Mechanisms of H2S Production from Cysteine and Cystine by Microorganisms Isolated from Soil by Selective Enrichment

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Hydrogen sulfide (H2S) is a major component of biogenic gaseous sulfur emissions from terrestrial environments. However, little is known concerning the pathways for H2S production from the likely substrates, cysteine and cystine. A mixed microbial culture obtained from cystine-enriched soils was used in assays (50 min, 37°C) with 0.05 M Tris-HCl (pH 8.5), 25 μmol of l-cysteine, 25 μmol of l-cystine, and 0.04 μmol of pyridoxal 5′-phosphate. Sulfide was trapped in a center well containing zinc acetate, while pyruvate was measured by derivatization with 2,4-dinitrophenylhydrazine. Sulfide and total pyruvate production were 17.6 and 17.2 nmol mg of protein-1 min-1, respectively. Dithiothreitol did not alter reaction stoichiometry or the amount of H2S and total pyruvate, whereas N-ethylmaleimide reduced both H2S and total pyruvate production equally. The amount of H2S produced was reduced by 96% when only l-cystine was included as the substrate in the assay and by 15% with the addition of propargylglycine, a specific suicide inhibitor of cystathionine γ-lyase. These data indicate that the substrate for the reaction was cysteine and the enzyme responsible for H2S and pyruvate production was cysteine desulfhydrase (EC 4.4.1.1). The enzyme had a Km of 1.32 mM and was inactivated by temperatures greater than 60°C. Because cysteine is present in soil and cysteine desulfhydrase is an inducible enzyme, the potential for H2S production by this mechanism exists in terrestrial environments. The relative importance of this mechanism compared with other processes involved in H2S production from soil is unknown.

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