EFFECT OF FERRIC AND CUPRIC IONS ON THE INACTIVATION RATE OF DEXTRANSUCRASE

When ferric ion was added to solutions of the enzyme dextransucrase, f irst-order followed by second-order inactivation behavior was observed . The initial rapid activity loss was attributed to a ferric ion inter acting with the thiol group of the native monomer to form a less activ e enzyme-ion complex; the second inactivation stage involved enzyme-io n complex aggregation and disulfide cross-link formation. In contrast, Cu2+ ion inactivation demonstrated simple first-order kinetics. As wi th Fe3+, Cu2+ ions can form complexes with enzyme thiol groups. Howeve r, unlike ferric ions, cupric ions can also strongly interact with the imidazole ring of histidine. Since the dextransucrase active site con tains two key histidines, imidazole-cupric-ion interactions could pote ntially inhibit enzymatic activity. Thus, it was hypothesized that fir st-order Cu2+ inactivation kinetics involved the adsorption of this io n to the enzyme's activity site, The addition of a reducing agent such as dithiothreitol can inhibit the second enzyme aggregation stage by breaking disulfide cross-links but cannot restrict the initial formati on of metal-enzyme complexes.