Myo‐Inositol, a Cofactor in the Biosynthesis of Stachyose

Abstract

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. Two tests to measure the enzyme activity are described. The reaction is reversible. An equilibrium constant of approximately 4 has been determined for . The enzyme catalyzes exchange reactions between galactinol and myo‐[14C]inositol and between stachyose and [14C]raffinose. Specifically labeled galactinol and stachyose can be prepared in this way. The Km values for galactinol and for the two best galactosyl acceptors, raffinose and melibiose, have been measured. 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. 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. The physiological importance of the results is discussed. Copyright © 1968, Wiley Blackwell. All rights reserved