FRUCTAN BIOSYNTHESIS IN EXCISED LEAVES OF LOLIUM-TEMULENTUM L .6. OPTIMIZATION AND STABILITY OF ENZYMATIC FRUCTAN SYNTHESIS

A crude enzyme preparation from excised, illuminated leaves of Lolium temulentum L. catalyzed the de novo synthesis of fructan of apparent degree of polymerization 3-20. In the absence of sucrose, the fructan synthetic activity (FSA) was labile at temperatures above 10 degrees C, but could be stored at 5 degrees C for 6 h with the loss of only 20% of the initial activity. Sucrose stabilized the FSA such that high rates of fructan synthesis continued for at least 6 h at 30 degrees C. By comparison, in the absence of sucrose 80% of the FSA was lost in 2 h at 30 degrees C. The FSA was maximal at pH 6.0 in citrate-phosphate buffer and at 43 degrees C. It was not possible to saturate the FSA even at 1500 mol m(-3) sucrose, or to calculate meaningful kinetic parameters for enzymatic fructan synthesis. 1500 mol m(-3) sucrose was the highest substrate concentration practically feasible and imposed an arbitrary maximum concentration for use in enzyme assays. Pyridoxal-hydrochloride was found to reduce invertase activity by 40%, but also inhibited FSA by 20% at 1500 mol m(-3) sucrose and pH 6.0.