Atmospheric H2S as sulfur source for Brassica oleracea:: kinetics of H2S uptake and activity of O-acetylserine (thiol)lyase as affected by sulfur nutrition

The uptake of hydrogen sulfide (H2S) by shoots of curly kale (Brassica oleracea) showed saturation kinetics with respect to the atmospheric concentration. The kinetics are largely determined by the rate of metabolism of the absorbed H,S into cysteine, catalyzed by O-acetylserine (thiol)lyase, and can be described by the Michaelis-Menten equation. When B. oleracea was grown under sulfate (SO42-)-deprived conditions, plants developed sulfur (S) deficiency symptoms and H,S uptake kinetics were substantially altered. Shoots of SO42- -deprived plants had a lower affinity to H2S uptake, whereas the maximal H2S uptake rate was higher. When SO42--deprived plants were simultaneously exposed to 0.2 mul l(-1) H2S all S deficiency symptoms disappeared and H,S uptake kinetics returned rapidly to values observed for S-sufficient shoots. The activity of the H,S-fixating enzyme O-acetylserine (thiol)lyase was hardly affected upon either prolonged H2S exposure or SO42- deprivation. Evidently, the activity of O-acetylserine (thiol)lyase was not the rate-limiting step in the H2S uptake by shoots. The significance of the in situ availability and rate of synthesis of the substrate O-acetylserine for O-acetylserine (thiol)lyase as determining factor in the uptake kinetics of H2S needs further evaluation. (C) 2001 Elsevier Science B.V. All rights reserved.