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.