The Electrochemical Proton Gradient in Mycoplasma Cells

Abstract

The electrochemical proton gradient, generated upon glycolysis by Mycoplasma mycoides var. Capri cells has been determined. The components, the transmembrane pH gradient, ΔpH, and the membrane potential, ΔΨ, were measured using several methods. The determination of the ΔpH 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+ 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 in M. mycoides var. Capri cells is thus about–115 mV. When the external pH was altered from 7.7 to 5.7, ΔΨ decreased from–90 mV to–60 mV. On other hand although the internal pH decreased, ΔpH was found to increase from 0.2 to 1.0 pH unit. Since the changes in ΔΨ were largely compensated by the changes in ΔpH, remained practically constant at about–115 mV throughout the pH range tested. Finally, inhibition of ΔpH 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. Copyright © 1981, Wiley Blackwell. All rights reserved