Mechanism of Oxidation of Reduce Sulfur Compounds by Sulfur-Grown Acidithiobacillus Caldus Strain GO-1

The oxidation of reduced sulfur compounds was studied by using resting cells of sulfur-grown Acidithiobacillus caldus strain GO-1. The optimum pHs for the oxidation of thiosulfate, tetrathionate, sulfur, sulfite and sulfide were 2, 3, 3-6, 7 and 7, respectively. The highest oxidation rate was observed with sulfite. The oxidation rates of the reduced sulfur compounds were measured in the absence or presence of inhibitors and uncouplers. 2, 4-dinitrophenol (DNP) and carbonyl cyanide-m-chlorophenylhydrazone (CCCP) strongly inhibited the oxidations of sulfur and sulfite. N-Ethylmaleimide (NEM) strongly inhibited the oxidation of tetrathionate and sulfur. 2-heptyl-4-hydroxy-quinoline-N-oxide(HQNO) inhibited the oxidation of sulfur and sulfite. The results suggested that tetrathionate was oxidized in the periplasmic space, and sulfur and sulfite were oxidized in the cytoplasm. Pyridine ferrohemochromes prepared from the membrane of strain GO-1 cell revealed the involvement of cytochromes b and c. Ubiquinol oxidase activity was detected in strain GO-1 cell, but cytochrome c oxidase measured by using mammalian cytochrome c as an electron donor was not detected in the cell. On the basis of the results a model for the metabolism of the reduced sulfur compounds by At. caldus strain GO-1 was proposed.

硫黄で増殖したAcidithiobacilluscaldusGO-1株の休止菌体を用いて，還元型硫黄化合物の酸化機構を検討した．GO-１株は，チオ硫酸，テトラチオン酸，元素硫黄，亜硫酸及び硫化物の酸化活性を持っており，その最適pHは，それぞれ２，３，３-６，７及び７であった．これらの酸化活性の中で，亜硫酸の酸化速度が最も高かった．阻害剤やアンカップラーを用いて，還元型硫黄化合物の酸化への影響を調べた結果，DNPとCCCPによる元素硫黄及び亜硫酸酸化活性の強い阻害，NEMによるテトラチオン酸及び硫黄酸化活性の強い阻害，およびHQNOによる元素硫黄及び亜硫酸酸化活性の阻害が観察された．これらの結果は，テトラチオン酸がペリプラズマで，硫黄及び亜流酸が細胞質で，それぞれ酸化されることを示唆した．細胞膜からピリジンフェロヘモクロームを調製して，シトクロームの構成成分を検討した結果，ヘムb及びcが検出された．また，末端酸化酵素の活性を検討した結果，ユビキノール酸化酵素の活性は検出されたが，哺乳類のシトクロームを用いて測定したシトクロームc酸化酵素の活性は検出されなかった．これらの結果に基づいて，At.caldusGO-１株の還元型硫黄化合物の酸化機構を考察した．