The alkaline reducing activity of glycated serum proteins and its relevance to diabetes mellitus.

We investigated the ability of the fructosamine assay to detect glycation of serum proteins. We incubated both whole human serum and serum protein fractions in vitro with [14C]glucose, and analyzed for reducing activity and for uptake of 14C by protein. In all experiments, the reducing activity increased linearly with time for seven days and was correlated with 14C uptake (r = 0.94-0.98). Protein ketoamines were about fivefold more actively reducing than equimolar concentrations of deoxymorpholinofructose, the fructosamine standard, which explains why values for fructosamine in serum are higher than the expected concentration of protein ketoamines. We also used [14C, 2-3H]glucose to assess the contribution of the aldimine component to 14C uptake. Whole human serum and albumin incubated with [14C, 2-3H]glucose showed little uptake of 3H in relation to 14C. We conclude that glycated protein can be simply and reliably quantified by the fructosamine assay, and we discuss the relevance of this conclusion to the monitoring of diabetes.