2-Carboxy-d-arabinitol 1-phosphate (CA1P) phosphatase de- grades CA1P, an inhibitor associated with the regulation of ribulose bisphosphate carboxylase/oxygenase in numerous plant species. CA1P phosphatase purified from Phaseolus vulgaris was partially inactivated by oxidizing conditions during dialysis in air-equilibrated buffer. Phosphatase activity could then be stimulated 1.3-fold by dithiothreitol and also by addition of reduced thioredoxin from Escherichia coli. These effects were enhanced synergistically by the positive effector, fructose 1,6-bisphosphate (FBP). Most notably, CA1P phosphatase activity was stimulated up to 35-fold by glutathione, and was sensitive to the ratio of reduced (GSH) to oxidized (GSSG) forms. At concentrations of glutathione approximating measured levels in chloroplasts of P. vulgaris (5 mM total S), CA1P phosphatase exhibited > 20-fold stimulation by a change in the redox status of glutathione from 60 to 100% GSH. This stimulation was augmented further by reduced E. coli thioredoxin. In contrast, FBP, which activates CA1P phosphatase under reducing conditions, was strongly inhibitory in the presence of GSSG. We propose that glutathione may have an appreciable role in the light/dark regulation of CA1P phosphatase in vivo. A model for the reversible activation of CA1P phosphatase by GSH was derived based upon the various responses of the enzyme's activity to a range of thiol reagents including N-ethylmaleimide, 5,5´-dithiobis-(2-nitrobenzoic acid) and arsenite. These data indicate that the bean enzyme contains two physically distinct sets of thiol groups that are critical to its redox regulation.

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