Stimulation of H₂S Emission from Pumpkin Leaves by Inhibition of Glutathione Synthesis

Abstract

The effect of inhibitors of glutathione (GSH) synthesis, namely γ-methyl glutamic acid, d-glutamic acid, cystamine, methionine-S-sulfoximine (MSX), buthionine-S-sulfoximine, and GSH itself, on the emission of H₂S was investigated. All these compounds stimulated H₂S emission from pumpkin (Cucurbita pepo L. cv Small Sugar Pumpkin) leaf discs in response to sulfate. MSX and GSH were the most effective compounds, stimulating H₂S emission from leaf discs of mature pumpkin leaves by about 80% in response to sulfate. Both inhibitors did not appreciably enhance H₂S emission in response to l-cysteine and inhibited H₂S emission in response to sulfite.

Treatment with MSX or GSH enhanced the uptake of sulfate by pumpkin leaf discs, but did not affect the incorporation of sulfate into reduced sulfur compounds. Inhibition of GSH synthesis by MSX or GSH caused an increase in the pool size of cysteine, and, simultaneously, reduced the incorporation of labeled sulfate into cysteine. The incorporation of labeled sulfate into the sulfite and sulfide pools of the cells are stimulated under these conditions.

These observations are consistent with the idea that inhibition of GSH synthesis leads to an elevated cysteine pool that inhibits further cysteine synthesis. The H₂S emitted under these conditions appears to arise from diversion of a precursor of the sulfur moiety of l-cysteine. Therefore, stimulation of H₂S emission in response to sulfate upon inhibition of GSH synthesis may reflect a role of H₂S emission in keeping the cysteine concentration below a critical level.