Characterization of glycosylated hemoglobins. Relevance to monitoring of diabetic control and analysis of other proteins.

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Abstract

Boronate affinity chromatography and ion exchange chromatography were used to measure the levels of glycosylated hemoglobins in normal and diabetic hemolysates, as well as the distribution of glucose adducts on alpha-NH2-valine and epsilon-NH2-lysine residues. When analyzed by ion exchange chromatography on BioRex 70 resin, the Hb Alc peak comprised 4.4 +/- 0.6% of 15 normal hemolysates and 9.1 +/- 2.1% of 15 diabetic hemolysates. The "Hb Alc" was rechromatographed on GlycoGel B boronate affinity resin that binds vicinal hydroxyl groups of covalently linked sugars. Only 70 +/- 5% of the hemoglobin adhered to the resin. Analysis by the thiobarbituric acid colorimetric test confirmed that the affinity resin effectively separated glycosylated from nonglycosylated hemoglobin. When corrected for nonglycosylated contaminants, the mean level of Hb Alc in normal hemolysates was 2.9 +/- 0.4%, a value considerably lower than those previously reported. In addition to Hb Alc, 5.2 +/- 0.5% of the remaining hemoglobin (Hb Ao) was glycosylated. In diabetics, glycosylated Ao was increased in parallel with Hb Alc. After reduction with [3H]borohydride and acid hydrolysis, glycosylated amino acids were first purified on Affi-Gel boronate affinity resin and then analyzed by ion exchange chromatography. The glucose adducts on Hb Ao were distributed as follows: alpha-chain N-terminal valine, 14%; alpha-chain lysines, 40%; beta-chain lysines, 46%. This study has revealed several pitfalls in the analysis of nonenzymatically glycosylated proteins. Peaks isolated by ion exchange chromatography or electrophoresis are likely to be contaminated by nonglycosylated proteins. Furthermore, both the thiobarbituric acid test and [3H]borohydride reduction show variable reactivity depending upon the site of the ketoamine-linked glucose.

Authors

R L Garlick, J S Mazer, P J Higgins, H F Bunn