Objective: Superoxide dismutase (SOD) (EC 1.15.1.1) is reported to decrease the reduction of nitroblue tetrazolium (NBT) in the fructosamine assay. The study was undertaken to find a method to eliminate this interference.
Design and methods: We studied the NBT reduction in the presence and absence of added bovine erythrocyte SOD during fructosamine assay. Formation of reduced NBT decreased with the increasing concentration of SOD. Various inhibitors of SOD were experimented with for effectively eliminating this interference.
Results: Cyanide eliminates the interference due to SOD, but is unsuitable because in it's presence glucose becomes reducing under the conditions of fructosamine assay. SOD inhibitors such as EDTA and Azide did not eliminate the effect of SOD. Guanidine. HCl gives opalescence in the reaction mixture. Addition of 2M HCl to the serum and incubation at 37 degrees C for 10 min eliminated the effect of added SOD (70 kU/L). The correlation between deltaA10-20 min of serum treated with HCI in presence and absence of added SOD is y = 0.9011x + 0.01055 with r = 0.9789 and S.E. (y) 0.007686.
Conclusion: SOD does not interfere in globin bound fructosamine assay as acid-acetone treatment in preparation of heme free globin inhibits SOD. Pretreatment of serum with HCl can satisfactorily eliminate the interference due to SOD in fructosamine assay. The acid treatment could be used to inhibit SOD in various other reactions that are followed with NBT reduction.