Effect of ferric and cupric ions on the inactivation rate of dextransucrase

When ferric ion was added to solutions of the enzyme dextransucrase, first-order followed by second-order inactivation behavior was observed. The initial rapid activity loss was attributed to a ferric ion interacting with the thiol group of the native monomer to form a less active enzyme-ion complex; the second inactivation stage involved enzyme-ion complex aggregation and disulfide cross-link formation. In contrast, Cu²⁺ ion inactivation demonstrated simple first-order kinetics. As with Fe³⁺, Cu²⁺ ions can form complexes with enzyme thiol groups. However, unlike ferric ions, cupric ions can also strongly interact with the imidazole ring of histidine. Since the dextransucrase active site contains two key histidines, imidazole-cupric-ion interactions could potentially inhibit enzymatic activity. Thus, it was hypothesized that first-order Cu²⁺ inactivation kinetics involved the adsorption of this ion 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 formation of metal-enzyme complexes.