Metabolism of L-[sulfane-34S]thiocystine by Escherichia coli

Abstract

The metabolism of L-thiocystine [bis(2-amino-2-carboxyethyl) trisulfide] by Escherichia coli was studied by using L-[sulfane-35S]thiocystine. This compound was found to serve as a source of sulfur for E. coli grown on a defined medium free of other sulfur sources and to incorporate its labeled sulfur into cysteine as well as the other sulfur-containing cellular components. For determination of the extent of the synthesis of new cysteine in these cells, cells were grown with [3,3-2H2]serine and L-[sulfane-34S]thiocystine, and the extent of incorporation of both deuterium and 34S into the cellular cysteine was measured by gas chromatography-mass spectrometry. The results show that approximately 50% of the cysteine which is incorporated into cellular macromolecules is derived from the thiocystine without cleavage of the carbon-sulfur bond, the remaining portion being newly biosynthesized from serine and 34S-enriched H2S. These results suggest that the first step in the metabolism of thiocystine by E. coli involves the beta elimination of pyruvate. This type of reaction is characteristic of the cleavage reactions catalyzed by beta-cystathionase.