Effect of uncouplers on endogenous respiration and ferrous iron oxidation in a chemolithoautotrophic bacterium Acidithiobacillus ( Thiobacillus) ferrooxidans

Abstract

Oxidation of ferrous iron (Fe 2+) to ferric iron (Fe 3+) with oxygen (O 2) by Acidithiobacillus ( Thiobacillus) ferrooxidans is considered to be inhibited by uncouplers. Oxidation of the endogenous substrates (presumably NADH) with O 2 or Fe 3+, on the other hand, was stimulated by uncouplers, 2,4-dinitrophenol (DNP) and carbonylcyanide- m-chlorophenyl-hydrazone (CCCP), as expected in respiratorily controlled mitochondria or heterotrophic bacteria. Amytal and rotenone were inhibitory. Fe 3+ reduction by endogenous substrates was studied extensively and was found to be stimulated by a permeable anion, SCN − and weak acids, as well as the above uncouplers. Proton translocating properties of some of these stimulators were shown by following a pH change in the cell suspension. It was concluded that any compounds that destroy proton electrochemical gradient, Δ p, stimulated endogenous respiration. Stimulation of Fe 2+ or ascorbate oxidation by lower concentrations of uncouplers was successfully demonstrated by shortening the reaction time, but only to a small extent. Uncouplers at concentrations stimulatory to endogenous respiration inhibited Fe 2+ oxidation if present before Fe 2+ addition. The inhibition by 10 μM CCCP was reversed by washing the cells in a buffer. Complex I inhibitors, atabrine, rotenone and amytal inhibited Fe 2+ oxidation, more strongly in the presence of 0.1 mM DNP. It is proposed that Fe 2+ oxidation required Δ p perhaps to climb an energetically uphill reaction or to reduce NAD + to NADH by reversed electron flow for CO 2 fixation. The latter interpretation implies some obligatory coupling between Fe 2+ oxidation and NAD + reduction.