Lysosomes are membrane-bound organelles found in animal cells; they are a dense organelle that contains both soluble and membrane- associated hydrolytic enzymes within their acidic lumen. Lysosomes also contain special proteins known as lysosomal–associated membrane proteins, which play an essential role in the maintenance of the lysosome structure. Functionally, lysosomes can digest unwanted materials and cellular debris by using acid hydrolase enzymes. Furthermore, these organelles are very important for some physiological processes for, such as secretion, immunological responses, autolysis and the inflammatory response. Over 30 hereditary disorders in human are caused by malfunction of the lysosome. The most important lysosomal storage disease is Chediak- Higashi syndrome. The role and the function of lysosomes are not limited to protein degradation, but also fusion with the plasma membrane during cell injury.
Lysosomal Enzymes Lysosomal enzymes can breakdown macromolecules and other materials such as bacteria, which is Dictyostelium’s original food source. The study of the biosynthesis of lysosomal enzymes has been characterized in many mammalian systems such as the human fibroblast. First, lysosomal enzymes are created on membrane- bound polysomes while, co-translationally, they receive N-linked high-mannose oligosaccharide side chains as they are inserted into the rough endoplasmic reticulum. These enzymes are later moved to the Golgi complex where the change to their carbohydrate moieties generates phosphomannosyl residues; these have been shown to serve as the recognized markers for mannose 6-phosphate receptors. Then, these receptors mediate transport of lysosomal enzymes to endosomal or pre-lysosomal vesicles. Finally, these receptors are transported back to the Golgi.
Several lines of proofs suggest that lysosomal enzymes can be correctly sorted independently of the Man-6-Preceptorc1-dependent pathway. For instance, lysosomal enzymes in special fibroblast cells of patients with I-cell disease are not phosphorylated. Moreover, in the absence of the mannose 6-phosphate recognition marker, these enzymes are secreted in their precursor forms. Conversely, many differentiated cells in these patients, such as leukocytes and hepatocytes, hold nearly normal levels of lysosomal enzymes. Furthermore, a variety of other cells have been identified that lack any detectable Man-6-P receptors, and they are capable to suitably arrange and sort lysosomal enzymes. A little is known about the sorting mechanisms, which it is by these cells or the role of acidic pH compartments in the targeting of enzymes to lysosomes.