Rate of transmembrane electron transfer in chromaffin-vesicle ghosts.
The chromaffin vesicle of the adrenal medulla contains a transmembrane electron carrier that may provide reducing equivalents for dopamine beta-hydroxylase in vivo. This electron-transfer system can be assayed by trapping ascorbic acid inside resealed membrane vesicles (ghosts), adding an external electron acceptor such as ferricytochrome c or ferricyanide, and following the reduction of these acceptors spectrophotometrically. Cytochrome c reduction is more rapid at high pH and is proportional to the amount of chromaffin-vesicle ghosts, at least at low ghost concentrations. At pH 7.0, ghosts loaded with 100 mM ascorbic acid reduce 60 microM cytochrome c at a rate of 0.035 +/- 0.010 mu equiv min-1 (mg of protein)-1 and 200 microM ferricyanide at a rate of 2.3 +/- 0.3 mu equiv min-1 (mg of protein)-1. The rate of cytochrome c reduction is accelerated to 0.105 +/- 0.021 mu equiv min-1 (mg of protein)-1 when cytochrome c is pretreated with equimolar ferrocyanide. Pretreatment of cytochrome c with ferricyanide also causes a rapid rate of reduction, but only after an initial delay. The ferrocyanide-stimulated rate of cytochrome c reduction is further accelerated by the protonophore carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone (FCCP), probably because FCCP dissipates the membrane potential generated by electron transfer. These rates of electron transfer are sufficient to account for electron transfer to dopamine beta-hydroxylase in vivo and are consistent with the mediation of electron transfer by cytochrome b-561.
Harnadek
G J
GJ
Ries
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EA
Njus
D
D
D000313:Adrenal Medulla / Q000378:metabolism*; D000818:Animals; D002259:Carbonyl Cyanide p-Trifluoromethoxyphenylhydrazone / Q000494:pharmacology; D002417:Cattle; D002837:Chromaffin Granules / Q000378:metabolism*; D002838:Chromaffin System / Q000378:metabolism*; D003574:Cytochrome c Group / Q000378:metabolism; D004579:Electron Transport; D005292:Ferricyanides / Q000494:pharmacology; D005295:Ferrocyanides / Q000494:pharmacology; D007425:Intracellular Membranes / Q000378:metabolism*; D007700:Kinetics; D010084:Oxidation-Reduction
D003574:Cytochrome c Group; D005292:Ferricyanides; D005295:Ferrocyanides; C007931:hexacyanoferrate III; D002259:Carbonyl Cyanide p-Trifluoromethoxyphenylhydrazone