It is now well established that incubation of mitochondria
at pH 8 or higher opens up an electrophoretic anion transport pathway in the
inner membrane. It is not known, however, whether this transport process has
any physiological relevance. In this communication we demonstrate that anion
uniport can take place at physiological pH if the mitochondria are depleted
of matrix divalent cations with A23187 and EDTA. Using the light-scattering
technique we have quantitated the rates of uniport of a wide variety of anions.
Inorganic anions such as Cl-, SO4(2-), and Fe(CN)6(4-) as well as physiologically
important anions such as HCO3-, Pi-, citrate, and malate are transported.
Some anions, however, such as gluconate and glucuronate do not appear to be
transported. On the basis of the finding that the rate of anion uniport assayed
in ammonium salts exhibits a dramatic decline associated with loss of matrix
K+ via K+/H+ antiport, we suggest that anion uniport is inhibited by matrix
protons. Direct inhibition of anion uniport by protons in divalent cation-depleted
mitochondria is demonstrated, and the apparent pK of the binding site is
shown to be about 7.8. From these properties we tentatively conclude that
anion uniport induced by divalent cation depletion and that induced by elevated
pH are catalyzed by the same transport pathway, which is regulated by both
matrix H+ and Mg2+.
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