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  • Title: Tracking of proton flow during transition from anaerobiosis to steady state in rat liver mitochondria.
    Author: Luvisetto S, Cola C, Conover TE, Azzone GF.
    Journal: Biochim Biophys Acta; 1990 Jul 17; 1018(1):77-90. PubMed ID: 2165420.
    Abstract:
    (1) The hydrophobic pH indicator Bromthymol blue and the hydrophilic pH indicator Phenol red have been used to follow the redox-pump-linked proton flows during transition from anaerobiosis to static head. The domains monitored by the pH indicators, whether external or internal, and the localization of the dye, whether free or membrane bound, have been identified by recording the absorbance changes following addition of nigericin or valinomycin to anaerobic or aerobic mitochondria and the effects of permeant and impermeant buffers. (2) After addition of the H+/K+ exchanger, nigericin, to anaerobic mitochondria. Phenol red and Bromthymol blue record an alkalinization and an acidification, respectively, indicating that while the hydrophilic pH indicator faces an external domain, the hydrophobic pH indicator faces, at least partly, an internal domain. The latter effect is sensitive to phosphate and to phosphate carrier inhibitors. On the other hand, addition of nigericin to aerobic mitochondria leads to an increased Bromthymol blue absorbance, which reflects an alkalinization, indicating that the pH indicator faces an external domain. The reorientation of the dye from the internal to the external domain is a function of the uncoupler concentration and thus of the membrane potential (cf. Mitchell et al. (1968) Eur. J. Biochem. 4, 9-19). (3) The amount of oxygen required for the transition from anaerobiosis to static head has been determined by following in parallel the extent of oxidation of cytochrome aa3 and the rise of delta mu H+. With succinate as substrate, 50% levels of cytochrome oxidation are obtained at 0.125 ngatom oxygen/mg and 50% of Safranine response at about 0.2 ngatom oxygen/mg. These amounts of oxygen correspond to an H+ displacement of about 0.8-1.2 ngatom/mg on the basis of the H+/O stoichiometry. It is concluded that mitochondria are in presteady state below, and in static head above, displacement of 2-3 ngatom H+/mg. This figure is very close to the original calculation of Mitchell (Mitchell, P. (1966) Biol. Rev. 41, 445-502). (4) Transition, by oxygen pulses, of EGTA-supplemented mitochondria from anaerobiosis to either presteady state or static head state results in a response of the hydrophilic pH indicator, Phenol red, which is negligible in amount and/or kinetically unrelated to the delta mu H+ rise. The fact that H+ extrusion in the bulk aqueous phase is negligible also in presteady state excludes proton cycling as an explanation. Addition of oxygen pulses to Sr2(+)-supplemented anaerobic mitochondria results in an H+ extrusion whose amount and rate is proportional to the Sr2+ concentration.(ABSTRACT TRUNCATED AT 400 WORDS)
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