Proton Selective Substate of the Mitochondrial Permeability Transition Pore:  Regulation by the Redox State of the Electron Transport Chain

The permeability transition pore of rat liver mitochondria can be closed by chelating free Ca2+, with respect to the passage of large molecules such as mannitol and sucrose. However, an apparent H+-conducting substate remains open under these conditions, as indicated by the persistence of maximal O2 consumption rates and by the failure to recover a membrane potential. Agents which favor a closed pore, such as cyclosporin A, ADP, Mg2+, or bovine serum albumin, do not close the H+-conducting substate, but it closes spontaneously when respiration becomes limited by the availability of O2. Closure provoked by an O2 limitation requires free Mg2+ in the sub-micromolar concentration range and becomes less efficient with increasing time spent in the presence of free Ca2+. The H+-conducting substate is apparently regulated by the redox status of the electron transport chain, with a reduced form favoring closure. A physical association (or equivalence) between the pore and one of the respiratory chain complexes is supported. These characteristics suggest that the transition is irreversible in vivo, if it involves a small fraction of total mitochondria, and would lead to their elimination and/or replacement by the cell. The implications of this proposal are considered, as they relate to a possible role for the transition in cellular apoptosis and the elimination of mitochondria containing mutated DNA.