Thermoplasma acidophilum Cell Membrane: Cytochrome b and Sulfate-stimulated ATPase

Abstract

The extreme acidophile Thermoplasma acidophilum is about an order of magnitude less sensitive to certain H⁺ ionophores than is a bacillus that lives in similar conditions. Thermoplasma is also totally insensitive to DCCD, a specific inhibitor of the H⁺-trans-locating ATPase, and a potent poison to virtually all other organisms that have been examined. Thus, the unusual metabolic properties of Thermoplasma membranes are of particular interest.

The membranes were purified from Thermoplasma by means of sonication and differential centrifugation. They contained respiratory enzymes capable of reducing O₂, using either NADH, D-lactate, or succinate as an electron donor. According to the reduced/ oxidized difference spectrum of the purified membranes, the respiratory chain appeared to consist of only a b-type cytochrome plus a quinone.

The purified membranes also contained a phosphatase activity capable of hydrolyzing ATP, ADP, and inorganic PPi, but not organic monophosphates such as AMP and 1-glycerol phosphate. This activity was specifically stimulated by MgS0₄, and was not inhibited by either DCCD or ouabain. It is proposed that this enzyme is a sulfate-exporting trans-locase, and that its activity may be the source of the inside-positive electrical potential of these cells. There was no evidence of any other membrane-bound ATPase. Presumably, the only mechanism for expelling H⁺ is by the respiratory chain described above, which could account for the observation that T. acidophilum is obligately aerobic.