Abstract
Mg:ATP-dependent H+ pumping has been studied in microsomal vesicles from 24-hour-old radish (Raphanus sativus L.) seedlings by monitoring both intravesicular acidification and the building up of an inside positive membrane potential difference (Δ ψ). ΔpH was measured as the decrease of absorbance of Acridine orange and Δ ψ as the shift of absorbance of bis(3-propyl-5-oxoisoxazol-4-yl)pentamethine oxonol. Both Mg:ATP-dependent Δ pH and Δ ψ generation are completely inhibited by vanadate and insensitive to oligomycin; moreover, Δ pH generation is not inhibited by NO3−. These findings indicate that this membrane preparation is virtually devoid of mitochondrial and tonoplast H+-ATPases. Both intravesicular acidification and Δ ψ generation are influenced by anions: Δ pH increases and Δ ψ decreases following the sequence SO42−, Cl−, Br−, NO3−. ATP-dependent H+ pumping strictly requires Mg2+. It is very specific for ATP (apparent Km 0.76 millimolar) compared to GTP, UTP, CTP, ITP. Δ pH generation is inhibited by CuSO4 and diethylstilbestrol as well as vanadate. Δ pH generation is specificially stimulated by K+ (+ 80%) and to a lesser extent by Na+ and choline (+28% and +14%, respectively). The characteristics of H+ pumping in these microsomal vesicles closely resemble those described for the plasma membrane ATPase partially purified from several plant materials.
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Selected References
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