Fructan metabolism in leaves of Loliwn rigidum Gaudin

II. Fructosyltransferase, invertase and fructan hydrolase activity

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This paper reports the conditions for the assay of sucrose–sucrose fructosyltransferase (SST); fructan–fructan fructosyltransferase (FFT), invertase and Fructan hydrolase (FH) in extracts of leaves of Loliutn rigidum Gaudin. Invertase had a temperature optimum between 30 and 35 °C and a pH optimum between 4.5 and 5.0 whereas the enzymes of fructan metabolism had temperature optima at 25 °C and pH optima at about 5.5. Analysis of the products of enzyme assays showed that crude SST activity produced mainly 1-kestose from 100 mM sucrose and when incubated with 500 mvt sucrose, neokestose and possibly 6-kestose were also formed in low concentrations. FFT produced two tetrasaccharides and neokestose after incubation with 100 mM 1-kestose and FH produced fructose exclusively after incubation with high-molecular-weight fructan extracted from Loliutn rigidum and Jlclianthus tuberosus L. and with a commercial preparation of bacterial levan. Enzyme activities were also measured during a phase of fructan accumulation. FH activity declined by 50%, but SST and FFT activities increased about 2.5-fold during this period. The data supported the proposed synthetic pathway of fructan accumulation from sucrose via 1 -kestose into high-molecular-weight fructan and indicated that fructan synthesis may occur at low substrate concentrations. Extracts from leaves which had not accumulated fructans contained measurable activities of SST and FFT.

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