title: Apparent equilibrium constant and mass-action ratio for sucrose-phosphate synthase in seeds of Pisum sativum.
abstract: The aim of this work was to use preparations from germinating seeds of Pisum sativum to determine the apparent equilibrium constant of the reaction catalysed by sucrose-phosphate synthase (EC 2.4.1.14) and to compare this with the mass-action ratio of the reaction in the seeds. The apparent equilibrium constant ranged from 5.3 at 0.25 mM-MgCl2, pH 7.0, to 62 at 10 mM-MgCl2, pH 7.5. The sucrose phosphate content of the seeds, 23 nmol/g fresh wt., was determined by separating sucrose phosphate from sucrose by ion-exchange chromatography and then measuring the sucrose released by alkaline phosphatase. Comparison of equilibrium constants and mass-action ratios in the cotyledons of 38 h-germinated seeds showed that the reactions catalysed by glucose-6-phosphate isomerase, phosphoglucomutase and UDP-glucose pyrophosphorylase are close to equilibrium, and those catalysed by sucrose-phosphate synthase and sucrose phosphatase are considerably displaced from equilibrium in vivo.
authors: Botany School, University of Cambridge, U.K.
authors: J E
authors: JE
authors: Lunn
authors: T
authors: T
authors: ap Rees
mesh_terms: D005964:Glucosyltransferases / Q000032:analysis*; D008274:Magnesium / Q000494:pharmacology; D012639:Seeds / Q000201:enzymology*
chemical_list: D005964:Glucosyltransferases; C022698:sucrose-phosphate synthase; D008274:Magnesium