Preferential inhibition of lysosomal beta-mannosidase by sucrose

The lysosomal storage disease beta-mannosidosis, described in both goats and humans, can be detected by measuring a deficiency in hydrolysis of the fluorogenic substrate 4-methylumbelliferyl-beta-D-mannoside. An inhibitor of guinea pig beta-mannosidase (beta-man) activity was detected when tissue was homogenized in phosphate-buffered-saline (pH 7.4) containing 0.25 mol/l sucrose. The existence of such an inhibitor was apparent when the enzyme was immunoprecipitated from tissue using a specific beta-man polyclonal antibody. There was up to a threefold increase in activity in the immunoprecipitated enzyme (antibody-enzyme complex) compared to the activity of the nonimmunoprecipitated enzyme. An extensive study was therefore undertaken to determine the nature and specificity of this inhibitor by analyzing the effect of a range of metal ions and sugars on beta-man activity compared to other lysosomal hydrolase activities. Although ferrous, ferric, cobalt, and manganese ions were highly inhibitory to beta-man, they also inhibited other lysosomal hydrolases to a similar extent. Likewise, mannose inhibited both alpha- and beta-man activities equally. The only compound to specifically inhibit beta-man in a manner similar to that observed in the tissue homogenate was glucosyl(beta, 2)fructofuranoside (sucrose). This is an important finding in that tissue samples are commonly prepared in buffers containing sucrose and this could lead to a wrong diagnosis of beta-man deficiency. In order to determine if the absence of an activator factor or alternatively the presence of a specific inhibitor was a contributing factor in the lack of beta-man activity in cultured fibroblasts from affected humans and goats, mixing studies with normal and affected cell extracts were performed but no restoration or inhibition of beta-man activity was found.