Lysosomal catabolic pathway of N-glycosylprotein glycans.

In vitro study of the initiation steps of catabolism of N-glycosylproteins in rat liver lysosomes has led to the evidence that the degradation of the carbohydrate chain is an ordered and bi-directional phenomenon: 1) The first one starts at the reducing terminus, immediately follows the degradation of the peptidic backbone by proteases and involves a serial reaction of 3 enzymes, respectively, 1) an alpha-L-fucosidase, 2) an aspartylglucosaminidase, and 3) an acidic 'oligosaccharide specific' endo-N-acetyl-beta-D-hexosaminidase, that we propose calling endo-chitobiase. 2) The second one is the commonly admitted sequential recurrent degradation by exoglycosidases. This process explains the presence of oligosaccharides sharing a single terminal reducing N-acetylglucosamine residue in human lysosomal storage diseases. Using a model glycoasparagine as a substrate, we followed the above mentioned hydrolysis reactions by 1H NMR spectroscopy. The kinetic data revealed a rapid hydrolysis of the substrate via pathway 1 prior to the action of exoglycosidases. Moreover, the latter act at different rates on the different antennae of the substrate.