Mechanism of fructosamine assay: evidence against role of superoxide as intermediate in nitroblue tetrazolium reduction.

We studied the chemistry of the fructosamine assay for glycated serum proteins by using the model Amadori compound N alpha-formyl-N epsilon-fructoselysine (fFL), an analog of glycated lysine residues in protein. Free lysine was formed at approximately 70% yield during a standard 20-min incubation of fFL with alkaline nitroblue tetrazolium (NBT) at 37 degrees C. Although superoxide dismutase (SOD; EC 1.15.1.1) and catalase (EC 1.11.1.6) decreased the yield of the product, monoformazan dye (MF+), the yield of MF+ was slightly greater under anaerobic than aerobic conditions, excluding a role for superoxide as an intermediate in the reduction of NBT during the fructosamine assay. SOD added to diabetic patients' sera at physiological concentrations also caused a significant (approximately 50%) inhibition of MF+ formation. This inhibition was reduced by addition of nonionic detergents, which contain organic peroxide inhibitors of SOD, to the fructosamine reagent. Overall, these data indicate that the Amadori compound is the direct reductant of NBT in the fructosamine assay and that superoxide is not an intermediate in the reaction. The inhibitory effects of SOD and catalase are most likely the result of oxygen regeneration in the assay mixture.