Yesterday I learned what “A1c” really is – you know, that thing you hear about in all those commercials for diabetes drugs to “ask your doctor about your A1c.” Turns out that it’s a glycated hemoglobin chain and its levels tell you about long-term glucose levels. Basically, if you have uncontrolled blood sugar (glucose) levels (such as happens with uncontrolled diabetes), glucose can latch onto the end of a hemoglobin protein chain (hemoglobin, abbreviated Hb, is that protein that carries oxygen through your blood & your red blood cells are chock full of it!) This latching-on of glucose occurs through something called a Schiff base linkage, which rearranges a little to give you a characteristic glycated hemoglobin that can be measured and which is called HbA1c. These measurements are useful because the glycation is “permanent” and red blood cells (where all that hemoglobin & sugar’s at) have a lifetime of a few months. So the A1c levels give you an idea about the blood sugar levels during the last few months as opposed to just “now” and therefore can give doctors information about how well someone’s blood sugar levels are being controlled and whether they might have prediabetes or diabetes or no longer have diabetes (if type 2).
not much text today because this was all kinda spur of the moment because I just learned about all this and was too excited not to share while it’s all fresh
The name is for historical reasons having to do with it being the 3rd fraction isolated when separating variants of adult hemoglobin by ion exchange chromatography. Ion exchange chromatography separates things based on differences in their charge by running them through a charged material and seeing “how hard they stick to it” – the glycosylation leads to a slight difference in the isoelectric point meaning it’s slightly more negative than the normal form at certain pHs, making it separable by the ion exchange. There are a bunch of ways to quantify it these days – some (HPLC ones) use separation like that or with affinity methods here the material has boronic acid groups that bind specifically to the glycans; there are various immunoassays (using antibodies specific to A1c to see how much there is); and there enzymatic assays that cut off the glycans & react them and measure the product.
Here’s the paper “defining” A1c – glycated hemoglobin had already been discovered (well, they didn’t know it was glycated, just that it moved faster than other fractions of hemoglobin in an agar starch electrophoresis (we know know this is because the glycation reduces its charge). (that was by Huisman and Meyering in 1958 but I can’t find their actual paper, just a citation). This group I’m linking to further separated the “fast hemoglobin” using ion exchange chromatography. When they did this they had 3 main fractions (A1a, A1b, & A1c).
Observations on the Chromatographic Heterogeneity of Normal Adult and Fetal Human Hemoglobin: A Study of the Effects of Crystallization and Chromatography on the Heterogeneity and Isoleucine Content. David W. Allen, W. A. Schroeder, and Joan Balog https://pubs.acs.org/doi/abs/10.1021/ja01540a030
Journal of the American Chemical Society 1958 80 (7), 1628-1634. DOI: 10.1021/ja01540a030
We now know that those other fractions corresponded to other glycations (not glucose addition). From this great (but old) review article… John WG. Glycated haemoglobin analysis. Ann Clin Biochem. 1997 Jan;34 ( Pt 1):17-31. doi: 10.1177/000456329703400105. PMID: 9022885. https://journals.sagepub.com/doi/pdf/10.1177/000456329703400105
“Haemoglobin A1 (HbA1) refers to all species derived from adducts (not necessarily glucose) to the N-terminal valine of haemoglobin, whereas haemoglobin A1c (HbA1c) is a specific adduct of glucose to the N-terminal valine of haemoglobin, which is the preferred site of binding. HbA1c usually constitutes 60%- 80% of HbA 1, the remainder being adducts with fructose 1,6-diphosphate (HbA1a1), glucose-6- phosphate (HbA1a2) and an unknown carbohydrate (HbA1b) , not of direct importance in diabetic monitoring. Total glycated haemoglobin includes (in analytical terms) HbA le and the species derived from glucose adducts at secondary sites, i.e. amino groups elsewhere in the haemoglobin molecule.”
Here are a couple other good reviews:
English E, Milosevich E, John WG. In vitro determination of hemoglobin A1c for diabetes diagnosis and management: technology update. Pathology and Laboratory Medicine International. 2014;6:21-31 https://doi.org/10.2147/PLMI.S48403
Glycation of Proteins, Motoko Takahashi: https://link.springer.com/content/pdf/10.1007%2F978-4-431-54841-6_182.pdf
more on diabetes & insulin: http://bit.ly/insulindiabetes
more on hemoglobin: http://bit.ly/bohreffect