Diagnosis of alpha-mannosidosis by measuring alpha-mannosidase in plasma.

alpha-Mannosidosis is a lysosomal storage disease resulting from a deficiency of lysosomal alpha-mannosidase activity. Diagnosis of alpha-mannosidosis has traditionally been accomplished by demonstrating reduced alpha-mannosidase activity in leukocytes. We describe a new assay of alpha-mannosidase in serum or plasma that allows specific detection of the enzyme deficiency in alpha-mannosidosis with small, easily obtained sample volumes. The assay utilizes 40 microL of serum or plasma and a fluorescent substrate, 4-methylumbelliferyl-alpha-D-mannopyranoside in sodium acetate buffer, pH 4.0. The mean activity of a control population was 194 (SD 67) mU/L, whereas the activities obtained for four alpha-mannosidosis patients were 0, 17, 17, and 33 mU/L. Comparison with the standard leukocyte alpha-mannosidase assay showed this serum or plasma assay to be equally effective in diagnosing alpha-mannosidosis.