Subcellular Location of O-Acetylserine Sulfhydrylase Isoenzymes in Cell Cultures and Plant Tissues of Datura innoxia Mill.

O-Acetylserine sulfhydrylase (OASS; EC 4.2.99.8) catalyzes the formation of L-cysteine from O-acetylserine and inorganic sulfide. Three OASS isoenzymes that differ in molecular mass and subunit structure are present in shoot and root tissues and in cadmium-resistant and cadmium-susceptible cell cultures of Datura innoxia Mill. Different OASS forms predominate in leaves, roots, and suspension-cell cultures. To determine the subcellular location of the OASS isoenzymes, purified mitochondria, chloroplasts, and cytosolic fractions from protoplasts were obtained. The isoenzymes are compartmentalized in D. innoxia cells, with a different isoenzyme predominant in the chloroplast, cytosol, and mitochondria, suggesting that they serve different functions in the plant cell. The chloroplast form is most abundant in green leaves and leaf protoplasts. The cytosolic form is most abundant in roots and cell cultures. A mitochondrial form is abundant in cell cultures, but is a minor form in leaves or roots. Cadmium-tolerant cell cultures contain 1.8 times as much constitutive OASS activity as the wild-type cell line, and 2.9 times more than the cadmium-hypersensitive cell line. This may facilitate rapid production of glutathione and metal-binding phytochelatins when these cultures are exposed to cadmium.