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Endocrinology and Metabolism |
a Address correspondence to this author at: Washington University School of Medicine, Division of Laboratory Medicine, Box 8118, St. Louis, MO 63110. Fax 314-362-1461; e-mail mscott{at}labmed.wustl.edu.
Irreversible glycation of the hemoglobin A0
(HbA0) ß chain leads to the production of
HbA1C, which can be used to monitor long-term blood glucose
control in patients with diabetes mellitus. HbA1C is less
positively charged than nonglycated 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(TM)) 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 ß
chain and genes revealed that the patient was heterozygotic for
Hb-Raleigh, a variant containing a valine
alanine substitution at
position 1 of the ß chain. The amino-terminal alanine in this variant
Hb is posttranslationally modified by acetylation, leading to a charge
difference similar to glycation and making the behavior of
HbA1C and Hb Raleigh virtually 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 the clinical picture, by an
independent HbA1C method such as immunoassay or boronic
acid affinity chromatography. However, for this particular variant Hb,
even these latter methods might be misleading, because the acetylated
N-terminal amino acid of the Hb-Raleigh ß chain cannot be glycated.
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