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Lysosomes Lysosomes are sites of intracellular digestion and turnover of cellular components.lysis, solution, + soma, body) are membrane-limited vesicles that contain about 40 different hydrolytic enzymes and are particularly abundant in cells with great phagocytic activity (eg, macrophages, neutrophils).Membrane receptors for M6P-containing proteins in the trans Golgi network then bind these proteins and divert them from the secretory pathway for segregation into lysosomes.Although the nature and activity of lysosomal enzymes vary depending on the cell type, the most common are acid hydrolyases such as proteases, nucleases, phosphatase, phospholipases, sulfatases, and ?-glucuronidase.Lysosomal hydrolases are synthesized and segregated in the RER and then transferred to the Golgi apparatus, where the enzymes are further modified and packaged in vacuoles that form lysosomes.The marker mannose-6-phosphate (M6P) is added by a phosphotransferase in the cis Golgi only to the N-linked oligosaccharides of the hydrolases destined for lysosomes.This mixes the endocytosed material with the lysosomal enzymes and activates proton pumps in the lysosomal membrane that acidify the contents, allowing digestion.They function to degrade denatured or otherwise nonfunctional polypeptides.Lysosomes (Gr.

Lysosomes
Lysosomes are sites of intracellular digestion and turnover of
cellular components. Lysosomes (Gr. lysis, solution, + soma,
body) are membrane-limited vesicles that contain about 40
different hydrolytic enzymes and are particularly abundant in
cells with great phagocytic activity (eg, macrophages, neutrophils).
Although the nature and activity of lysosomal enzymes
vary depending on the cell type, the most common are acid
hydrolyases such as proteases, nucleases, phosphatase, phospholipases,
sulfatases, and β-glucuronidase. As can be seen
from this list, lysosomal enzymes are capable of breaking
down most macromolecules.
Lysosomes, which are usually spherical, range in diameter
from 0.05 to 0.5 μm and present a uniformly granular, electrondense
appearance in the TEM (Figure 2–16). In macrophages
and neutrophils, lysosomes are slightly larger and visible with
the light microscope, especially after histochemical staining.
Cytosolic components are protected from these enzymes
by the membrane surrounding lysosomes and because the
enzymes have optimal activity at an acidic pH (~5.0). Any
leaked lysosomal enzymes are practically inactive at the pH of
cytosol (~7.2) and harmless to the cell.
Lysosomal hydrolases are synthesized and segregated in
the RER and then transferred to the Golgi apparatus, where the
enzymes are further modified and packaged in vacuoles that
form lysosomes. The marker mannose-6-phosphate (M6P)
is added by a phosphotransferase in the cis Golgi only to the N-linked oligosaccharides of the hydrolases destined for lysosomes.
Membrane receptors for M6P-containing proteins in the
trans Golgi network then bind these proteins and divert them
from the secretory pathway for segregation into lysosomes.
Material taken from outside the cell by endocytosis
is digested when the membrane of the phagosome
or pinocytotic vesicle fuses with a lysosome. This mixes
the endocytosed material with the lysosomal enzymes
and activates proton pumps in the lysosomal membrane
that acidify the contents, allowing digestion. Proteasomes
Proteasomes are very small abundant protein complexes not
associated with membrane, each approximately the size of the
small ribosomal subunit. They function to degrade denatured
or otherwise nonfunctional polypeptides. Proteasomes also
remove proteins no longer needed by the cell and provide an
important mechanism for restricting activity of a specific protein
to a certain window of time. Whereas lysosomes digest
organelles or membranes by autophagy, proteasomes deal primarily
with free proteins as individual molecules.