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Appl Environ Microbiol. 1991 May; 57(5): 1413-1417
Copyright © 1991, American Society for Microbiology. All Rights Reserved.

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

Matthew J. Morra and Warren A. Dick^*

Department of Agronomy, Ohio Agricultural Research and Development Center, The Ohio State University, Wooster, Ohio 44691-6900

ABSTRACT

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 -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 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 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.

^* Corresponding author.

Present address: Division of Soil Science, University of Idaho, Moscow, ID 83843.