Physiological role and membrane lipid modulation of the membrane-bound (Mg2+, na+)-adenosine triphosphatase activity in Acholeplasma laidlawii

Abstract

The membrane-bound adenosine triphosphatase (ATPase) activity of Acholeplasma laidlawii B differs in many respects from the common (Mg2+, Ca2+)-ATPase activity of higher bacteria, most notably in that it is specifically activated by Mg2+ and strongly and specifically stimulated by Na+ (or Li+). Various inhibitors diminish the ATPase activity with a concentration dependence which suggests that a single enzyme species is responsible for all of the observed ATP hydrolytic activity (both basal and Na+ stimulated). The Km for ATP is influenced by temperature but not by membrane lipid fatty acid composition. Vmax is influenced by both of these factors, showing a break in Arrhenius plots which falls below the lipid phase transition midpoint but well above the lower boundary when a phase transition occurs within the temperature range studied. The apparent energy of activation for Vma, is strongly influenced by lipid fatty acid composition both above and below the break. When whole cells of A. laidlawii B are incubated in KCI or NaCl buffers, they rapidly swell and lyse if deprived of an energy source or treated with ATPase inhibitors at concentrations which significantly inhibit enzyme activity in isolated membranes, whereas in sucrose or MgSO4 buffers of equal osmolarity, the cells are stable under these conditions. These results suggest that the membrane ATPase of A. laidlawii B is intimately associated with the membrane lipids and that it functions as a monovalent cation pump which regulates intracellular osmolarity as the (Na+, K+)-ATPase does in eucaryotes. 1027

Proton motive force across the membrane of Mycoplasma gallisepticum and its possible role in cell volume regulation

“…Although most of the bacterial ATPases resemble the proton-translocating ATPases of mitochondria and chloroplasts (5), ATPase of A. laidlawii is quite different. Jinks et al (6) have found that this enzyme is stimulated fourfold by Na+ but unaffected by K+ or by ouabain. It cannot be removed from the membrane by low ionic buffers or EDTA and is less sensitive to DCCD than bacterial ATPases.…”

Sodium and proton transport in Mycoplasma gallisepticum

“…The absence of Na+ or K+ stimulation of the ATPase in vitro (13) argues against the existence of an ATP-driven Na+ (or K+ or both) pump in M. gallisepticum. Jinks et al (8), on the other hand, reported a fourfold stimulation of the membrane ATPase of A. laidlawii by Na+ and postulated that this enzyme functioned as an Na+ pump. Benyoucef et al (4) reported K+ stimulation of Na+ extrusion in M. mycoides var.…”

“…This is due to the inward diffusion of NaCl and water as a result of colloidosmotic and Donnan forces caused by intracellular nondiffusable macromolecules (11,14,22). The two bacteria without cell walls in which this phenomenon has been documented are Acholeplasma laidlawii (8) and Mycoplasma gallisepticum (12,17). Both species are known to be able to maintain their volume in the presence of glucose by an energy-dependent extrusion of NaCl and water from the cell.…”

Inorganic Cation Transport and Energy Transduction in Enterococcus hirae and Other Streptococci

“…An Na ϩ -translocating ATPase, recently discovered in the thermophilic Clostridium fervidus (237), is very likely to be of the vacuolar type as judged by the enzymatic and biochemical properties of the purified enzyme (97). Furthermore, the enzymatic features of an Na ϩ -stimulated-ATPase observed in Mycoplasma mycoides (a parasitic glycolytic organism) are virtually identical to those of the E. hirae enzyme (25), and an Na ϩ -stimulated ATPase from Acholeplasma laidlawii seems to be of the V type as judged by its subunit composition (104,159).…”