Atmospheric H(2)S as sulfur source for Brassica oleracea: kinetics of H(2)S uptake and activity of O-acetylserine (thiol)lyase as affected by sulfur nutrition.

The uptake of hydrogen sulfide (H(2)S) 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(2)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 (SO(4)(2-))-deprived conditions, plants developed sulfur (S) deficiency symptoms and H(2)S uptake kinetics were substantially altered. Shoots of SO(4)(2-)-deprived plants had a lower affinity to H(2)S uptake, whereas the maximal H(2)S uptake rate was higher. When SO(4)(2-)-deprived plants were simultaneously exposed to 0.2 &mgr;l l(-1) H(2)S all S deficiency symptoms disappeared and H(2)S uptake kinetics returned rapidly to values observed for S-sufficient shoots. The activity of the H(2)S-fixating enzyme O-acetylserine (thiol)lyase was hardly affected upon either prolonged H(2)S exposure or SO(4)(2-) deprivation. Evidently, the activity of O-acetylserine (thiol)lyase was not the rate-limiting step in the H(2)S 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 H(2)S needs further evaluation.