Abstract
Biochemical, enzymatic and metabolic studies of purified lysosomal fractions from rat kidney and liver have shown that lysosomes contain two types of glycoproteins, enzyme glycoproteins, and soluble acidic lipoglycoproteins (SALGP) without catalytic properties. The SALGP comprise about 50% of the lysosol protein, are readily labelled by precursors of phospholipids, aminosugars, sialic acid and peptides, have a molecular weight of about 15,000 and a pi of about 4. Most or all of the acid hydrolases are glycoenzymes that are synthesized in a restricted portion of the rough endoplasmic reticulum in the form of basic glycoproteins containing N-acetylglucosamine and mannose. N-Acetylneuraminic acid (NANA) and additional N-acetylglucosamine are attached to the nascent glycoenzymes in the Golgi apparatus. The lysosomal enzymes are packaged in lysosomes exclusively as acidic sialoglycoproteins with pis between 3.5 and 4.9. The basic forms of the lysosomal hydrolases apparently originate from the corresponding acidic forms during biodégradation through a partial autolytic cleavage of NANA, carbohydrate and (glyco)peptide. Similar changes occur during incubation of lysosomal extracts in vitro at an acid pH.
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