Effect of Inhibitors of Pyridoxal-5-Phosphate-Dependent Enzymes on Cysteine Synthase in Echinochloa crusgalli L

The effect of inhibitors of pyridoxal-5ʹ-phosphate-dependent enzymes (IPEs) on cysteine synthase (CS; EC 4.2.99.8), which synthesizes cysteine from O-acetylserine and sulfide, was examined. CS was extracted from the leaves of Echinochloa crusgalli L., fractionated with 30–70% ammonium sulfate, and then used for the enzyme assay with seven IPEs. When substrates of CS and 10 mM IPEs were added simultaneously, -allylglycine, -propargylglycine, BETA-chloro- -alanine, 3bromopropionate, amino-oxyacetate (AOA), and hydroxylamine inhibited CS by 64, 45, 62, 61, 89, and 33%, respectively. All the IPEs inhibited shoot elongation of E. crus-galli in a seedling growth bioassay; however, there was a very low correlation between the inhibition of CS and that of shoot elongation (R2 = 0.173). When CS was preincubated with 10 mM IPEs at 30°C for 1 h, -allylglycine, -propargylglycine, BETA-chloro- -alanine, 3-bromopropionate, AOA, and hydroxylamine inhibited CS by 47, 27, 49, 40, 96, and 98%, respectively. CS inhibition by AOA and hydroxylamine increased during the preincubation period, suggesting that AOA and hydroxyamine might be irreversible inhibitors of CS. The correlation coefficient between CS inhibition by preincubation and inhibition of shoot elongation was R2 = 0.630. These results suggest that CS inhibition in E. crus-galli might affect its growth.