Pre-steady-state and steady-state function of the ileal brush border SO4(2-)-OH- exchanger.

Abstract

Rapid-sampling analysis of the detailed time course of 35SO4(2-) uptake under pH-gradient (pHin = 7.5; pHout = 5.5) conditions converged to a model of an initial burstlike pre-steady-state with relaxation to linear steady-state uptake across pig ileal brush border vesicles. A model of low affinity transport (K(m) = 7.7 mM) plus an unsaturable component described the steady-state component of 35SO4(2-) uptake. Steady-state transport was maximal under pH-gradient conditions and sensitive to inhibition by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid. Significant steady-state 35SO4(2-) transport sensitive to 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid was found under acidic (pHin = pHout = 5.5) and neutral (pHin = pHout = 7.5) pH-equilibrated conditions. Varying conditions from pH gradient to neutral pH equilibrated had no effect on the amplitude of the pre-steady-state burst or on the time constant for relaxation from pre-steady-state to steady-state conditions. However, maximal amplitude was obtained under acidic pH-equilibrated conditions. These results are incorporated into a model for the low affinity ileal brush border SO4(2-)-OH- exchanger whereby, under 0-trans (0 internal substrate concentration) conditions, translocation of inner-facing to outer-facing conformations defines the overall rate-limiting step of the transporter. Provision for slippage of the unloaded carrier could account for 35SO4(2-) transport under acidic pH-equilibrated conditions.