The Electrochemical Potential across Mycoplasmal Membranes

The electrochemical proton gradient across mycoplasmal membranes was studied. The transmembrane proton-motive potential, Δp, is composed of two parameters, a transmembrane electric potential difference, ΔΨ, and a transmembrane proton gradient, ΔpH, according to the formula Δp = ΔΨ - (A × ΔpH). Membrane potentials were determined with use of potential-sensitive cyanine dyes. The ΔΨ for both Mycoplasma mycoides subspecies capri and Mycoplasma gallisepticum was -48 mV ± 10%, with the inside negative; the ΔΨ of Acholeplasma laidlawii was -28mV ± 20%. The ΔpH was determined by measuring the distribution of [¹⁴C]5,5-dimethyl oxazolidine-2,4-dione between the intracellular space and the medium. The intracellular pH of glycolyzing mycoplasmas was generally more alkaline than the extracellular medium: at an external pH of 7.0, the internal pH was 7.4 and hence ΔpH = 0.4, a value corresponding to -24 mV. Thus, the Δp of both M. mycoides subspecies capri and M. gallisepticum was calculated to be -72 mV and that of A. laidlawii, to be -52 mV. The data further indicate that the Δp is generated by a membrane-bound electrogenic, proton-translocating adenosine triphosphatase that operates in the direction of hydrolysis of adenosine triphosphate, which formed by glycolysis, and leads to proton extrusion.