The Ki values of cyclosporins A, G and H for the peptidyl-prolyl cis-trans isomerase (PPIase) of liver and heart mitochondria are about 2, 20 and 500 nM respectively. This parallels their profile as inhibitors of non-specific pore opening of mitochondria induced by supraphysiological Ca2+ concentrations. The novel immunosuppressant FK-506 gave little inhibition of either process at 5 microM. These data support our previous hypothesis [Halestrap & Davidson (1990) Biochem. J. 268, 153-160] that pore opening involves an interaction between matrix PPIase and the adenine nucleotide translocase. It is suggested that this model may help to clarify the mechanism of action of cyclosporin as an immunosuppressant and its toxic effects on the liver and kidney following prolonged therapy.
