Irreversible glycation of the HbA0( chain leads to the productin of HbA1C, which can be used to monitor long-term blood glucose control in patients with diabetes mellitus. HbA1C is less positively charged than non-glycated HbA0 and this decrease in charge is the basis of ion-exchange and electrophoretic methods that measure HbA1C. We recently identified a sample that appeared to contain 46% HbA1C by an automated ion-exchange HPLC method (Bio-Rad Variant) but only 3.8% by an immunoinhibition latex agglutination method. A combination of traditional and mass spectrometric protein analysis and genomic DNA analysis of the Hb ( chain and genes revealed that the patient was heterozygote for Hb-Raleigh, a variant containing a valine->alanine substitution at position 1 of the ( chain. The amino-terminal alanine in this variant Hb is posttranslationally modified by acetylation leading to a charge difference similar to glycation, making the behavior of HbA1C and Hb Raleig h virtual ly identical in the ion-exchange HPLC method. This observation suggests that it is important to confirm HbA1C values in excess of 15%, especially if they are not consistent with clinical picture, by an independent HbA1C method, such as an immunoassay or boronic acid affinity chromatography. However, for this particular variant Hb even these latter methods might be misleading since the acetylated N-terminal amino acid of the Hb-Raleigh ( chain cannot be glycated.
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