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 formati on of L-cysteine from O-acetylserine and inorganic sulfide. Three OASS isoenzymes that differ in molecular mass and subunit structure are pr esent in shoot and root tissues and in cadmium-resistant and cadmium-s usceptible cell cultures of Datura innoxia Mill. Different OASS forms predominate in leaves, roots, and suspension-cell cultures. To determi ne the subcellular location of the OASS isoenzymes, purified mitochond ria, chloroplasts, and cytosolic fractions from protoplasts were obtai ned. The isoenzymes are compartmentalized in D. innoxia cells, with a different isoenzyme predominant in the chloroplast, cytosol, and mitoc hondria, suggesting that they serve different functions in the plant c ell. The chloroplast form is most abundant in green leaves and leaf pr otoplasts. The cytosolic form is most abundant in roots and cell cultu res. A mitochondrial form is abundant in cell cultures, but is a minor form in leaves or roots. Cadmium-tolerant cell cultures contain 1.8 t imes as much constitutive OASS activity as the wild-type cell line, an d 2.9 times more than the cadmium-hypersensitive cell line. This may f acilitate rapid production of glutathione and metal-binding phytochela tins when these cultures are exposed to cadmium.