Kinetic analysis of the nonenzymatic glycosylation of hemoglobin

Abstract

The rate constants have been derived for (a) the condensation of glucose with hemoglobin to form the labile Schiff base intermediate, pre-AIc; (b) the dissociation of this complex to hemoglobin and glucose; (c) the rearrangement of this complex to form the stable ketoamine, Hb AIc. These measurements required the purification of commercially available D-[14C]glucose in order to remove a rapidly reacting contaminant. The initial condensation reaction rate (kappa'1) was measured by incubating column purified Hb A0 for up to 8 h under physiologic conditions with purified D-[14C]glucose in the presence of cyanoborohydride which traps the Schiff base and reduces it to a stable adduct. A parallel incubation utilizing Hb AIc revealed the contribution of the beta-NH2-terminal amino group (kappa 1) to the overall value for kappa'1. The reverse reaction rate (kappa -1) was determined from incubations carried ut in the absence of cyanoborohydride. The rate of the Amadori rearrangement (kappa 2) was determined from longer (6-21 days) incubations under identical conditions, followed by chromatographic isolation of Hb AIc. This value for kappa 2 agrees well with one we previously obtained from in vivo data. These experiments provide direct chemical evidence for an aldimine precursor in the nonenzymatic glycosylation of protein. Furthermore, the use of these rate constants provides a reasonable estimate of the distribution of the labile aldimine (pre-AIc) and the stable ketoamine (Hb AIc) in normal and diabetic red cells. This information is useful in the interpretation of measurements of glycosylated hemoglobin in diabetic patients.