The human urine mannose 6-phosphate glycoproteome

Glycoproteins containing the mannose 6-phosphate (Man-6-P) modification represent a class of proteins of considerable biomedical importance. They include over sixty different soluble lysosomal hydrolases and accessory proteins, deficiencies of which result in over forty different known human genetic diseases. In addition, there are patients with lysosomal storage diseases of unknown etiology and lysosomal proteins have been implicated in pathophysiological processes associated with Alzheimer disease, arthritis, and cancer. The aim of this study was to explore urine as a source for the proteomic investigation of lysosomal storage disorders as well as for biomarker studies on the role of Man-6-P containing proteins in other human diseases. To this end, urinary proteins were affinity purified on immobilized Man-6-P receptors, digested with trypsin, and analyzed using nanospray LC/MS/MS. This resulted in identification of 67 proteins, including 48 known lysosomal proteins and 9 proteins that may be lysosomal. The identification of a large proportion of the known set of soluble lysosomal proteins with relatively few contaminants suggests that urine represents a promising substrate for the development of comparative proteomic methods for the investigation of lysosomal disorders and other diseases involving Man-6-P glycoproteins.