Submitted to: Functional Plant Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 30, 2001
Publication Date: January 20, 2002
Citation: Zhou, R., Sicher Jr, R.C., Quebedeaux, B. 2002. Apple leaf sucrose-phosphate synthase is inhibited by sorbitol-6-phosphate. Functional Plant Biology. 29:569-514.
Interpretive Summary: The normal end-products of photosynthesis in leaves are starch and sucrose. In most plant species sucrose is transported from the sites of photosynthesis to growth and storage areas on the plant. However, important commercial tree species in the Rose family, i.e., apple, peach, pear, etc., differ from normal plants in that a reduced carbohydrate, sorbitol, is the principal photosynthetic end-product. The metabolism of sorbitol in these horticultural species is dependent upon a small number of unique enzymes. Because members of the Rose family also synthesize low levels of starch and sucrose, it was hypothesized that there must be some unique way of coordinating the regulation of these important carbohydrate synthesizing pathways in plants. In the current study, sucrose-phosphate synthase (SPS) was isolated and characterized from mature apple leaves. This enzyme performs an important role in sucrose synthesis. The results indicated that sucrose synthesis by decreasing the activity of SPS. These results help to explain the hierarchy of sugar biosynthesis in apple and other orchard crops and may lead to a better understanding of how to manipulate crop yields in these commercially important species. They also advance our understanding of sorbitol, mannitol and other polyols which perform a vital role as osmotic protectants during drought or frost. These results should benefit plant physiologists, biochemists, horticulturists and possibly crop modelers.
Sucrose-phosphate synthase (SPS) from mature apples (Malus domestica Borhk. Cv. Gala) leaves were purified 34-fold to a final specific activity of 15.3 (mol per (mg protein) per hour. 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 an activator of the 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 species, inorganic phosphate (Pi) had little or no inhibitory effect on the apple SPS. The apple leaf enzyme was inhibited seven to 10 percent by 10 mM of Pi when F6P concentrations were in the range of two to 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. Thus, sucrose biosynthesis may be regulated by the end-products of sorbitol biosynthesis in apple leaves.