Membrane permeability of formats in human erythrocytes: NMR measurements

The rate of the rapid exchange of formate mediated by band 3 in human eryth rocytes, under equilibrium exchange conditions. was measured by using a T-1 relaxation method with C-13-labelled formate and C-13 NMR. and a pulsed fi eld-gradient spin-echo (PFGSE) method using H-1 NMR. The former analysis wa s based on large differences in T-1 between the inside and the outside of t he cells brought about by added Mn2+; the latter was based on large differe nces in the apparent diffusion coefficient inside and outside the cells. Th ere was close agreement in the estimates of the membrane permeabilities mad e using both methods, suggesting a lack of interference of the exchange pro cess by Mn2+. Regression analysis yielded estimates (under the specified co nditions, including 37 degrees C) of V-max of 3.5+/-0.3x10(-9) and 3.8+/-0. 4x10(-9) mol cm(-2) s(-1), and K-m of 9.8+/-0.2 and 8.1+/-0.2 mM, for the T -1 and the PFGSE methods. respectively. These are new estimates made using methodology that has not previously been applied to measuring rapid (sub-se cond time scale) formate exchange in cells.