Subject Matter of Lysosomes:
In the cytoplasmic matrix of the cells, there occur variously shaped bodies usually bounded by a single surface membrane and containing hydrolytic enzymes. These are called lysosomes. These enzyme containing bodies play important role in the digestion or lysis of intracellular substances, so they are called lysosomes.
Lysosomes were first reported by Christian de Duve and co-workers in Belgium in 1955 following their extensive work on the biochemical identification of certain hydrolytic enzymes in the liver cells of rats. During that period pioneering works were carried out by Novikoff in America when he was able to complement the biochemical work on Lysosome with observations at light microscopic and ultra-structural levels.
Occurrence of Lysosomes:
Lysosomes appear to be absent in prokaryotes. Cytochemical and electron microscopic studies have revealed the presence of membrane bound enzyme containing bodies in animal tissues. It is not certain whether the structures equivalent to animal Lysosomes are present in plant cells.
This uncertainty is partly due to strict adherence to the biochemical definition of lysosomes and partly because of technical difficulties in sub-cellular fractionation of plant tissues. Particles containing hydrolases and having characters of lysosomes have been visualized in many plant cells such as tobacco and maize.
The presence of lysosomes has been demonstrated in several slime moulds, fungal hyphae and algae. In 1964, P. Matile reported the occurrence of lysosomes in neumspora. Among the algae acid phosphatase has been located in lysosomes of Euglena and a few other species.
Structure of Lysosomes:
Lysosomes represent a class of morphologically heterogeneous cytoplasmic particles. The polymorphic nature of lysosomes has been attributed to specific functions, substances they contain and stage of digestion of those substances. Their size ranges from 0.25 to 0.8µ in diameter. In mammalian kidney cells they may be as large as 5µ and they may be even larger in phagocytes.
The density of lysosomes lies between that of mitochondria and microsomes (fragments of endoplasmic reticulum). Initially they were discovered by differential centrifugation and not by electron microscopes. They were originally obtained by sub-fractionation of the classical mitochondrial fraction. The classical mitochondrial fraction was sub-fractionated into the heavier and lighter components.
The light mitochondrial sub-fraction or L-fraction was later named lysosome (lytic bodies).
The lighter fraction (lysosomes) lacked cytochrome oxidase but had high concentration of acid phosphatase which became active only after severe mechanical disruption. The absence of cytochrome oxidase in lysosomes distinguishes them from mitochondria.
Lysosomes are bounded by a single limiting membrane of lipoprotein that is homologous with the unit membrane. Chemically lysosome is a bag packed with a variety of hydrolysing enzymes. The surface membrane is impermeable or very little permeable to substrates of the enzymes contained in the lysosome.
The internal organization is quite variable; some lysosomes have solid or very dense content, others have a dense outer zone with a less dense core and still others have a cavity or vacuoles.
When the surface membrane is dissolved the enzymes are set free in the cell saps which greatly alter the whole metabolic system of the cell. The membrane of lysosome is easily dissolved by the action of protolytic enzymes in hypotonic media.
The substances which cause instability of the membrane are called labilizers. Other substances, called stabilizers, stabilize the membrane. Among the many substances known to labilize lysosome membrane are vitamins A, E and K, progesterone, testosterone, β -estradiol, proteases, digitonin, ubiquinone, x-irradiation, ultraviolet irradiation, polyene antibiotics, endotoxins and bile salts.
Though Tie exact mechanism of functioning of labilizers is not known, these substances probably affect the membrane by (i) changing the fluidity of the lipid part of the membrane; and (ii) increasing the permeability of the lysosome membrane to small solutes and disruption of membrane due to osmotic swelling. Some of the membrane stabilizers are cholesterol, cortisone, chloroquine, heparin.
The lysosome membranes have limited permeability which possibly checks the direct access of hydrolytic enzymes to cellular components.
On the basis of morphological and functional criteria, a variety of lysosomes can be recognised in different cells as well as within a single cell. Two basic forms of lysosomes have teen distinguished; (i) the primary lysosomes; and (ii) the secondary lysosomes.
i. Primary Lysosomes (Storage Granules):
They originate from the endoplasmic reticulum or are cut off indirectly from the tips of golgi saccules and have not yet been involved in digestive process.
ii. Secondary Lysosomes:
These are digestive vacuoles which are sites of digestive activity.
Lysosomes belonging to this group may be classified into two separate types;
(a) Heterophagic; and
(b) Autophagic on the basis of exo or endogenous origin of the material undergoing digestion.
Each of these types may be further sub-divided as follows:
(i) Pre-Lysosomes whose enzymes have never been engaged in hydrolysis.
(ii) Lysosomes which are sites of present digestive activity.
(iii) Post-Lysosomes which have lost their enzymes.
Lysosomes start functioning when the cell takes up substances either by phagocytosis or pinocytosis. Undigested substances or residues within the lysosomes usually remain packed with different inclusions. Under certain circumstances lysosomes start digesting the very cell in which they occur by opening their membrane and setting free their digestive acid hydrolases.
The Lysosomes contain some 40 enzymes. Some of the important acid hydrolases identified from lysosomal fractions are as follows (Fig. 7.2):
It is not necessary that all the hydrolytic enzymes should occur in a single lysosome. These enzymes have been located in a variety of tissue types. Lysosomes obtained from one source do not present a homogeneous picture under the electron microscope. This suggests that a particular lysosome may contain only a few or even a single species of enzymes.
Some evidences do indicate that various hydrolytic enzymes are associated with different lysosomes in a variety of cells. These hydrolases have an acid optimum of about pH5. The enzymes of lysosomes become active only when the surface membrane is ruptured.
Functions of Lysosomes:
1. Extracellular digestion:
Lysosomes are small bags containing digestive enzymes. They behave like tiny time bombs waiting for their explosion in the cytoplasm. When the limiting membrane ruptures, the digestive enzymes are released which take part in the digestion.
Sometimes lysosomal enzymes may be released outside the cell where they digest extracellular substances. Saprophytic fungi and other micro-organisms utilize extracellular digestion of complex substrates in the habitat and degrade them into simpler soluble forms which are then absorbed.
2. Intracellular digestion:
The digestive enzymes released in the cytoplasm may be involved in autophagy or heterophagy.
(a) Autophagy refers to digestion of endogenous materials or breakdown of molecules and pieces of cytoplasmic materials within the cell. The simpler substances formed after the digestion are then utilized in the synthesis of some other substances.
This recycling of cell component is called turnover. Actually the digestive materials are non-functional part of other organelles like mitochondria, endoplasmic tubules, enclosed in a vesicle or digestive vacuole called autophagic vesicle or autophagosome.
Primary lysosomes fuse with the autophagic vesicles as a result of which lysosomal enzymes are discharged into them. In this way secondary Lysosomes are formed. The presence of hydrolytic enzymes in the autophagosomes and progressive disintegration of enclosed organelles give indication of digestive activity (Fig. 7.3).
Autophagy may bring about cellular digestion after the death of a cell and so it brings about the self-clearance of dead cells. This is why Christian de-Duve called lysosomes as the suicide bags.
(b) Heterophagy refers to intake of extraneous matter into the cell and subsequent breakdown of that material by acid hydrolases. The bulk intake of exogenous material is called endocytosis (Fig. 7.4). The intake of liquid material is pinocytosis while the intake of solid matter is referred to as phagocytosis. The process of phagocytosis is illustrated by Fig. 7.4.
3. Role in the release of hormones:
There is evidence that lysosomal acid hydrolases are involved in release of certain hormones from secretory cells of certain glands, e.g., thyroid hormones are released by hydrolysis of thyroglobulin.
4. Role in the penetration of sperm nucleus into the egg:
The enzymes released from acrosome vesicle, the giant lysosomes of sperms, dissolve the cortical granules, the structure surrounding the egg nucleus and help in the penetration of sperm nucleus into the egg.
5. Role in metamorphosis:
During the development of embryo, several tissues become functionless which are digested by the enzymes released from lysosomes and the digested materials are then absorbed by the surrounding cells. Rudolph Weber has reported the absorption tail of tadpole in this way.
6. Lysosomal activity in relation to pathology:
Several recent pathological studies have indicated that irregularities in lysosomal activity may cause fever, congestive heart failure, hepatitis, Pylonephritis, hypertension, joint injuries, leucocyte granules and tissue injuries.
Release of nucleases (DNAase and RNAase) which attack DNA and RNA respectively may cause chromosomal breaks and rearrangement and in this way it can result in structural abnormalities in chromosomes.
The chromosomal abnormality may induce certain harmful mutations in characters. Sometime it may lead to carcinogenesis (cancer formation). Malignant cells are also found to have abnormal chromosomes. Partial deletion of chromosome 21 of man has been associated with chronic myeloid leukemia
Accumulation of certain indigestible materials such as silica, asbestos particles, crystals of sodium ureate in the cells under certain conditions may result in cell inflammation. The inflammations of cells in such cases results due to release of enzymes after lysosomal break down in the cells containing ingested particles.
The actively dividing cells have been found to contain only a few lysosomes which are located generally near the periphery of the cell rather than near the nucleus. This suggests that the release of hydrolases from lysosomes has something to do with the division of cell.
Lysosomes of leucocytes help in defence against infection by bacteria and other microbes and guard against toxic molecules by digesting them. A mature leucocyte or white blood cell entering the circulation contains many lysosomes. During the life-time a leucocyte may ingest a foreign body such as a bacterium.
In this event the lysosomes disappear releasing their enzymes into the digestive vacuoles containing the bacterial cell. The lysosomes rupture, bacterium is digested, and the leucocyte ultimately dies having performed its major function in the body.
In animal body several kinds of cells have short lives such as, outer layer of skin and the mucous membrane lining the body. The short lived cells are being continuously replaced and the lysosomes of dead or degenerating cells release their enzymes into the body of the cells so that the whole cell may be digested. The process of tissue degeneration or necrosis can be attributed partly to lysosomal function.