OPPOSITE EFFECTS OF UREA ON HEMOGLOBIN-OXYGEN AFFINITY IN ANEMIA OF CHRONIC-RENAL-FAILURE

We studied the action of urea on the spin-spin relaxation rate of 2,3- diphosphoglycerate (2,3-DPG) phosphorus atoms in normal and uremic ery throcytes. At concentrations from 10 to 60 mM, urea increased the rela xation rates of 2,3-DPG P-3 phosphorus atoms. This evidenced a stronge r binding of 2,3-DPG to hemoglobin (Hb), suggesting that the deoxyform of Hb was stabilized. This hypothesis was confirmed by measurements o f the association constant of oxygen to hemoglobin (K) in normal eryth rocytes in presence of urea concentrations in the range of those obser ved in uremic patients (30 mM). Indeed, the observed decrease in K sug gests that the T structure of hemoglobin is stabilized. By contrast, w ith higher urea concentrations (120 mM), measurements of P-50 showed a n increase in the hemoglobin affinity for oxygen (decrease in P-50). M oreover, the relaxation rates of 2,3-DPG P-3 phosphorus atoms were not modified, which is consistent with the simultaneous increase of K. Th is may be attributed to the formation of carbamylated hemoglobin in pr esence of urea. These results suggest two opposite effects of urea on Hb-O-2 affinity: the first reinforces 2,3-DPG-Hb binding and leads to a decrease in O-2 affinity; the second, mediated by carbamylation of H b, hinders the binding of 2,3-DPG and increases the O-2 affinity. Thes e findings are consistent with the fact that, despite the presence of carbamylated hemoglobin, uremic patients do not present increased Hb-O -2 affinity.