Goat liver beta-mannosidases: molecular properties, inhibition and inactivation of the lysosomal and nonlysosomal forms.

The two caprine hepatic beta-mannosidases have been partially purified and their properties have been compared. The lysosomal beta-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(beta 1-4)GlcNAc and Man(beta 1-4)GlcNAc(beta 1-4)GlcNAc. The nonlysosomal beta-mannosidase B had an apparent molecular weight of 43,000 +/- 2,000 and an isoelectric point of pH 5.5. beta-Mannosidase B was activated by Triton X-100 (0.1%) and was inhibited by cysteine (20 mM). Hydrolysis of Man(beta 1-4)GlcNAc, but not of Man(beta 1-4)GlcNAc(beta 1-4)GlcNAc, followed incubation with beta-mannosidase B. 1,5-Dideoxy-1,5-imino-D-mannitol did not inhibit the A enzyme and only feebly (Ki = 0.3 mM) inhibited the B enzyme; beta-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.