Action of intestinal sucrase-isomaltase and its free monomers on an alpha-limit dextrin.

The concurrent action of the sucrase and isomaltase active sites of the hybrid sucrase-isomaltase from rat intestine on an a-limit dextrin was studied by use of 63 maltotriosylmaltotriose isolated from controlled action of pullulanase on pullulan. Hydrolysis of the a-dextrin hexasaccharide occurred by sequential removal of a glucose residue from its nonreducing end. The initial pentasaccharide product was shown to be 63 maltosylmaltotriose since reaction of its sodium [3H]borohydride derivative with pullulanase produced maltose and [3H]maltotriitol. Further hydrolysis by sucrase-isomaltase yielded 63 glucosylmaltotriose + maltotriose -+ maltose -+ glucose. The free isomaltase monomer was capable of hydrolyzing the initial substrate and all intermediate products; it displayed maximal specificity for removal of the a-( 1 + 6)-linked glucose stub from 63 glucosylmaltotriose but little specificity for maltose. The free sucrase monomer was incapable of hydrolyzing the a-(1 + 6)-linked glucose stub but possessed appreciable activity against all a-(1 4 4) linkages and was particularly active against maltotriose and maltose. Based on calculation of the relative contribution of free sucrase and free isomaltase to the hydrolytic rate constants (Kc,*) for the parent and intermediate substrates, either active site can initiate a-dextrin hydrolysis by removing the nonreducing a-(1 + 4)-linked glucosyl residue, but the isomaltase site is essential for cleavage of the a-(1 + 6)-linked glucose stub. The suerase site is primarily responsible for the final cleavage of the released malto-oligosaccharides to glucose. Instead of acting synergystically, the two a-glucosidase sites appear to cleave a-limit dextrins by virtue of their complementary specificity.