NMR study of -17-O from H2-17-O in human erythrocytes

Human erythrocytes were incubated in a Ringer's solution enriched with 10--18 per cent H2-17-O. The longitudinal relaxation time (T1) of the -17-O was determined separately in samples of red cell suspensions, packed cells, and supernatant. The longitudinal relaxation of -17-O in erythrocyte suspensions was non-exponential, reflecting water exchange across the cell membranes as well as relaxation processes inside and outside the cell. The T1 of intracellular -17-O is 4--5 times shorter than in the supernatant, similar to the enhancement of proton relaxation by hemoglobin in erythrocytes and free solution at the frequency applied (8.13 MHz). This datum is consistent with tht thesis that hemoglovin modifies the NMR relaxation behavior of water inside cells and in free solution in the same way. The rate constant (kx) for water exchange was calculated to be 60 and 107 s- minus 1 at 25 and at 37 degrees C, respectively. The apparent activation energy for kx over the temperature range 23--37 degrees C was 8.7 plus or minus 1.0 kcal/mole.