Fructosyltransferase activities from Lolium rigidum Gaudin

Abstract

A partial separation of sucrose: sucrose fructosyltransferase (SST) activity from invertase activity extracted from leaves of Lolium rigidum Gaudin was achieved by gel filtration. The SST activity was purified 54‐fold with a ratio of SST activity to invertase activity of 1.1:1. The partially purified SST activity eluted with an apparent molecular mass of about 68000, had an optimum pH of 55 and a Km of 0.2 M. When incubated with sucrose concentrations ranging from 10 to 600 mM the partially purified SST activity produced mainly 1‐kestose and some 6‐kestose (10–15% of trisaccharide produced). SST and invertase activities declined during storage at — 25 °C with invertase activity declining at a faster rate. After the loss of invertase activity in storage, the partially purified SST activity synthesized only 1‐kestose. When incubated with 1‐kestose the partially purified SST activity synthesized neokestose (1.9 nkat ml−1), nystose (2.2 nkat ml−1) and an unidentified tetrasaccharide (1.4 nkat ml−1) indicating that the fructosyltransferase preparation also had FFT‐like activity. The partially purified invertase had significant sucrose‐hydrolyzing activity (25 nkat ml−1) and contained no measurable SST activity when incubated with 10–400 mM sucrose, but produced trisaccharide (1‐kestose and 6‐kestose, ratio 15:1) when incubated with 600 mM sucrose. L. rigidum does not normally accumulate 6‐kestose. The optimum pH of the sucrose‐hydrolyzing activity was 4–5 and the optimum pH of the fructosyltransferase activity of the invertase preparation was 55. The partially purified invertase did not have significant FFT‐like activity. The results support the hypothesis that SST activity synthesizes only 1‐kestose, which is a key intermediate in the synthesis of fructan in L. rigidum. Copyright © 1993, Wiley Blackwell. All rights reserved