The Electrochemical Proton Gradient in Mycoplasma Cells

Publisher Website
Google Scholar
Abstract
The electrochemical proton gradient, .DELTA.~.mu.H+, generated on glycolysis by M. mycoides var. capri cells was determined. The transmembrane pH gradient (.DELTA.pH) and the membrane potential (.DELTA..psi.), components of the electrochemical proton gradient, were measured using several methods. The determination of the .DELTA.pH was conducted by measuring the transmembrane distribution of weak acids (acetate and butyrate) and of a weak base (methylamine), using flow dialysis and filtration techniques. The transmembrane electrical potential was determined from the distribution of the lipophilic cation Ph3MeP+ [triphenylmethylphosphonium ion] and of Rb+ or K+ in the presence of valinomycin. At extracellular pH 7.2, glycolyzing Mycoplasma cells maintain an internal pH more alkaline (0.5 pH unit) than that of the milieu and an electrical potential of -85 mV, interior negative. The .DELTA.~.mu.H+ in M. mycoides var. capri cells is thus about -115mV. When the external pH was altered from 7.7 to 5.7, .DELTA..psi. decreased from -90 to -60 mV. The internal pH decreased, but .DELTA.pH increased from 0.2 to 1.0 pH unit. Since the changes in .DELTA..psi. were largely compensated by the changes in .DELTA.pH, .DELTA.~.mu.H+ remained practically constant at about -115 mV throughout the pH range tested. Inhibition of .DELTA.pH by N,N''-dicyclohexylcarbodiimide, carbonylcyanide-p-trifluoromethoxyphenylhydrazone or nigericin confirmed that chemiosmotic phenomena contribute to energy transduction across the membranes of M. mycoides var. capri cells.