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

Incubation of glucose-6-phosphate dehydrogenase from Leuconostoc 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.