Characterization of the mutant α-mannosidase in bovine mannosidosis

Abstract

Residual acidic .alpha.-mannosidase, varying in amount up to approximately 15% of normal values, can be measured in various organs of a calf with mannosidosis. The highest specific activity and relative proportion of residual activity were found in the liver. Chromatography on DEAE-cellulose showed that the residual activity was associated with 2 components which eluted at comparable positions with those found in normal tissues. The residual activity had a lower thermal stability and a higher Km value for a synthetic substrate than did the normal enzyme. No differences in MW or electrophoretic mobility between normal acidic .alpha.-mannosidase and the residual activity were observed by gel filtration and electrophoresis on cellulose acetate, respectively. The isoelectric focusing profiles for the .alpha.-mannosidase in the normal and pathological livers were very similar. A mutant enzyme, apparently resulting from a mutation in a structural gene, accounts for the residual acidic .alpha.-mannosidase in mannosidosis. The mutant enzyme, which cross-reacts with antiserum raised against normal bovine acidic .alpha.-mannosidase, is present at a decreased concentration compared with the normal enzyme. There is a correlation between the concentrations of residual activity and cross-reacting material in mannosidosis. .alpha.-Mannosidase with a pH optimum of 5.75 and which is activated by Zn2+ was detected in the liver of the calf with mannosidosis. It is probably not a product of the defective gene because addition of Zn2+ indicated that it was also present in normal tissues.