A candidate for the permeability pathway of the outer mitochondrial membrane

Abstract

UNLIKE most other membranes, the outer mitochondrial membrane seems to be freely permeable to various small molecules and has therefore been called ‘leaky’. The intactness of this membrane is demonstrated by observations^1–4 which show it to be impermeable to large polymers. This ‘sieving’ property is consistent with the presence of size-selecting units such as channels. Parsons and co-workers^5,6 have published electron micrographs showing what looked like channels in the outer membrane of mitochondria (particularly those from plants), and recent X-ray diffraction studies by Mannella and Bonner⁷ have confirmed the existence of structures of similar dimensions to those seen in the electron micrographs. I now report the extraction of channel-forming material from the outer membrane of rat liver mitochondria. This material has properties which are essentially identical with those reported previously⁸ for voltage-dependent anion-selective channels (VDAC) from mitochondria of Paramecium. VDAC could be responsible for the permeability of the outer membrane to small molecules and may be involved in regulating mitochondrial metabolism.