Evidence for Interindividual Heterogeneity in the Glucose Gradient Across the Human Red Blood Cell Membrane and Its Relationship to Hemoglobin Glycation

Abstract

OBJECTIVE-To determine whether interindividual heterogeneity in the erythrocyte (red blood cell [RBC]) transmembrane glucose gradient might explain discordances between A1C and glycemic control based on measured fructosamine. RESEARCH DESIGN AND METHODS-We modeled the relationship between plasma glucose and RBC glucose as the concentration distribution (C i -to-C o ratio) of a nonmetabolizable glucose analog 14 C-3-O-methyl glucose ( 14 C-3OMG) inside (C i ) and outside (C o ) RBCs in vitro. We examined the relationship between that distribution and the degree of glycation of hemoglobin in comparison with glycation of serum proteins (fructosamine), the glycation gap. A1C, fructosamine, and in vitro determination of the 14 C-3OMG distribution in glucose-depleted RBCs were measured in 26 fasted subjects.RESULTS-The C i -to-C o ratio 0.89 Ϯ 0.07 for 3-O-methyl-Dglucopyranose (3OMG) ranged widely (0.72-1.04, n ϭ 26). In contrast, urea C i -to-C o (1.015 Ϯ 0.022 [range 0.98 -1.07], P Ͻ 0.0001) did not. Concerning mechanism, in a representative subset of subjects, the C i -to-C o ratio was retained in RBC ghosts, was not dependent on ATP or external cations, and was reestablished after reversal of the glucose gradient. The 3OMG C i -to-C o ratio was not correlated with serum fructosamine, suggesting that it was independent of mean plasma glucose. However, C i -to-C o did correlate with A1C (R 2 ϭ 0.19) and with the glycation gap (R 2 ϭ 0.20), consistent with a model in which differences in internal glucose concentration at a given mean plasma glucose contribute to differences in A1C for given level of glycemic control.CONCLUSIONS-The data demonstrate interindividual heterogeneity in glucose gradients across RBC membranes that may affect hemoglobin glycation and have implications for diabetes complications risk and risk assessment. Diabetes 57: [2445][2446][2447][2448][2449][2450][2451][2452] 2008 A 1C is the current gold standard for determination of chronic glycemic control in people with diabetes. Yet it is common to find hematologically normal people with diabetes in whom A1C appears discordant from other measures of glycemic control. Some have suggested the notion of a "hemoglobin glycation index" to assess A1C discordance from mean blood glucose (1-3). We have quantitated this discordance with the use of the glycation gap (GG), a measure of the disparity between two integrated measures of glycemic control, one intracellular in red blood cells (RBCs), A1C, and the other extracellular, glycated serum protein (GSP) measured as fructosamine (4). A non-zero GG could result from differences between the ambient glucose concentrations or rates of glycation in the intracellular and extracellular compartments, and/or interindividual differences in the turnover/metabolism of underlying proteins. We have demonstrated that the GG is reproducible within subjects over time and is associated with important clinical end points. In longstanding type 1 diabetes, there is a over a twofold rise in prevalence of nephropathy in pati...

Progression of Nephropathy in Type 2 Diabetes: The Glycation Gap Is a Significant Predictor after Adjustment for Glycohemoglobin (Hb A1c)

“…These genetic results reinforce the implications of the temporal stability of gg and HGI in the sense that Hb A 1c levels must be determined by multiple factors, not all of which are related to glycemia. Candidate factors include erythrocyte 2,3-diphosphoglycerate levels, intra-erythrocyte pH, and total plasma amino acid levels (8 ); lipoperoxides and antagonistic antioxidants (12 ); glycolytic and deglycating enzymes (6,7,21 ); processes related to glucose transporter 1-mediated transport (11 ); and erythrocyte transmembrane glucose gradient (27 ). When factors such as these have been more definitely related to the glycability of hemoglobin and/or of other proteins glycated in diabetes, they will doubtlessly be used in evaluating risk of complications.…”

Red cell life span heterogeneity in hematologically normal people is sufficient to alter HbA1c

“…45 For example, we have proposed heterogeneity in the glucose gradient across the RBC membrane independent of glycemic control as one possibility. 46,47 Whereas some authors interpret data as indicating minimal biologic variation in HbA1c between individuals, 4 others have found evidence for more marked differences resulting from poorly understood genetic factors. 48,49 After the elucidation of the nature of nonenzymatic glycation, 50,51 glycation rates have been shown to be subject to variation between individuals by factors that are independent of glucose concentration, including pH, 32 inorganic phosphate, 52 oxidative stress, 53 deglycation, 43,54 and Schiff base inhibitors.…”

Current controversies in the use of haemoglobin A1c

“…Genetics are thought to play a significant role in the determination of HbA 1c , with research conducted in twins estimating that 62% of the population variance in HbA 1c is explained by heritability [26]. Recent research suggests that, even amongst healthy individuals, heterogeneity exists both in the glucose concentration gradient across the erythrocyte membrane and in the average erythrocyte lifespan [27,28]. Obviously, many disease states also alter erythrocyte turnover through changes in the rate of either erythropoiesis or haemolysis.…”