Myo‐Inositol, a Cofactor in the Biosynthesis of Stachyose

1 In extracts from Phaseolvs vulgaris seeds an enzyme has been found which transfers the galactosyl moiety of galactinol to raffinose giving rise to stachyose and myo‐inositol. 2 Two tests to measure the enzyme activity are described. 3 The reaction is reversible. An equilibrium constant of approximately 4 has been determined for . The enzyme catalyzes exchange reactions between galactinol and myo‐[ 14 C]inositol and between stachyose and [ 14 C]raffinose. Specifically labeled galactinol and stachyose can be prepared in this way. 4 The K m values for galactinol and for the two best galactosyl acceptors, raffinose and melibiose, have been measured. 5 Besides the galactinol: raffinose‐6‐galactosyl transferase the extract of the seeds also contains an α‐galactosidase, which separates from the transferase during the purification. The transferase is strongly inhibited by p ‐chloromercuribenzoate. 6 Ripening seeds contain an enzyme which catalyzes the reaction UDP‐galactose + myoinositol → galactinol + UDP. However, no direct transfer of galactose from UDP‐galactose to raffinose has been observed. 7 The physiological importance of the results is discussed.