The Function of myo-Inositol in the Biosynthesis of Raffinose

Purification and Characterization of Galactinol:Sucrose 6-Galactosyltransferase from Vicia faba Seeds


  • Trivial Name. Galactinol, l-1-(O-α-d-galactopyranosyl)-myo-inositol.


1. An enzyme from Vicia faba seeds is described which transfers the galactosyl moiety of galactinol to sucrose giving rise to raffinose and myo-inositol.

2. The enzyme was purified about 400-fold through 6 steps. A molecular weight of 80000 has been determined by gel-filtration and of 100000 by glycerol density gradient centrifugation.

3. The enzyme galactinol:sucrose 6-galactosyl transferase is different from α-galactosidase; these two activities as well as the stachyose-synthesizing enzyme separate during purification.

4. The transferase showed a high acceptor specificity. Out of 10 acceptors tested a transfer only to sucrose took place. This transfer was 5 times faster than the hydrolysis of galctinol. Galactinol, p-nitrophenyl-α-d-galactopyranoside and raffinose, but not UDP-galactose, could act as donors.

5. The enzyme catalyzes an exchange reaction between raffinose and [14C]sucrose. This partial reaction is less sensitive towards heat inactivation and SH-poisons than the total reaction.

6. The pH-optimum of the reaction was found to be pH 7.0, the temperature optimum 42 °C. Heat inactivation could be prevented to some extent by galactinol and raffinose. In the presence of 0.4 mM sucrose the Km-value for galactinol was 7 mM and for raffinose 10 mM. For sucrose a Km-value of 1 mM in the synthesis reaction has been determined.

7. The transferase activity is high enough to explain the synthesis rate in vivo of all the raffinose-type sugars present in the seeds.

8. The physiological meaning of the results as well as the metabolic function of myo-inositol is discussed.