Protein-Catalyzed Isotopic Exchange Reaction between Cysteine and Sulfide in Spinach Leaves

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Abstract
A protein has been isolated from spinach leaves which catalyzes the following isotopic exchange reaction: Cys-SH+H2 35S ↔ Cys-35SH + H2S . This enzyme has an pH-optimum above 9.0; its molecular weight has been estimated on a Sephadex-G-100 column to be around 64 000 daltons. During purification this exchange reaction activity follows cysteine synthase activity on DEAE-cellulose and Sephadex-G-100 column chromato­ graphy; however this enzyme fraction has not been purified to homogeneity to prove that both activities are catalyzed by the same protein. The apparent Km for a) H2S has been determined to be 0.86 mM using cysteine as substrate and 0.6 mM using O-acetylserine as substrate; b) for cysteine was found to be 3.3 mM; and c) for O-acetylserine to be 3.3 mM. For catalysis no metal ion is required and the reaction proceeds without addition of pyridoxalphosphat. This exchange reaction might prove to be a simple method to prepare labelled cysteine from non-labelled cysteine and labelled H2S. The exchange reaction was found too in Chlorella pyrenoidosa and in Rhodospirillum rubrum.

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