Reduced susceptibility of glycosylated hemoglobin to proteolytic cleavage.

Four different experimental designs were selected to study, whether glycosylation of hemoglobin alters susceptibility against proteolytic degradation. We compared the resistance of different hemoglobin fractions against degradation by pure proteolytic enzymes, by liver cell culture, by live organ culture and the resistance of in vitro glycosylated vs. "non-glycosylated" hemoglobin solutions. The hemolysates were characterized by high performance liquid chromatography. The degree of glycosylation of hemoglobin was further estimated by the thiobarbituric-acid assay. From the data presented it is concluded, that the glucose moiety linked to the N-terminal valine and lysine residues of the alpha- and beta chain of the hemoglobin exerts a protective effect against proteolytic cleavage. This could explain the extended lag phase in response of glycosylated hemoglobin to an improved metabolic control in diabetic patients and the discrepancy between the rate of synthesis and the rate of catabolism.