Substrates of Hexokinase, Glucose-6-Phosphate Dehydrogenase, and Glyceraldehyde-3-Phosphate Dehydrogenase Prevent the Inhibitory Response Induced by Ascorbic Acid/Iron and Dehydroascorbic Acid in Rabbit Erythrocytes

Exposure of intact rabbit erythrocytes or erythrocyte lysates to ascorbic acid/FeCl3in a glucose-free saline promoted a rapid decline in reduced glutathione and this response was paralleled by inactivation of hexokinase. Under the same conditions, the activity of the enzymes glyceraldehyde-3-phosphate dehydrogenase and glucose-6-phosphate dehydrogenase did not show appreciablevariations in intact cells, but was severely inhibited in the cell-free system. Similar results were obtained by replacing ascorbic acid/FeCl3with dehydroascorbic acid. In addition, both treatments effectively inhibited the activity of purified hexokinase as well as those of glucose-6-phosphate dehydrogenase and glyceraldehyde-3-phosphate dehydrogenase. Further studies using the cell-free system indicated that the inhibition of enzyme activities elicited by either of the two treatments was effectively counteracted by the specific substrates of these enzymes. The fact that the hexokinase substrate glucose freely permeates the plasma membrane, unlike the substrates of glucose-6-phosphate dehydrogenase and glyceraldehyde-3-phosphatedehydrogenase, explains the selective inhibition of hexokinase observed in intact cells. The above results also indicate that dehydroascorbic acid is an inhibitor of these enzymes and strongly suggest that it is at least in part responsible for the effects mediated by the cocktail ascorbic acid/FeCl3.