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J Membr Biol. 2002 Feb 1;185(3):209-20. Epub 2002 Feb 13.

Regulation of the lemon-fruit V-ATPase by variable stoichiometry and organic acids.

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  • 1Biology Department, Sinsheimer Laboratories, University of California, Santa Cruz, CA 95064, USA. mathias_muller@maxygen.com

Abstract

The lemon-fruit V-ATPase can exist in two forms: nitrate-sensitive and nitrate-insensitive. Here we report the results of measurements of H+ /ATP stoichiometries using two kinetic methods: one based on steady-state DpH and one based on initial rates of H+-pumping. Our findings indicate that the nitrate-insensitive fruit V-ATPase has an H+ /ATP stoichiometry of ~1, while both the nitrate-sensitive fruit V-ATPase and the epicotyl V-ATPase have stoichiometries of 2, under zero-load conditions. As DpH increases, the stoichiometry of the nitrate-sensitive fruit V-ATPase decreases to 1. Under similar conditions, the stoichiometry of the epicotyl enzyme remains 2. Thus, the pH-dependent variable stoichiometry of the lemon-fruit V-ATPase may represent a key factor in juice sac vacuolar hyperacidification. On the other hand, the H+ /ATP stoichiometry of the epicotyl V-ATPase can decrease from 2 to 1 in the presence of a membrane potential. The low pH of the fruit vacuole is not due solely to the lower H+/ATP stoichiometry of its pump. We show that lumenal citrate and malate improve the coupling of both the epicotyl and fruit V-ATPases and enhance their ability to generate a pH gradient. Since citrate accumulation is restricted to fruit vacuoles, it may be another important determinant of vacuolar pH.

PMID:
11891579
[PubMed - indexed for MEDLINE]
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