A novel alkaline alpha-galactosidase from melon fruit with a substrate preference for raffinose

The cucurbits translocate the galactosyl-sucrose oligosaccharides raffinose and stachyose, therefore, alpha-galactosidase (alpha-D-galactoside galacto hydrolase, EC 3.2.1.22) is expected to function as the initial enzyme of ph otoassimilate catabolism. However, the previously described alkaline alpha- galactosidase is specific for the tetrasaccharide stachyose, leaving raffin ose catabolism in these tissues as an enigma. In this paper we report the p artial purification and characterization of three alpha-galactosidases, inc luding a novel alkaline alpha-galactosidase (form I) from melon (Cucumis me lo) fruit tissue. The form I enzyme showed preferred activity with raffinos e and significant activity with stachyose. Other unique characteristics of this enzyme, such as weak product inhibition by galactose (in contrast to t he other alpha-galactosidases, which show stronger product inhibition), als o impart physiological significance. Using raffinose and stachyose as subst rates in the assays, the activities of the three alpha-galactosidases (alka line form I, alkaline form II, and the acid form) were measured at differen t stages of fruit development. The form I enzyme activity increased during the early stages of ovary development and fruit set, in contrast to the oth er alpha-galactosidase enzymes, both of which declined in activity during t his period. In the mature, sucrose-accumulating mesocarp, the alkaline form I enzyme was the major cu-galactosidase present. We also observed hydrolys is of raffinose at alkaline conditions in enzyme extracts from other cucurb it sink tissues, as well as from young Coleus blumei leaves. Our results su ggest different physiological roles for the alpha-galactosidase forms in th e developing cucurbit fruit, and show that the newly discovered enzyme play s a physiologically significant role in photoassimilate partitioning in cuc urbit sink tissue.