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A Potential Pathway for Galactose Metabolism in Cucumis sativus L., A Stachyose Transporting Species ¹

Department of Horticultural Science, North Carolina State University, Raleigh, North Carolina 27650

Conversion of [¹⁴C]galactose (Gal) 1-P, UDP-[¹⁴C]Gal, or UDP-[¹⁴C]glucose to [¹⁴C]sucrose was observed when cell-free homogenates of cucumber (Cucumis sativus L.) fruit peduncles were incubated with individual ¹⁴C-labeled substrates, appropriate cofactors, and fructose. The sucrose product was labeled only in the glucose moiety. Conversion of [¹⁴C]Gal-1-P to [¹⁴C]sucrose was catalyzed by extracts of peduncles from all other stachyose transporting species tested, as well as green bean (a sucrose transporter) but was not catalyzed by peduncle extracts from three other sucrose transporting species. In cucumber, the ability of extracts to form [¹⁴C]sucrose from [¹⁴C]Gal-1-P was greater when peduncles were harvested from growing fruit than from unpollinated ovaries. [¹⁴C]Sucrose formation from [¹⁴C]Gal-1-P was inhibited by Mg · PPi, Mg · UDP, UMP, and sucrose. -Galactosidase, galactokinase, UDP-gal pyrophosphorylase, UDP-Gal-4'-epimerase, UDP-glucose pyrophosphorylase, and sucrose synthase activities were detected in peduncle extracts. Neither sucrose phosphate synthetase nor hexose-1-P uridyltransferase were detected. Peduncle tissue contained a small pool of free galactose. These results suggest a potential pathway for the metabolism of galactose moieties hydrolyzed from stachyose, the major sugar transported by cucumber plants.

¹ Supported in part by funds from United States Department of Agriculture, Science and Education Administration cooperative agreement No. 58-7B30-9-140. Paper 6885 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, NC. The use of trade names in this publication does not imply endorsement by the North Carolina Agricultural Research Service of products named, nor criticism of similar ones not mentioned.

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