Unreliability of carotenoid electrochromism for the measure of electrical potential differences induced by ATP hydrolysis in bacterial chromatophores

Abstract

ATP hydrolysis induces the activation of the proton ATPase in chromatophores of Rhodobacter capsulatus supplemented with nigericine and 50 mM K + (i.e. when ΔpH < 0.2 units). The value of transmembrane electric potential (Δϕ) driving this activation was measured using three different approaches: carotenoid electrochromism, uptake of SCN − and responses of the dye oxonol VI. The value of Δϕ calculated from the SCN − uptake, on the basis of an internal volume determined experimentally, was about 140 mV, while that indicated by the electrochromic signal ranged between 35 and 70 mV. Only the value indicated by SCN - distribution is consistent with the energetic requirement for the activation of H +-ATPase.