Production of Thiocysteine (Sulfide) in Cysteine Amended Soils

Abstract

AbstractLittle information is currently available concerning the formation in soils of hydrogen sulfide, an important intermediate in the cycling of sulfur within the earth's environment. Studies were performed in which 3 g of air‐dry soil were treated with 10 mM cysteine HCl (pH 6.0) and incubated for 24 h at 37°C. Results indicated that a dithio sulfur compound, thiocysteine (2‐amino‐2‐carboxyethyl hydrodisulfide), was produced. Production of thiocysteine occurs as a result of cystathionine γ‐lyase (EC 4.4.1.1) activity and the actual substrate of the enzyme is cystine, the oxidized form of cysteine. The procedure to measure thiocysteine involved a second addition of cysteine, used as a reagent, after the 24‐h incubation. Cysteine causes a nonenzymatic breakdown of thiocysteine, producing hydrogen sulfide, which was then collected in zinc acetate and determined colorimetrically. The amount of thiocysteine (sulfide) measured in the four soils studied after the 24‐h incubation period ranged from 7.8 to 16 mg kg−1 soil. The biological nature of the reaction was demonstrated when the production of thiocysteine (sulfide) was inhibited when soils were treated with 0.1 mL of 0.5 M sodium azide or steam sterilized for 1 h at 121°C. Complete inhibition also occurred upon the addition of 0.2 mL of toluene or formaldehyde. Addition of 10 mM propargylglycine (16.7 µmol g soil−1), a specific inhibitor of cystathionine γ‐lyase, reduced thiocysteine (sulfide) production by 58 to 91%. The formation of hydrogen sulfide as a result of the combination of cystathionine γ‐lyase activity and the nonenzymatic degradation of thiocysteine to produce hydrogen sulfide may represent an important pathway in the cycling of sulfur through the soil system.