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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-methylrnyo-inositol), yielding galactosylononitol (O-a-D-galactopyranosyl-(l -* 3)-4-0-methyl-D-m,yo-inositol) and myoinositol. 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 chromatography, 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 oligosaccharides. 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 //mol seed" 1 ), followed by sucrose (1.5/anol seed" 1 ) and galactosylononitol (1.2 /tmol seed" 1 ). A metabolic pathway for galactosylononitol in the seeds of V. umbellata is proposed.

Metabolism of Galactosylononitol in Seeds of Vigna umbellata