Decreased pyrimidine nucleoside monophosphate kinase activity in sickle erythrocytes

We have previously shown that physiologic concentrations of hemin cause marked inhibition of several red blood cell (RBC) enzymes. Because endogenous heme content is elevated in sickle RBCs, we have examined the activity of hemin-sensitive enzymes in these RBCs. One of the hemin-sensitive enzymes, pyrimidine nucleoside monophosphate kinase (PNMK), was shown to have decreased activity in sickle RBCs relative to RBCs of equivalent cell age. The other hemin-sensitive enzymes, including adenylate kinase (AK), pyrimidine 5'-nucleotidase (P5N), 6-phosphogluconate dehydrogenase (6PGD), and aldolase, had activities that were appropriate for cell age. We have also examined the affinity of the hemin-sensitive enzymes to hemin. Using two different methods, PNMK was shown to have the highest binding affinity to hemin. The exquisite sensitivity of PNMK to inhibition by hemin, coupled with the enzyme's high affinity to hemin, may account for the decrease in PNMK activity and the lack of significant decrease in the other hemin-sensitive enzymes in sickle RBCs. These results suggest that the increased endogenous heme content in sickle RBCs may be responsible for the decrease in PNMK activity. Whether the increased endogenous heme content of sickle RBCs can cause hemolysis indirectly by inhibiting RBC enzymes remains to be determined.