Mechanisms of H 2 S Production from Cysteine and Cystine by Microorganisms Isolated from Soil by Selective Enrichment

Abstract

Hydrogen sulfide (H 2 S) is a major component of biogenic gaseous sulfur emissions from terrestrial environments. However, little is known concerning the pathways for H 2 S 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 H 2 S and total pyruvate, whereas N -ethylmaleimide reduced both H 2 S and total pyruvate production equally. The amount of H 2 S 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 H 2 S and pyruvate production was cysteine desulfhydrase (EC 4.4.1.1). The enzyme had a K m 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 H 2 S production by this mechanism exists in terrestrial environments. The relative importance of this mechanism compared with other processes involved in H 2 S production from soil is unknown.