The Function of <i>myo</i>‐Inositol in the Biosynthesis of Raffinose

<jats:p>1. An enzyme from <jats:italic>Vicia faba</jats:italic> seeds is described which transfers the galactosyl moiety of galactinol to sucrose giving rise to raffinose and <jats:italic>myo</jats:italic>‐inositol.</jats:p><jats:p>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.</jats:p><jats:p>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.</jats:p><jats:p>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, <jats:italic>p</jats:italic>‐nitrophenyl‐α‐<jats:sc>d</jats:sc>‐galactopyranoside and raffinose, but not UDP‐galactose, could act as donors.</jats:p><jats:p>5. The enzyme catalyzes an exchange reaction between raffinose and [<jats:sup>14</jats:sup>C]sucrose. This partial reaction is less sensitive towards heat inactivation and SH‐poisons than the total reaction.</jats:p><jats:p>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 <jats:italic>K</jats:italic><jats:sub>m</jats:sub>‐value for galactinol was 7 mM and for raffinose 10 mM. For sucrose a <jats:italic>K</jats:italic><jats:sub>m</jats:sub>‐value of 1 mM in the synthesis reaction has been determined.</jats:p><jats:p>7. The transferase activity is high enough to explain the synthesis rate <jats:italic>in vivo</jats:italic> of all the raffinose‐type sugars present in the seeds.</jats:p><jats:p>8. The physiological meaning of the results as well as the metabolic function of <jats:italic>myo</jats:italic>‐inositol is discussed.</jats:p>