REDUCED SULFUR COMPOUND OXIDATION BY THIOBACILLUS-CALDUS

The oxidation of reduced inorganic sulfur compounds was studied by usi ng resting cells of the moderate thermophile Thiobacillus caldus strai n KU. The oxygen consumption rate and total oxygen consumed were deter mined for the reduced sulfur compounds thiosulfate, tetrathionate, sul fur, sulfide, and sulfite in the absence and in the presence of inhibi tors and uncouplers. The uncouplers 2,4-dinitrophenol and carbonyl cya nide m-chlorophenyl-hydrazone had no affect on the oxidation of thiosu lfate, suggesting that thiosulfate is metabolized periplasmically. In contrast, the uncouplers completely inhibited the oxidation of tetrath ionate, sulfide, sulfur, and sulfite, indicating that these compounds are metabolized in the cytoplasm of T. caldus KU. N-Ethylmaleimide inh ibited the oxidation of tetrathionate and thiosulfate at the stage of elemental sulfur, while 2-heptyl-4-hydroxyquinoline-N-oxide stopped th e oxidation of thiosulfate, tetrathionate, and elemental sulfur at the stage of sulfite. The following intermediates in the oxidation of the sulfur compounds were found by using uncouplers and inhibitors: thios ulfate was oxidized to tetrathionate, elemental sulfur was formed duri ng the oxidation of tetrathionate and sulfide, and sulfite was found a s an intermediate of tetrathionate and sulfur metabolism. On the basis of these data we propose a model for the metabolism of the reduced in organic sulfur compounds by T. caldus KU.