Membrane permeability of formate in human erythrocytes: NMR measurements

The rate of the rapid exchange of formate mediated by band 3 in human erythrocytes, under equilibrium exchange conditions, was measured by using a T ₁ relaxation method with ¹³C-labelled formate and ¹³C NMR, and a pulsed field-gradient spin-echo (PFGSE) method using ¹H NMR. The former analysis was based on large differences in T ₁ between the inside and the outside of the cells brought about by added Mn²⁺; the latter was based on large differences in the apparent diffusion coefficient inside and outside the cells. There was close agreement in the estimates of the membrane permeabilities made using both methods, suggesting a lack of interference of the exchange process by Mn²⁺. Regression analysis yielded estimates (under the specified conditions, including 37°C) of V _max of 3.5±0.3×10^–9 and 3.8±0.4×10^–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-second time scale) formate exchange in cells. Received: 8 May 1998 / Accepted: 16 July 1998