Mannosidosis: Clinical, Morphologic, Immunologic, and Biochemical Studies

Extract: The primary metabolic defect in mannosidosis is the deficiency of the acidic α-mannosidase A and B activities which results in the lysosomal accumulation of mannose-rich substrates. Our studies demonstrate that the enzymatic diagnosis of suspect homozygotes can be made reliably using plasma, isolated leukocytes, or cultured skin fibroblasts assayed carefully at the appropriate acidic pH. Immunologic studies of a mannosidosis homozygote revealed significant abnormalities of neutrophil function; these included a depressed chemotactic responsiveness and impaired phagocytosis of bacteria. Lymphocyte transformation studies showed a 20% of normal response to purified phytohemagglutinin and a 25% of normal response to concanavalin A. Three major components of α-mannosidase activity in normal human liver were resolved by ion exchange chromatography on DEAE-cellulose and electrophoresis on cellulose acetate gels. Electrophoresis of the liver extract from homozygote 1 with mannosidosis revealed only one band of activity which coelectro-phoresed with the α-mannosidase C isozyme partially purified from normal liver. However, ion exchange chromatography revealed the presence of residual hepatic acidic activities; the residual A isozyme was eluted in a position corresponding to that of normal α-mannosidase A whereas the residual B activity was eluted at a slightly more electronegative position than that of normal B isozyme. The apparent Km values for α-mannosidase activity as determined from Lineweaver-Burk plots were 1.1 mM for normal liver and 0.9 mM for normal leukocytes. In contrast, the residual activity in these sources from homozygote 1 could not be saturated within the solubility range of the substrate; the apparent Km value was estimated at 15.4 mM in liver extracts. Zinc significantly lowered the apparent Km value of the acidic activity in normal liver (from 1.2 to 0.24 mM), whereas this metallic ion had little effect on the values for mannosidosis hepatic activity (from 15.4 to 12.3 mM). Unlike zinc, cobalt had its major effect on the acidic activity in the mannosidosis liver extract, lowering the apparent Km from 15.4 to 3.9 mM, whereas the apparent Km fot the normal activity was increased from 1.2 to 1.9 mM. The residual acidic activitieswere markedly stimulated by zinc in both leukocytes (∼300%)and plasma (∼400%)from the homozygotes and to a lesser extent in those sources from normal individuals. In contrast, cobalt enhanced the residual acidic activities in leukocytes (∼500%) and plasma (∼200%) from the homozygotes while inhibiting these acidic activities (78.9%and 47.7%, respectively) in normal individuals. Speculation: The susceptibility of homozygotes with mannosidosis to severe recurrent infections may be the direct consequence of impaired leukocyte membrane recognition processes resulting from the defective catabolism of substrates with a-D-mannose residues. The metabolic defect in mannosidosis presumably results from a structural gene mutation which alters the kinetic and metal binding properties of the lysosomal a-mannosidase activities. Appropriate metal cation supplementation may stimulate the residual activity of the mutant isozymes and provide an effective therapeutic approach to patients with this inborn error of metabolism.