Variation among Species in Light Activation of Sucrose-Phosphate Synthase

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Abstract
Some species exhibit light activation of sucrose-phosphate synthase (SPS) in intact leaves. Twelve species which vary in extent of light modulation of SPS were studied to identify factors which regulate activation of the enzyme in situ. Leaves were harvested in light and darkness, and SPS was assayed under high substrate (Vmax) or limiting substrate conditions (in the presence of th inhibitor inorganic phosphate). The species tested fell into three groups. In some species (Group I; barley and maize) light activation involved an increase in Vmax of the enzyme. In other species (Group II; spinach, swiss chard, sugarbeet, broad bean) light activation had no effect on Vmax but altered certain kinetic properties such that activation was only apparent when SPS was assayed with the limiting substration condition. Significantly, in some species (Group HI; soybean, pea, tobacco, Arabidopsis thaliana cucumber and melon) SPS activity was essentially unaffected by light/dark transitions. Phosphate sequestering agents (mannose or gluco-samine) activated SPS in darkness in species which show light modulation (Groups I and II) but not species of Group III which do not light activate. Thus, changes in inorganic phosphate level may be one of the factors which regulate the activation of SPS in situ. In addition, the catalytic activity of SPS from species which exhibit light modulation (Groups I and II) is more strongly inhibited in vitro by inorganic phosphate (an allosteric effector) compared with the enzyme extracted from Group III species, which indicates that certain properties of the enzyme itself may also vary among species. We conclude that species which do not exhibit light activation of SPS in intact leaves lack the mechanism responsible for the covalent modification of enzyme activity (Walker and Huber 1989, Planta, in press) involved in light activation process.