High Concentrations of Glucose can Activate or Inhibit Human Erythrocyte Aminolevulinate Dehydratase in vitro Depending Exposure Time

Abstract

Hyperglycemia can cause oxidative stress and inactivate susceptible enzymes. In the present investigation we examined whether short-term incubations (24 or 48 h) with high concentrations of glucose (10-200 mmol L^-1) inhibit ALA-D from human erythrocytes. Incubations of erythrocytes for 24 h with glucose (10 up to 40 mmol L^-1) resulted in an increase of δ-ALA-D activity (P<0,001). Incubations of erythrocytes with 100 to 200 mmol L^-1 glucose for 48h inhibited δ-ALA-D activity (P<0,001). DTT (2 mmol L^-1) increased glucose-inhibitedδ-ALA-D activity 120%, but the activity did not return to the control level. These results indicated that the inhibitory effect of glucose depends on time of exposure and its concentration. A significant positive correlation was found between δ-ALA-D activity and NPSH groups from erythrocytes incubated 24h with different glucose concentrations. (r=0.65, P<0.0001. In contrast to the results of 24 h, incubation of erythrocytes for 48 h with 100, 150 and 200 mmol L^-1 of glucose did not modify significantly the NPSH content from erythrocytes. Incubations of erythrocytes for 48h with increasing concentrations of glucose (100 to 200 mmol L^-1) resulted in a significant concentration-dependent increase of TBARS content compared with control group. The TBARS content of erythrocytes incubated for 24 h with 5 (control), 10, 20, 30, 40 and 100 mmol L^-1 of glucose tend to increase as the concentration of glucose increased; however, the values were significantly higher than control only after incubation with 30, 40 and 100 mmol L^-1 of glucose. The results of this study indicate that the use of high concentrations of glucose (above 30 mmol L^-1) for short periods is of little pathophysiological significance for the study of molecular mechanism underlining enzymes inhibition caused by glucose. Thus, further in vitro studies (using glucose concentrations lower than 30 mmol L^-1 for higher periods of cells exposure to glucose) are necessary to established whether or not in vitro incubation of erythrocytes with glucose can be considered a reliable model for the study of the toxicity of glucose to proteins under pathophysiological conditions.