Determination of Cysteine and its Accumulation in Spinach Leaf Tissue upon Exposure to Excess Sulfur

Abstract

Summary The cysteine level in plant leaf tissue was determined by measuring the difference in the sulfhydryl content of a deproteinized 30,000 g supernatant extract before and after reaction of the cysteine sulfhydryl with methylglyoxal. In leaves of red clover, sugar beet and spinach, cysteine accounted for 5, 23 and 19 % of the water-soluble non-protein sulfhydryl compounds, respectively. A 24 h exposure of spinach leaf tissue to excess sulfur in the form of atmospheric 0.25 μll −1 HZS or 0.33 μll −1 SO 2 or to 50 mM Na 2 SO 4 (liquid phase) resulted in an increase of the non-protein sulfhydryl levels in light and in darkness. The cysteine level increased relatively more than the total sulfhydryl content and it could account for more than 30% of the total sulfhydryl. Although glutathione was the major water-soluble non-protein sulfhydryl compound in spinach leaf tissue, the high levels of cysteine after exposure to excess sulfur indicate a rather poor metabolic control of intracellular levels of this compound.