Chaotropic Agents and Increased Matrix Volume Enhance Binding of Mitochondrial Cyclophilin to the Inner Mitochondrial Membrane and Sensitize the Mitochondrial Permeability Transition to [Ca2+]

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Abstract
Binding of mitochondrial cyclophilin (CyP) to the inner mitochondrial membrane is induced by treatment of mitochondria with thiol reagents or oxidative stress and correlates with a sensitization to [Ca2+] of the cyclosporin A-sensitive mitochondrial permeability transition pore (MTP) [Connern, C. P., & Halestrap, A. P. (1994) Biochem. J. 303, 321−324]. Here we show that detection of the bound CyP by Western blotting is greatly enhanced by fixing the CyP to the blotting membrane with glutaraldehyde. CyP binding was only observed when mitochondria were incubated and then frozen in KSCN medium before preparation of the membrane fraction, but not when KCl medium was used. However, incubation of mitochondria (energized or deenergized) in KCl medium followed by KSCN addition immediately prior to freezing did allow CyP binding to be detected. The action of KSCN could be mimicked by guanidinium chloride, implying that the chaotropic action of these agents stabilized the bound complex. The sensitivity to [Ca2+] of the MTP in deenergized mitochondria was greatly enhanced in KSCN medium as compared to KCl medium. Binding of CyP to the mitochondrial membrane was increased by treatment with tert-butylhydroperoxide, phenylarsine oxide, and diamide and by hypoosmotic KCl medium. These conditions all increased the sensitivity of the MTP to [Ca2+]. Conditions known to increase the mitochondrial NADH/NAD+ ratio decreased CyP binding. In contrast, the effects of mitochondrial membrane potential, matrix pH, and adenine nucleotide translocase conformation on the sensitivity of the MTP to [Ca2+] were not associated with a change in CyP binding. Our data imply that there may be two independent mechanisms of altering the Ca2+ sensitivity of the MTP, one brought about by CyP binding which is stabilized by chaotropic agents and another involving additional regulatory sites on the pore complex.