A Novel Alkaline 
-Galactosidase from Melon Fruit with a
Substrate Preference for Raffinose1
Zhifang Gao and
Arthur A. Schaffer*
Department of Vegetable Crops, Institute of Field and Garden Crops,
The Volcani Center, Bet Dagan, 50250, Israel
The cucurbits translocate the
galactosyl-sucrose oligosaccharides raffinose and stachyose, therefore,
-galactosidase (-D-galactoside galactohydrolase, EC
3.2.1.22) is expected to function as the initial enzyme of
photoassimilate catabolism. However, the previously described alkaline
-galactosidase is specific for the tetrasaccharide stachyose,
leaving raffinose catabolism in these tissues as an enigma. In this
paper we report the partial purification and characterization of three
-galactosidases, including a novel alkaline -galactosidase (form
I) from melon (Cucumis melo) fruit tissue. The form I
enzyme showed preferred activity with raffinose and significant
activity with stachyose. Other unique characteristics of this enzyme,
such as weak product inhibition by galactose (in contrast to the other -galactosidases, which show stronger product inhibition), also impart physiological significance. Using raffinose and stachyose as
substrates in the assays, the activities of the three
-galactosidases (alkaline form I, alkaline form II, and the acid
form) were measured at different stages of fruit development. The form
I enzyme activity increased during the early stages of ovary
development and fruit set, in contrast to the other -galactosidase
enzymes, both of which declined in activity during this period. In the
mature, sucrose-accumulating mesocarp, the alkaline form I enzyme was the major -galactosidase present. We also observed hydrolysis of
raffinose at alkaline conditions in enzyme extracts from other cucurbit
sink tissues, as well as from young Coleus blumei
leaves. Our results suggest different physiological roles for the
-galactosidase forms in the developing cucurbit fruit, and show that
the newly discovered enzyme plays a physiologically significant role in photoassimilate partitioning in cucurbit sink tissue.
1
This research was partially supported by the
United States-Israel Binational Agricultural Research and Development
Fund (grant no. 2270-93). This work is contribution no. 146/98, 1998 series, from the Agricultural Research Organization, The Volcani
Center, Bet Dagan, Israel.
*
Corresponding author; VCARIS{at}volvsni.agri.gov.il; fax
1-972-966-9642.
Plant Physiol. (1999) 119: 979-988
Copyright Clearance Center: 0032-0889/99/119//10
© 1999 American Society of Plant Physiologists
