Larger Loss of Activity in Taka-amylase A Modified with Starch-dialdehyde than with Dextran-dialdehyde

Cross-linked modifications of Taka-amylase A (TAA) with various dialdehyde derivatives including starch- and dextran-dialdehyde were examined. A rapid and significant decrease in the amount of free amino groups and enzyme activity of TAA was attained by starch-dialdehyde, whereas dextran dialdehyde caused much lower loss in activity of TAA in spite of a similar extent of amino group modification. The molecular weight of the dextran-modified TAA increased by about 10, 000-20, 000 Da, which was measured by SDS-gel electrophoresis. A comparison of the activity of modified TAA on various substrates indicated a slight but definite increase in susceptibility to maltose and phenyl α-glucoside according to the increase in the extent of modification of TAA. In contrast, the activity of the modified TAA on maltotriose, maltotetraose, and γ-cyclodextrin decreased upon modification. Here, again, TAA modified with dextran-dialdehyde retained higher activity to those oligosaccharides than that modified with starch-dialdehyde. Moreover, inactivation caused by the fluorescent reagent, o-phthalaldehyde, was retarded by previous modification with dextran-dialdehyde, indicating that reaction between TAAand dialdehyde derivatives occurred through non-critical amino groups for enzyme activity.