Evidence for two distinct forms of mammalian beta-mannosidase.

Liver from goats with an inherited deficiency of beta-D-mannosidase appears only partially deficient in beta-mannosidase (40% of normal values) when assayed with synthetic beta-mannoside substrates at pH 5.0. Other tissues such as brain and cultured skin fibroblasts show an almost complete deficiency of beta-mannosidase activity. Fractionation of supernatant solutions of normal goat liver on columns of concanavalin A bound to Sepharose 4B resolved beta-mannosidase into a bound (acidic) form (pH optimum, 5.0 to 5.5) and an unbound (neutral) form (broad pH optimum from 5.0 to 8.0). Both forms were heat-labile, inhibited by sodium taurocholate (0.1%) and insensitive to divalent cations such as zinc. However, only the acid lysosomal) form was able to hydrolyze a Man beta GlcNAc beta [3H]GlcNAc trisaccharide. Comparable fractionation of liver from affected goats revealed normal levels of the unbound (neutral) form but a complete absence of the bound (acidic, lysosomal) form. Fractionation of liver from an obligate heterozygote goat revealed normal neutral and 50% of the acidic. These studies suggest that goat liver contains both lysosomal beta-mannosidase (acidic form; deficient in beta-mannosidosis) and nonlysosomal beta-mannosidase (neutral) activity.