The Regulative Properties of UDPglucose: d ‐Fructose‐6‐phosphate 2‐Glucosyltransferase (Sucrose Phosphate Synthetase) from Vicia faba Cotyledons

Sucrose phosphate synthetase of cotyledons of Vicia faba was purified about 130‐fold by means of ammonium sulfate and protamine fractionation. After freezing and thawing an activator could be separated by centrifugation from the purified enzyme. The regulatory properties of the activator‐bound and activator‐devoid enzyme were compared. Under the normal conditions of assay the activator‐devoid enzyme had scarcely any activity, but could be activated by citrate, a number of dicarboxylic acids and protamin. The saturation curves of these effectors were sigmoidal. Also the substrate saturation curves were sigmoid shaped in absence of activator but the presence of activator converted them to a hyperbolic form. Quite different was the effect of citrate on the activity of the activator‐bound enzyme: at low concentration it inhibited the production of sucrose‐6‐phosphate. At 10–20 mM citrate a minimum of activity could be measured. At higher citrate concentrations a reactivation occurred. This activator‐bound preparation could be further inhibited, but not reactivated, by UDP, UTP, ATP, ADP and other phosphates which had scarcely any influence on the activity of the activator‐devoid enzyme. Experiences where the influence of citrate concentration was investigated on the UTP‐inhibited enzyme suggest that both effectors share binding sites. The role of citrate in the control of the activities of sucrose phosphate synthetase (activation) and phosphofructokinase (inhibition), both enzymes competing for the branchpoint metabolite fructose‐6‐phosphate, is discussed.