Immunoelectron microscopy was performed to study the biosynthesis of lysosomal 
beta-galactosidase (beta-gal) in normal and mutant human fibroblasts. Using 
polyclonal and monoclonal antibodies we show in normal cells precursor forms of 
beta-gal in the rough endoplasmic reticulum (RER) and in the Golgi apparatus 
throughout the stack of cisternae. In the lysosomes virtually all beta-gal 
exists as a high molecular weight multimer of mature enzyme. In the autosomal 
recessive disease GM1-gangliosidosis caused by a beta-gal deficiency and in 
galactosialidosis, associated with a combined deficiency of lysosomal 
neuraminidase and beta-gal, precursor forms of the latter enzyme are found in 
RER, Golgi and some labeling is present at the cell surface. The lysosomes 
remain unlabeled, indicative for the absence of enzyme molecules in this 
organelle. In galactosialidosis fibroblasts also no mature beta-gal is found in 
the lysosomes but in these cells the presence of the monomeric form can be 
increased by leupeptin (inhibition of proteolysis) whereas addition of a partly 
purified 32 kDa "protective protein" results in the restoration of high 
molecular weight beta-gal multimers in the lysosomes.