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The diffusion of metabolites across the outer mitochondrial membrane is essential for coupled cellular respiration. The outer membrane of mitochondria isolated from growth factor-deprived cells is impaired in its ability to exchange metabolic anions. When added to mitochondria, recombinant Bcl-x_L restores metabolite exchange across the outer membrane without inducing the loss of cytochrome c from the intermembrane space. Restoration of outer membrane permeability to anionic metabolites does not occur directly through Bcl-x_Lion channels. Instead, recombinant Bcl-x_L maintains the outer mitochondrial membrane channel, VDAC, in an open configuration. Consistent with these findings, when ADP-induced oxidative phosphorylation is limited by exogenous β-NADH, recombinant Bcl-x_L can sustain outer mitochondrial membrane permeability to ADP. β-NADH limits respiration by promoting the closed …