Goat Liver ß-Mannosidases: Molecular Properties, Inhibition and Inactivation of the Lysosomal and Nonlysosomal Forms

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Abstract
The two caprine hepatic ß-mannosidases have been partially purified and their properties have been compared. The lysosomal ß-mannosidase A had an apparent molecular weight of 127,000 ± 10,000 and an isoelectric point of pH 6-7. Its activity was unaffected by incubation with Triton X-100 (0.1 %) and cysteine (20 mM) and it hydrolyzed the presumed natural substrates, Man(ß1-4)GlcNAc and Man(ß1—4)GlcNaC(ß1-4)GlcNAc. The nonlysosomal ß-mannosidase B had an apparent molecular weight of 43,000 ± 2,000 and an isoelectric point of pH 5.5. ß-Mannosidase B was activated by Triton X-100 (0.1 %) and was inhibited by cysteine (20 mM). Hydrolysis of Man(ß1-4)GlcNAc, but not of Man(ß1- 4)GlcNAc(ß1-4)GlcNAc, followed incubation with ß-mannosidase B. 1,5-Dideoxy-1,5-imino- D-mannitol did not inhibit the A enzyme and only feebly (K(i) = 0.3 mM) inhibited the B enzyme; ß-D-mannopyranosylmethyl p-nitrophenyl triazene did not inactivate either enzyme but 1,2-anhydro-1,2,3,5,6/4-cyclohexane hexol inactivated the B enzyme only. The radical mechanistic differences between the two enzymes argue against their having the same genetic origin.