Hydrolytic activity of alpha-mannosidase against deoxy derivatives of p-nitrophenyl alpha-D-mannopyranoside

Deoxy derivatives of p-nitrophenyl (PNP) alpha-D-mannopyranoside, PNP 2-deoxy-alpha-D-arabino-hexopyranoside, 3-deoxy-alpha-D-arabino-hexopyranoside, 4-deoxy-alpha-lyxo-hexopyranoside, and alpha-D-rhamnopyranoside, were synthesized and hydrolytic activities of jack bean and almond alpha-mannosidases against them were investigated. These alpha-mannosidases scarcely acted on the 2-, 3-, and 4-deoxy derivatives, while the 6-deoxy one was hydrolyzed by the enzymes as fast as PNP alpha-D-mannopyranoside, which is a common substrate for alpha-mannosidase. These results indicate that the hydroxyl groups at C-2, 3, and 4 of the mannopyranoside are necessary to be recognized as a substrate by these enzymes, while that at C-6 does not have so a crucial role in substrate discrimination. Values of k(m) and V-max of the enzymes on the hydrolysis of PNP-alpha-D-rhamnopyranoside were obtained from kinetic studies.