cardiac glycogen deposition disease.

Rheumatoid arthritis

The etiology of rheumatoid arthritis is still unclear, but the inflammatory changes of articular periosteum and the erosion of articular chondrocytes shown by this disease are believed to be caused by the local release of lysosomes in cells. The reason may be that certain rheumatoid factors, such as anti IgG, are affected by macrophages Neutrophils Etc. phagocytosis, promoting lysosomal enzyme escape. Some of these enzymes, such as collagenase, can corrode cartilage and cause local joint damage, while the metabolic products of cartilage digestion, such as chondroitin sulfate, can promote the production of kinin and participate in the inflammatory reaction of joints.

shock

In the process of shock, the microcirculation of the body is disordered, tissue ischemia and hypoxia affect the energy supply system, make the membrane unstable, cause the leakage of lysosomal enzymes, and cause cell and body damage. During shock, the lysosomes in the cells of the body increased, the volume increased, and the phagosomes increased significantly. Enzymes in lysozyme body are released inside and outside tissues, mostly in liver and mesentery, causing autolysis of cells and tissues. Therefore, in shock, measuring the content of lysosomal enzymes in lymph and blood can be used as a quantitative indicator of mild and severe cell damage. Usually, acid phosphatase, β -- Glucuronic acid Enzyme and cathepsin were used as indicators. As for the mechanism of lysosome release during shock, it was suggested that the reason was the decrease of pH and the obstruction of tricarboxylic acid cycle. Ischemia and hypoxia during shock cause the decrease of cell pH value (about pH5), the activation of acid hydrolase, hydrolysis of lysosomal membrane, and finally lead to lysosomal membrane lysis, lysosomal release, and cell and tissue autolysis.

tumour

The relationship between lysosomes and tumors has increasingly attracted people's attention, and there are generally the following views:

(1) Carcinogens cause cell division regulation dysfunction and chromosome aberration, which may be related to the role of lysosome releasing hydrolase;

(2) Some effects on lysosomal membrane permeability Substances such as croton oil, some detergents, hyperbaric oxygen, etc. are cofactors promoting carcinogenesis and can also cause abnormal cell division;

(3) When the nuclear membrane is incomplete, the protection of the nuclear membrane to the nucleus is lost, and the lysosome can dissolve the chromatin, causing cell mutation;

(4) Some products of lysosome metabolism are the material basis of tumor cell proliferation;

(5) Carcinogens are always stored in lysosomes before they are integrated with chromosomes when entering cells Autoradiography Confirmed.

In conclusion, whether lysosomes are directly related to tumorigenesis remains to be further explored^[3] 。

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