Digestive Activity of Lysosomes: II. THE DIGESTION OF MACROMOLECULAR CARBOHYDRATES BY EXTRACTS OF RAT LIVER LYSOSOMES

Abstract

The ability of liver lysosomes to degrade hyaluronic acid and glycoproteins has been investigated. Purified liver lysosomes, prepared from rats previously treated, by injection, with Triton WR-1339, were incubated with the substrates at acid pH and 37°. Hyaluronic acid was degraded to the extent of about 85% to an equimolecular mixture of glucuronic acid and N-acetylglucosamine, with 15% of the molecule remaining in the form of hyalobiuronic acid, which proved resistant to attack by the lysosomal mixture of enzymes. With orosomucoid and fetuin as substrates, cleavage of about half the peptide bonds and removal of the larger part of the sialyl end groups of the carbohydrate chains occurred rapidly. This was followed more slowly by the release of galactose and of N-acetylglucosamine, which, however, did not exceed 30% of the theoretical yield under the conditions tried. Mannose could not be detected in the hydrolysis mixture. Apparently, considerable resistance to digestion occurs at or near the branching points in the carbohydrate portions of these glycoproteins. With submaxillary mucin as substrate, up to 80% of the disaccharide side chains were split off in the form of free N-acetylneuraminic acid and N-acetylgalactosamine. Peptide hydrolysis did not start until about half the carbohydrate portion had been removed, and then proceeded slowly, reaching 30% of theoretical yield at the end of the incubation period. Some 60% of the mannose residues were detached from ovalbumin glycopeptides. Thus, lysosomes can extensively degrade both the carbohydrate and the peptide portions of glycoproteins, but may lack the ability to bring their digestion to completion, at least with some substrates.