Unusual regulatory properties of sucrose‐phosphate synthase purified from soybean (Glycine max) leaves

Sucrose‐phosphate (SPS) from source leaves of soybean (Glycine max (L.) Merr. cv. Ransom II) was purified 74‐fold to a final specific activity of 1.8 U (mg protein)¹. The partially purified preparation was free from phosphoglucoseisomerase (EC 5.3.1.9), pyrophosphatase (EC 3.6.1.1), phosphoenolpyruvate‐phosphatase (EC 3.1.3.‐), phosphofructokinase (EC 2.7.1.11), and uridine diphosphatase (EC 3.6.1.6), and was used for characterization of the kinetic and regulatory properties of the enzyme. The enzyme showed hyperbolic saturation kinetics for both fructose‐6‐phosphate (K_m=0.57 mM) and UDPGlucose (UDPG) (K_m=4.8 mM). The activity of SPS was inhibited by the product UDP. In vitro this inhibition could be partially overcome by the presence of Mg²⁺. Inorganic orthophosphate was only slightly inhibitory (35% inhibition at 25 mM phosphate). Glucose‐6‐phosphate (up to 20 mM) had no effect on activity, and did not show any significant interaction with phosphate inhibition. A range of potential effectors was tested and had no effect on SPS activity: Glucose‐1‐phosphate, fructose‐1, 6‐bisphosphate, α‐glycero‐phosphate, dihydroxyacetone‐phosphate, 3‐phosphoglyceric acid, (all at 5 mM), sucrose at 100 mM and pyrophosphate at 0.1 mM. The apparent lack of allosteric regulation of soybean SPS makes this enzyme markedly different from SPS previously characterized from spinach and maize.