PROTONATION EQUILIBRIUM OF ELLIPTICINE BOUND TO THE ENERGY-TRANSDUCING MEMBRANE OF MITOCHONDRIA

The relationship between the protonation state of ellipticine (5,11-di methyl-6H-pyrido[4,3-b]carbazole), an antitumoral alkaloid, and the tr ansmembrane proton electrochemical gradient associated with mitochondr ial membranes has been investigated. The binding parameters of ellipti cine to mitochondria are independent of either the transmembrane Delta pH or membrane potential, which are both components of proton gradien t (association constant = 1.5 X 10(6) M(-1); maximal binding ratio = o ne bound ellipticine per 25 phospholipids). Since the apparent pK of b ound ellipticine is 7.1, its two protonation states can be detected fr om their specific fluorescence emission spectra near physiological pH. It is shown that a shift in the drug protonation equilibrium toward t he neutral form occurs in mitochondrial membrane during the generation of the proton electrochemical gradient. This shift is sensitive to tr ansmembrane Delta pH but insensitive to membrane potential, indicting that the protonation equilibrium of membrane bound ellipticine detects the H+ movement in mitochondria.