Pyrimidine nucleoside monophosphate kinase hyperactivity in hereditary erythrocyte pyrimidine 5'-nucleotidase...

The pyrimidine nucleoside triphosphates (CTP, UTP) increase in the pyrimidine 5'-nucleotidase (P5N) deficient red blood cell (RBC) to a greater degree than do the pyrimidine nucleoside monophosphates (CMP, UMP). Pyrimidine nucleoside monophosphate (PNMP) kinase phosphorylates CMP and UMP to their respective phosphodiesters. We tested the hypothesis that increased PNMP kinase activity contributes to the disproportionate increase in CTP and UTP in the P5N deficient RBC. CMP and UMP kinase activities were increased in high reticulocyte (4.4 +/- 2.1 and 8.5 +/- 3.3 mumol/ml RBC per minute) compared to normal RBC (2.8 +/- 1.0 and 6.0 +/- 2.5 mumol/ml RBC per minute). P5N deficient RBC (n = 2) had significantly increased CMP and UMP kinase activities (14.0 and 26.5 mumol/ml RBC per minute). UMP and CDP-ethanolamine were able to increase the activity of CMP kinase in crude haemolysate and the activity of partially purified enzyme. Since the Km for CMP of CMP kinase was 33 mumol/l in P5N deficient RBC and since the CMP concentration is 25-90 mumol/l in the P5N deficient RBC, the enzyme should be nearly saturated with CMP in the P5N deficient RBC. Thus, PNMP kinase hyperactivity appears to contribute to the disproportionate increase in CTP and UTP in the P5N deficient RBC.