Effect of cortisol on erythrocyte and reticulocyte enzymes: modulation of phosphofructokinase properties.

After incubation of rabbit red blood cells with 10 micrograms/ml of cortisol, a decrease of the activity of glucose-6-phosphate dehydrogenase, phosphofructokinase and acid phosphatase was observed. Glucose-6-phosphate dehydrogenase and phosphofructokinase were isolated from hemolysate and investigated. No changes in the affinity of both enzymes towards substrates and coenzymes were found. Cortisol modified allosteric properties of phosphofructokinase. Differences were seen between the erythrocyte and reticulocyte enzyme. In the erythrocyte enzyme the inhibitory effect of ATP and citrate was enhanced and the activatory effect of AMP was abolished after incubation with cortisol. Cortisol showed no effect on the inhibition of reticulocyte phosphofructokinase by ATP or activation by AMP, and protected the enzyme toward inhibition by citrate.