Salt activation of sucrose-phosphate synthase from darkened leaves of maize and other C-4 plants

Maize leaf sucrose-phosphate synthase (SPS) has been shown to be inactivated by protein phosphorylation in vitro, which appears to be the mechanism of light modulation in situ [Huber and Huber (1991) Plant Cell Physiol. 32: 319-326]. The catalytic activity of the inactivated enzyme (dark form or in vitro inactivated form) was strongly stimulated by high ionic strength in the assay mixture and at 0.4 M KC1 reached activities similar to those obtained from illuminated leaves. Numerous salts were effective, but for most studies, 0.3 M KC1 was used. The salt-stimulation of enzyme activity was rapid and readily reversible and was antagonized by the presence of ethylene glycol in the assay. The presence of salt was also found to reduce the IC₅₀ (concentration required for 50% inhibition) for p-chloromercuribenzenesulfonic acid. We postulate that phosphorylation of maize SPS induces a conformational change in the protein (that affects both maximum catalytic activity and sensitivity to Pⁱ either through electrostatic or hydrophobic interactions which are affected by high ionic strength. Salt stimulation of the deactivated enzyme extracted from darkened leaves was observed for a variety of C-4 plants, but not for any of the C-3 species tested.