Fructan metabolism in leaves of Loliwn rigidum Gaudin

II. Fructosyltransferase, invertase and fructan hydrolase activity

Authors


summary

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.

Abbreviations
BSA

bovine serum albumin

DP

degree of polymerization

DDT

dithiothreitol

FFT

fructan–fructan fructosyltransferase

FH

fruction hydrolase

Fruf

fructofuranose

Glp

glucopyranose

HPLC

high performance liquid chromatography

SST

sucrose–sucrose fructoryltransferase

TLC

thin-layer chromatography

WSC

water-soluble carbohydrates

Ancillary

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