SPECIFIC RECOGNITION AND UPTAKE OF LYSOSOMAL ENZYMES AND MODIFIED GLYCOPROTEINS BY RAT TISSUES

Abstract

This chapter discusses the specific recognition and uptake of lysosomal enzymes and modified glycoproteins by rat tissues. Lysosomal glycosidases comprise a family of enzymes having certain features in common including subcellular localization, acid pH optima, and the presence of covalently bound carbohydrate. The fate of rapidly cleared enzyme has been resolved by liver subcellular fractionation experiments following large doses of β-glucuronidase, which demonstrate that within a few minutes the cleared activity appears in the large-granule fraction. The rapid clearance of lysosomal glycosidases displays saturability, suggesting a finite number of enzyme recognition components and tissue-binding sites that mediate the uptake process. The notion that sugar residues may be important in the recognition of lysosomal enzymes in vivo is suggested by the observation that, with the exception of cathepsin B, all lysosomal enzymes studied far are either suspected to be or have been shown to be glycoproteins.