Oxidative Modification of Glucose-6-phosphate Dehydrogenase from Leuconostoc mesenteroides by an Iron(II)-Citrate Complex

Incubation of glucose-6-phosphate dehydrogenase fromLeuconostoc mesenteroides with Fe2+ and citrate results in rapid O2-dependent inactivation of the enzyme. Maximal rate of inactivation occurred at equimolar concentrations of Fe2+ and citrate. Loss of enzyme activity appears to be the result of selective oxidative modification, as evidenced by a corresponding increase in protein carbonyl content. Partially inactivated enzyme remained predominantly in the dimeric form with no change in the apparent affinity of the remaining active subunits for substrate. Modified Glu-6-PDH was, however, more susceptible to heat denaturation. Our results suggest that the Fe2+-citrate complex binds to the glucose 6-phosphate binding site and then undergoes reaction with H2O2 formed in solution leading to the oxidative modification of amino acids essential for enzyme activity.