The electrochemical potential across mycoplasmal membranes.

The electrochemical proton gradient across mycoplasmal membranes was studied. The transmembrane proton-motive potential, delta p, is composed of two parameters, a transmembrane electric potential difference, delta psi, and a transmembrane proton gradient, delta pH, according to the formula delta p = delta psi -(A x delta pH). Membrane potentials were determined with use of potential-sensitive cyanine dyes. The delta psi for both Mycoplasma mycoides subspecies capri and Mycoplasma gallisepticum was -48 mV +/- 10%, with the inside negative; the delta psi of Acholeplasma laidlawii was -28mV +/- 20%. The delta pH 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 delta pH = 0.4, a value corresponding to -24 mV. Thus, the delta p 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 delta p is generated by a membrane-bound electrogenic, proton-translocating adenosine triphosphatase that operates in the direction of hydrolysis of adenosine triphosphatase, which formed by glycolysis, and leads to proton extrusion.