Kinetic properties of the potassium/hydrogen ion antiport of heart mitochondria

The fluorescence of 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein (BCECF) has been used to follow a K(+)-dependent alkaline shift in the matrix pH (pHi) of isolated heart mitochondria. The K(+)-dependent pHi change has properties consistent with an inward exchange of K+ for matrix H+ on the K+/H+ antiport of the mitochondrion. The reaction is activated by depletion of matrix Mg2+ with A23187 and by an alkaline external pH (pHo) and hypotonic conditions. The exchange is inhibited by quinine, dicyclohexylcarbodiimide, and exogenous Mg2+, but not by Li+. The rate of K+/H+ antiport measured in this way increases with increasing pHo to a maximum near pHo 9. The rate is a hyperbolic function of [K+] at pHo values above 8.3 with an apparent Km of 30 mM at pHo 8.4 and 14 mM at pHo 8.8. External H+ acts as a mixed-type inhibitor of the K+/H+ antiport under these conditions with a Ki equivalent to pHo 8.6-8.8. When pHo is kept constant, the reaction is relatively insensitive to matrix pH (pHi) in the range from 7.0 to 7.5. Above this pHi, the K(+)-dependent H+ extrusion shows a hyperbolic dependence on [H+]i with an apparent Km equivalent to pHi 8.1. The activated antiport shows an affinity sequence of Li+ greater than K+ = Rb+ greater than Cs+. The inward antiport of K+ is inhibited noncompetitively by NH4+ and is also sensitive to benzamil and to 5-N-substituted amiloride analogues with I50 values near 20 microM. Both NH4+ and the amiloride analogues increase pHi at constant pHo and appear to be concentrated in the matrix under these conditions.