mitochondrial inner membrane
A family of antiport, ATP-ADP translocases (SLC25A4,5,6), preferentially export ATP from the matrix while importing ADP from the cytosol, thereby maintaining a high ADP:ATP ratio in the matrix. When there are increased energy demands on the body, such as under heavy exercise, cytosolic ADP rises and is exchanged with mitochondrial matrix ATP via the transmembrane ADP:ATP translocase. Increased ADP causes the proton-motive force to be discharged and protons enter via ATPase, thereby regenerating the ATP pool.
There are 3 isoforms of translocases in humans; isoform 1 (SLC25A4) is the heart/skeletal muscle form, isoform 2 (SLC25A5) is the fibroblast form and isoform 3 (SLC25A6) is the liver form. All isoforms exist as homodimers.