Role of the Mitochondrial Outer Membrane in Dynamic Compartmentation of Adertine Nucleotides

Abstract

Small molecules such as adenine nucleotides can pass the porin pores of mitochondrial outer membrane (VDAC) for exchange between mitochondria and cytosol. Estimated to range from 1.25 to 2 × 10^-9 m (Colombini et al., 1987), the radius of the pores is sufficient to allow the passage of molecules up to a molecular weight of 6000 (Zalman et al., 1980). The effective cross-section of pores is influenced by voltage-dependent processes (Colombini et al., 1987) or colloid-osmotic effects (Zimmerberg & Parsegian, 1986). Nevertheless, the intermembrane space together with the extramitochondrial compartment is commonly believed to form a homogeneous pool for adenine nucleotides. However, it may be assumed that the transport of adenine nucleotides between the cytosol and the AdN translocator occurs by diffusion along concentration gradients, but the question arises whether or not these gradients are high enough to cause a measurable AdN compartmentation in the intermembrane space. Since concentration gradients are necessarily connected with fluxes and disappear with them, the term dynamic compartmentation has been proposed for diffusion-dependent inhomogeneities (Gellerich et al., 1987).