Substrate Specificity of Bovine Liver Cytosolic Neutral -Mannosidase Activated by Co2+

Abstract

A cytosolic neutral α-mannosidase was purified from bovine liver. Its molecular weight was found to be 500,000 on gel filtration. The activity of the enzyme toward Manα1-6(Manα1-3)Manα1-6(Manα1-3)Manβ1-4GlcNAc-PA was increased 26-fold by preincubation with 1 mM Co²⁺. Manα1-6(Manα1-3)Manβ1-4GlcNAc was hydrolyzed by the enzyme to Manα1-3Manα1-6(Manα1-3)Manβ1-4GlcNAc, which was further hydrolyzed to Manα1-6(Manα1-3)Manβ1-4GlcNAc. The rate of hydrolysis was 15-fold greater than that of Manα1-6(Manα1-3)Manα1-6(Manα1-3)Manβ1-4GlcNAc. This substrate specificity suggested that the enzyme could be involved in the degradation of oligomannose-type sugar chains with one GlcNAc residue released from glycoproteins by endo- β-N-acetylglucosaminidase, and supported a pathway for glycoprotein catabolism via oligomannosyl glycans with one GlcNAc residue proposed on the basis of an earlier study on a cytosolic neutral α-mannosidase from Japanese quail oviduct [Oku, H. and Hase, S. (1991) J. Biochem 110, 982-989].