Modification of the Activity of the Plasma Membrane Redox System of Zea mays L. Roots by Vitamin K3 and Dicumarol

The correlation of the effects of vitamin K₃ and dicumarol (ananti-vitamin K in pharmaceutical applications) on the transplasmamembrane electrical potential difference of maize roots withthe reduction of the artificial electron acceptors hexacyanoferrate(III) or hexabromoiridate (IV) and the concomitant enhancementof acidification of the incubation medium was investigated. Vitamin K₃ depolarized the plasma membrane of Zea mays L. roots,while dicumarol had no significant effect on the membrane potential.Plants treated with vitamin K₃ for 30 min followed by intenserinsing showed higher reduction of hexabromoiridate (IV) thanhexacyanoferrate (III), as well as a stimulated acidificationof the incubation medium. Depolarization of the plasma membraneby hexacyanoferrate (III) or hexabromoiridate (IV) decreasedafter an incubation with vitamin K₃. Pretreatment with dicumarolcaused an inhibition of hexacyanoferrate (III) reduction andmedium acidification as well as depolarization by K₃. The reductionof hexabromoiridate (IV) was not affected by dicumarol pretreatment.The proton secretion associated with the reduction was slightlylowered. According to our results, it seems possible that vitaminK₃ acts as an electron acceptor for the plasmalemma electrontransport system of maize roots whereas dicumarol appears toinhibit electron and proton transport. Key words: Vitamin K3, dicumarol, plasmalemma redox system, Zea mays L., membrane potential     

The Effects of Vitamin K3 and Dicumarol on the Plasma Membrane Redox System and H+ Pumping Activity of Zea mays L Roots Measured over a Long Time Scale

The effects of vitamin K₃ or dicumarol on plasma membrane boundhexacyanoferrate (III) and hexabromoiridate (IV) reductase activityand on the H⁺ pumping rate were investigated. Incubation withvitamin K₃ followed by intense rinsing stimulated the subsequentreduction of hexabromoiridate (IV) and hexacyanoferrate (III)as well as proton secretion induced by external electron-acceptors,while pretreatment with dicumarol inhibited proton secretioninduced by redox activity and hexacyanoferrate (III) reductionrate, but not the effects of hexabromoiridate (IV). A 30 minincubation in 0·2 mM K₃ or dicumarol, followed by rinsing,inhibited H⁺ secretion for about 2 d. Incubation for more than12 h in 0·1 mM dicumarol or 0·2 mM K₃ caused lethalinjury to the root cells. Key words: Vitamin K.₃, dicumarol, plasmalemma redox system, Zea mays L., proton pump     

Membrane Depolarization by Hexacyanoferrate (III), Hexabromoiridate (IV) and Hexachloroiridate (IV)

Three artificial electron acceptors of different E_o and charge,hexacyanoferrate (III) (K₃Fe(CN)₆), hexachloroiridate (IV) (K₂IrCl₆),and hexabromoiridate (IV) (K₂IrBr₆), were compared with respectto their rate of reduction by roots of Zea mays L., the concomitantproton secretion, and to the effect on plasmalemma depolarization. It has been shown that these plasma membrane impermeable electronacceptors were reduced by a plasmalemma reductase activity.At low concentrations proton secretion was slightly inhibited,at higher concentrations, however, the rate of proton secretionwas stimulated. The root cell plasmalemma showed a transientdepolarization after addition of all three electron acceptors.The depolarization was concentration-dependent for the iridatecomplexes but not for hexacyanoferrate (III). For both iridatecomplexes maximum depolarization was reached at 50 µmoldm^–3. A hypothetical model as an explanation of the redox dependentproton secretion will be given. Key words: Hexachloroiridate (IV), hexabromoiridate (IV), hexacyanoferrate (III), plasmalemma redox, membrane potential, Zea mays