Journal Article

Metabolism of Galactosylononitol in Seeds of <i>Vigna umbellata</i>

Thomas Peterbauer, Markus Puschenreiter and Andreas Richter

in Plant and Cell Physiology

Published on behalf of Japanese Society of Plant Physiologists

Volume 39, issue 3, pages 334-341
Published in print March 1998 | ISSN: 0032-0781
Published online March 1998 | e-ISSN: 1471-9053 | DOI: http://dx.doi.org/10.1093/oxfordjournals.pcp.a029374
Metabolism of Galactosylononitol in Seeds of Vigna umbellata

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In this paper, we describe the occurrence of a new galactosylononitol synthase activity (GOS) in seeds of Vigna umbellata. The enzyme catalyzed the reversible galactosyl transfer from galactinol to D-ononitol (lo-4-O-methyl-rnyo-inositol), yielding galactosylononitol (0-α-D-galac-topyranosyl-(l -* 3)-4-0-methyl-myo-inositol) and myo-inositol. A brief characterization of the enzyme indicated a native molecular mass of 96 kDa and Km values for galactinol and ononitol of 3.7 mM and 10.2 mM, respectively. The GOS activity could not be separated from stachyose synthase (STS) by anion-exchange and size-exclusion chro-matography, suggesting that the two reactions are catalyzed by very similar enzymes or by the same enzyme. An equilibrium constant of 0.8 was determined for galactosylononitol synthesis. Thus, the group transfer potential of galactosylononitol is similar to that of galactinol and sufficient for galactosylation of raffinose family oligosac-charides. It was demonstrated that galactosylononitol can substitute for galactinol in the STS catalyzed synthesis of stachyose. Galactosylononitol and GOS activity were initially detected halfway through seed development. In mature seeds stachyose was the predominant sugar (3.6 μmol seed−1), followed by sucrose (1.5μmol seed−1) and galactosylononitol (1.2 μmol seed−1). A metabolic pathway for galactosylononitol in the seeds of V. umbellata is proposed.

Keywords: Cyclitols; Galactosylononitol synthase; Stachyose synthase; Vigna umbellata

Journal Article.  0 words. 

Subjects: Biochemistry ; Molecular and Cell Biology ; Plant Sciences and Forestry

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