LIGHT AND HEAVY LYSOSOMES - CHARACTERIZATION OF N-ACETYL-BETA-D-HEXOSAMINIDASE ISOLATED FROM NORMAL AND I-CELL DISEASE LYMPHOBLASTS

We previously reported that I-cell disease lymphoblasts maintain norma l or near-normal intracellular levels of lysosomal enzymes, even thoug h N-acetylglucosamine-1-phosphotransferase activity is severely depres sed or absent (Little et al., Biochem. J., 248, 151-159, 1987). The pr esent study, employing subcellular fractionation on colloidal silica g radients, indicates that both light and heavy lysosomes isolated from I-cell disease and pseudo-Hurler polydystrophy lymphoblasts possess no rmal specific activity levels of N-acetyl-beta-D-hexosaminidase, alpha -D-mannosidase and beta-D-glucuronidase. These current findings are in contrast to those of cultured fibroblasts from the same patients, whe re decreased intralysosomal enzyme activities are found. Column chroma tography on Ricinus communis revealed that N-acetyl-beta-D-hexosaminid ase in both heavy and fight I-cell disease lysosomal fractions from ly mphoblasts possesses an increased number of accessible galactose resid ues (30-50%) as compared to the enzyme from the corresponding normal c ontrols. Endo-beta-N-acetylglucosaminidase H treatment of N-acetyl-bet a-D-hexosaminidase from the I-cell lysosomal fractions suggests that t he majority of newly synthesized high-mannose-type oligosaccharide cha ins are modified to complex-type carbohydrates prior to being transpor ted to lysosomes. This result from lymphoblasts differs from previous findings with fibroblasts, where N-acetyl-beta-D-hexosaminidase from I -cell disease and pseudo-Hurler polydystrophy lysosomes exhibited prop erties associated with predominantly high-mannose-type oligosaccharide chains. The current results imply that different cell types may modif y the carbohydrate side chains of lysosomal enzymes in a differential manner, and that selected cell types may also employ mechanisms other than the mannose-6-phosphate pathway for targeting lysosomal enzymes t o lysosomes.