Glycation of proteins as a source of superoxide

For the monitoring of diabetes mellitus, the concentration of serum glycated proteins is currently measured by a colorimetric reaction using the dye nitroblue tetrazolium. As this reduction may depend on superoxide ions (O2-), we checked whether glycated proteins were capable of generating O2- in vitro. We incubated ferricytochrome c with glucose, fructose, 1-deoxy-1-morpholino-D-fructose (an analog of the 1-desoxy-1-amino-fructose radical found in glycated proteins) and glycated proteins prepared from diabetic blood serum. We found that these substances, except free glucose, were all able to generate O2- not only at alkaline pH, but even at pH 7.4 with a slower rate. The possibility of O2- formation from glycated proteins may explain some long term complications of diabetes mellitus.