Inhibition of Sucrose:Sucrose Fructosyl Transferase by Cations and Ionic Strength

Fructans are storage carbohydrates found in many temperate grasses. The first enzyme in the biosynthetic pathway of most fructans is sucrose:sucrose fructosyl transferase (SST). In this report, we demonstrate that K+ and ionic strength noncompetitively inhibit the activity of SST from wheat (Triticum aestivum L.) stems. The Ki for this inhibition is high, 122 mM, but in the range of concentrations of K+ found in the tissue (205-314 mM). Addition of KCl to the assay system had no effect on the pH optimum (5.5) or the Km for sucrose (266 mM) but reduced the Vmax. At equivalent ionic strengths, inhibition by choline chloride was about half that of KCl, indicating that inhibition by ionic strength might be responsible for approximately 50% of the KCl inhibition. Inhibition by LiCl and (NH4)2SO4 was similar to that by choline chloride. Soluble invertase activity found in the SST preparations was less sensitive to KCl and more sensitive to choline chloride than was SST. SST from barley (Hordeum vulgare L.) stems and leaves, as well as SST from leaves of orchardgrass (Dactylis glomerata), was also inhibited by KCl. SST from onion (Allium cepa L.) bulbs and asparagus (Asparagus officinalis L.) stems was not inhibited by KCl; thus, inhibition of activity by KCl is not a universal characteristic of SST from all sources.