Lysosomes other activities expressed

The availability of biologically active i Sj.iabelled EPO facilitated the detection and study of cell surface receptors. In addition to erythroid precursors, various other cell lines were shown to express EPO receptors, at least when cultured in vitro. Many harboured two classes of receptors a high-affinity and a low-affinity form. Most appeared to express between 1000 and 3000 receptors/cell. Radiotracer experiments illustrated that the EPO receptor is rapidly internalized after ligand binding and the EPO-receptor complex is subsequently degraded within lysosomes. 

Interestingly, all of the enzymes that comprise the sphingolipid-catabolic pathway are glycoproteins and may be found in different places within the organelle. For example, glucocerebrosidase is firmly associated with the lysosomal membrane, whereas others, like hexoseaminidase A, exist largely in soluble form in the lysosomal matrix. A common property of the lysosomal hydrolases, however, is their expression of maximum activity at a relatively acidic pH (i.e., pH 4.0-5.5), hence the term acid hydrolase. This is not unexpected because ATP-driven proton pumps sustain an acidic milieu (pH 5.2) within the lysosome. 

The only nonbone condition in which elevated activities of TR-ACP are found in serum is Gaucher s disease of spleen, a lysosomal storage disorder. Its source in this disease is the abnormal macrophages in spleen and other tissues, which overexpress this normal macrophage constituent. The hairy cells of hairy-ceU leukemia (leukemic reticuloendotheliosis) also express the osteoclast-type AGP, providing a useful histological marker. However, in this condition, the isoenzyme does not enter the plasma in increased amounts. 

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