Role of O-Acetylserine in Hydrogen Sulfide Emission from

In the presence of excess sulfate, cysteine synthesis in pumpkin (Cucurbita pepo) leaves is not limited by sulfate reduction, but by the availability of O-acetylserine. Feeding of O-acetylserine or its metabolic precursors S-acetyl-coenzyme-A and coenzyme A to leaf discs enhanced the incorportion of [35Sjsulfate into reduced sulfur compounds, mainly into cysteine, at the cost of lowered H2S emission; the uptake and reduction of sulfate is not affected by these treatments. /8-Fluoropyruvate, an inhibitor of the generation of S-acetyl-coenzyme A via pyruvate dehydrogenase, stimulated H2S emission in response to sulfate. This stimulation is overcompensated by addition of O-acetylserine, S-acetylcoenzyme A, or coenzyme A. These results indicate that, in the presence of high amounts of sulfate, excess sulfur is reduced and emitted as H2S into the atmosphere. The H2S emitted seems to be produced by liberation from a precursor of cysteine rather than by cysteine desulfhydration.