Enzymes of Sucrose Breakdown in Soybean Nodules

The specific activities of acid and alkaline invertases (beta-d-fructofuranoside fructohydrolase, EC 3.2.1.26), sucrose synthase (UDPglucose: d-fructose 2-alpha-d-glucosyltransferase, EC 2.4.1.13), hexokinase (ATP: d-hexose 6-phosphotransferase, EC 2.7.1.1), and fructokinase (ATP: d-fructose 6-phosphotransferase, EC 2.7.1.4) were determined in soybean (Glycine max L. Merr cv Williams) nodules at different stages of development and, for comparison, in roots of nonnodulated soybeans. Alkaline invertase and sucrose synthase were both involved in sucrose metabolism in the nodules, but there was only a small amount of acid invertase present. The nodules contained more phosphorylating activity with fructose than glucose. Essentially all of the alkaline invertase, sucrose synthase, and fructokinase were in the soluble fraction of nodule extracts whereas hexokinase was in the bacteroid, plant particulate, and soluble fractions.Soybean nodule alkaline invertase was partially purified and shown to be a beta-d-fructofuranosidase which was specific for sucrose. The pH optimum was 7.6 and the K(m) for sucrose was 10 millimolar. Fructose was a competitive inhibitor. Tris was a noncompetitive inhibitor and the enzyme was very sensitive to inhibition by heavy metals.