Confirmation of fructans biosynthesized in vitro from [1-13C]glucose in asparagus tissues using MALDI–TOF MS and ESI–MS

Accumulation of fructans was confirmed in asparagus tissues that had been cultured for 2 days on media supplemented with glucose. It is very common that fructans are biosynthesized from sucrose. We hypothesized however that fructans could also be biosynthesized from glucose. Stem tissues of in vitro-cultured asparagus were subcultured for 72 h on a medium containing 0.5 M of [1-13C]glucose. A medium containing 0.5 M of normal (12C) glucose was used as control. Carbohydrates were extracted from the tissues and analyzed using HPLC, MALDI–TOF MS and ESI–MS. HPLC results indicated that the accumulation of short-chain fructans was similar in both 13C-labelled and control samples. Short-chain fructans of DP = 3–7 were detected using MALDI–TOF MS. The molecular mass of each oligomer in the 13C-labelled sample was higher than the mass of the natural sample by 1 m/z unit per sugar moiety. The results of ESI–MS on the HPLC fractions of neokestose and 1-kestose showed that these oligomers (DP = 3) were biosynthesized from exogenous glucose added to the medium. We conclude that not only exogenous sucrose but glucose can induce fructan biosynthesis; fructans of both inulin type and inulin neoseries are also biosynthesized from glucose accumulated in asparagus tissues; the glucose molecules (or its metabolic products) were incorporated into fructans as structural monomers.