Stachyose synthase (STS) (EC 2.4.1.67) was purified 313-fold from mature seeds of lentil. The final preparation had a specific activity of 9.09 nkat stachyose formed per milligram of protein. The enzyme was a monomeric protein with a molecular mass of 88.6 kDa (SDS-PAGE) and an isoelectric point of 4.8 (chromatofocusing). Western analysis revealed cross-reactivity of polyclonal antibodies raised against STS from adzuki bean with the lentil enzyme. The purified enzyme catalyzed a range of different galactosyl transfer reactions. In addition to the genuine STS reaction (raffinose + galactinol --> stachyose + myo-inositol), the enzyme catalyzed the reversible galactosyl transfer from galactinol to d-pinitol (1d-3-O-methyl-chiro-inositol), yielding galactopinitol A (O-alpha-d-galactopyranosyl-(1 --> 2)-4-O-methyl-d-chiro-inositol) and myo-inositol. Galactopinitol A could be further galactosylated by STS to give ciceritol (O-alpha-d-galactopyranosyl-(1 --> 6)-O-alpha-d-galactopyranosyl-(1 --> 2)-4-O-methyl-d-chiro-inositol). Enzymatic synthesis of galactopinitol A and ciceritol is a new observation. However, STS was not only able to utilize galactopinitol A as galactosyl acceptor, but also as galactosyl donor to form stachyose from raffinose. The role of STS in the metabolism of galactosyl cyclitols and oligosaccharides in plant seeds is discussed.
Copyright 1999 Academic Press.