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Volume 271, Number 43, Issue of October 25, 1996 pp. 26724-26731
©1996 by The American Society for Biochemistry and Molecular Biology, Inc.

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The Role of Pyridine Dinucleotides in Regulating the Permeability of the Mitochondrial Outer Membrane

(Received for publication, March 18, 1996, and in revised form, August 13, 1996)

From the Laboratories of Cell Biology, Department of Zoology, University of Maryland, College Park, Maryland 20742

Both NADH and NADPH reduce the permeability of the mitochondrial outer membrane to ADP. This is specific for the outer membrane and uncorrelated with the respiratory control ratio. This could result in a 7-fold difference between the concentration of ADP in the intermembrane space and that in the external environment (at 5 µM ADP). In both cases the permeability declines by a factor of 5, but NADH is more potent: K_D = 86 µM for NADH versus 580 µM for NADPH. The lower apparent affinity for NADPH is partly explained by Mg²⁺-NADPH being the active species, and under our conditions only 30% of the NADPH is in this form. The corrected K_D is 184 µM. Free NADH has the same charge as the Mg²⁺-NADPH complex, and thus both likely bind to the same site. The ability of NADH and NADPH to induce the closure of reconstituted VDAC channels is consistent with VDAC being the main pathway for metabolite flow across the outer membrane. Oncotic pressure, effective at inducing VDAC closure, also decreases the outer membrane permeability. Thus, in the presence of cytosolic colloidal osmotic pressure NAD(P)H may inhibit mitochondrial catabolic pathways and divert reducing equivalents to anabolic pathways.

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