Inspection of human salivary alpha-amylase action by its transglycosylation action.
The course of the action of human salivary alpha-amylase (HSA) on a substrate was examined taking advantage of its transglycosylation action. IG5 phi (IG-G-G-G-G-phi), IG4 phi (IG-G-G-G-phi), and GIG4 phi (G-IG-G-G-G-phi) were used as the substrates and p-nitrophenyl alpha-glucoside (GP, G-P) as the acceptor. HSA hydrolyzes IG5 phi, IG4 phi, and GIG4 phi to IG3 (IG-G-G) and G2 phi (G-G-phi), to IG3 and G phi (G-phi), and to GIG3 (G-IG-G-G) and G phi, respectively. In the presence of GP, a part of the glycon residues, IG3 and GIG3, were transferred to the acceptor to give IG4P (IG-G-G-G-P) and GIG4P (G-IG-G-G-G-P), respectively. Whenever the enzyme attacks the substrate, G phi or G2 phi is liberated in both transglycosylation and hydrolysis. The extent of transglycosylation can be, therefore, estimated from the molar ratio of the transfer product to the liberated aglycon, G phi or G2 phi. HPLC analysis of the reaction mixtures revealed that the value of IG4P/G phi in the digest of IG4 phi was nearly equal to that of GIG4P/G phi in the digest of GIG4 phi and these values were ten times larger than that of IG4P/G2 phi in the digest of IG5 phi. These data suggested that G phi residue would fall away from aglycon binding site more rapidly than G2 phi residue after the cleavage of the alpha-1,4-glycosidic linkage to offer GP more chance to attack to the activated glycon and also indicated that the space of the glycon binding site corresponds to three glucose residues.