Deficient lysosomal carboxypeptidase activity in galactosialidosis

In the lysosome, the glycosidases neuraminidase (EC 3.2.1.18) and beta-galactosidase (EC 3.2.1.23) are associated to a 52 kDa "protective protein" to form a large multi-enzymatic complex. Deficient synthesis or inactivation of this protective protein causes galactosialidosis, a lysosomal storage disorder in man in which both neuraminidase and beta-galactosidase activities are deficient. Since the protective protein possesses extensive sequence homology with carboxypeptidase Y (carb Y) and the KEX 1 gene product from yeast, we have used the artificial substrate N-CBZ-Phe-Leu to detect and characterize the peptidase activity of the lysosomal carboxypeptidase (carb L). Using both a purified preparation of the lysosomal multi-enzymatic complex and cultured skin fibroblasts of patients affected with galactosialidosis, we demonstrate that the 52 kDa protective protein is responsible for carb L activity. The fibroblasts of three patients affected with late infantile and juvenile galactosialidosis were found to be deficient in carb L activity (1.4% of normal mean value).