The role of lysosomal sialidase and β-galactosidase in processing the complex carbohydrate chains on lysosomal enzymes and possibly other glycoproteins



Previous studies using the lectin RCA-I from Ricinus communis have indicated that several lysosomal enzymes in the fibroblasts of patients deficient in β-galactosidase carry excess terminal galactose. Electrophoretic studies have shown that the same enzymes and the non-lysosomal adenosine deaminase also show excess terminal sialic acid in patients deficient in sialidase. In this paper we confirm, using Jack-bean β-galactosidase, that the binding to RCA-I of the purified N-acetyl-β-d-hexosaminidase from a patient with Gm1 gangliosidosis depends on a terminal β-linked galactose. We provide evidence, using bacterial sialidase and measuring the binding to RCA-I, for excess subterminal galactose on the enzymes of patients deficient in sialidase. We also show that adenosine deaminase from the fibroblasts of patients deficient in β-galactosidase has increased binding to RCA-I. These observations suggest that in healthy individuals the carbohydrate structure of the precursors of lysosomal enzymes and possibly some other glycoproteins also includes extended carbohydrate side chains with terminal sialic acid and subterminal galactose, and that the mature enzyme extracted from tissues is the product of degradation.