Open AccessOn the Lack of ATP-Induced Midpoint Potential Shift for Cytochrome b -566 in Plant Mitochondria
Author(s) -
Alan M. Lambowitz,
Walter D. Bonner,
Mårten Wikström
Publication year - 1974
Publication title -
proceedings of the national academy of sciences of the united states of america
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 5.011
H-Index - 771
eISSN - 1091-6490
pISSN - 0027-8424
DOI - 10.1073/pnas.71.4.1183
Subject(s) - electron transport chain , mitochondrion , redox , atp synthase , chemiosmosis , biophysics , chemistry , cytochrome , biochemistry , biology , enzyme , inorganic chemistry
Recently proposed mechanisms of site II energy transduction that assign a key role to cytochromeb -566 are based on the finding that the apparent midpoint potential ofb -566 in animal mitochondria increases by more than 250 mV upon addition of ATP [Chanceet al. (1970)Proc. Nat. Acad. Sci. USA 66, 1175-1182]. However, since it has never been shown that the redox mediators used in the midpoint potential measurements equilibrate directly withb -566, the observed midpoint potential shift could merely reflect reversed electron transport. In mung bean mitochondria, the apparent midpoint potential ofb -566 is known to be unaffected by addition of ATP [Dutton and Storey (1971)Plant Physiol. 47, 282-288]. In the present work, mung beanb -566 is shown to undergo an ATP-induced reduction similar to that observed forb -566 in animal mitochondria. However, in mung bean mitochondria the reduction is found to be rapidly relaxed by addition of redox mediator (phenazine methosulfate, PMS) and concomitantly PMS causes a marked, antimycinsensitive stimulation of ATPase activity. These results suggest that the ATP-induced reduction in mung bean mitochondria is due to reversed electron transport and that PMS can effectively short-circuit reversed electron transport in this system, bringing it close to equilibrium. Moreover, since mung bean and animalb -566 are identical in all other respects tested, the results support the idea that the apparent midpoint potential shift in animal mitochondria is also merely due to reversed electron transport, and that the mediators are now not effective enough to bring the system to equilibrium.