Apple leaf sucrose-phosphate synthase is inhibited by sorbitol-6-phosphate

Rui Zhou; Richard C Sicher; Bruno Quebedeaux

doi:10.1071/PP01123

Apple leaf sucrose-phosphate synthase is inhibited by sorbitol-6-phosphate

Funct Plant Biol. 2002 May;29(5):569-574. doi: 10.1071/PP01123.

Authors

Rui Zhou¹, Richard C Sicher², Bruno Quebedeaux³

Affiliations

¹ Department of Natural Resource Sciences and Landscape Architecture, University of Maryland, College Park, MD 20742, USA.
² US Department of Agriculture, Agricultural Research Service, Alternate Crops and Systems Laboratory, Beltsville, MD 20705, USA.
³ Department of Natural Resource Sciences and Landscape Architecture, University of Maryland, College Park, MD 20742, USA.Corresponding author; email: [email protected].

PMID: 32689502
DOI: 10.1071/PP01123

Abstract

Sucrose-phosphate synthase (SPS) from mature apple (Malus domestica Borhk. cv. Gala) leaves was purified 34-fold to a final specific activity of 15.3 μmol mg^-1 protein h^-1. The enzyme showed hyperbolic saturation kinetics for both fructose-6-phosphate (F6P) (Km= 0.36 mM) and uridine-5'-diphosphoglucose (UDPG) (Km = 6.49 mM). Glucose-6-phosphate (G6P) was found to be an activator of apple SPS, and the activation was dependent upon the F6P concentration. At a concentration of 2 mM, G6P significantly decreased the Km for F6P and increased SPS activity. However, higher concentrations of G6P did not further stimulate SPS activity. In contrast to SPS from other plant species, inorganic phosphate (Pi) had little or no inhibitory effect on apple SPS. The apple leaf enzyme was inhibited 7-10% by 10 mM Pi when F6P concentrations were in the range of 2-10 mM. We observed that sorbitol-6-phosphate, an intermediate metabolite in sorbitol biosynthesis, was a competitive inhibitor of SPS with a Ki of 1.83 mM. Sorbitol-6-phosphate also inhibited G6P activation of SPS. Our results suggest that sucrose biosynthesis may be altered by the products of sorbitol biosynthesis in apple leaves.