Separation and characteristics of galactose-1-phosphate and glucose-1-phosphate uridyltransferase from fruit peduncles of cucumber

Galactose-1-phosphate uridyltransferase (EC 2.7.7.10), responsible for the conversion of galactose-1-phosphate (Gal-1-P) to uridine diphosphate galactose (UDPgal) was examined in fruit peduncles of Cucumis sativus L. Two uridyltransferases (pyrophosphorylases), from I and II, were partially purified and resolved on a diethylamino-ethyl-cellulose column. Form I can utilize glucose-1-phosphate (Glc-1-P), while form II can utilize either Gal-1-P or Glc-1-P, with a preference for Gal-1-P. Form I was more heat stable than form II. Both Glc-1-P and Gal-1-P activities of form II were inactivated at the same rate by heating. The finding of a uridyltransferase with preference for Gal-1-P indicates that cucumber may have a Gal-1-P uridyltransferase (pyrophosphorylase) pathway for the catabolism of stachyose in the peduncles. The absence of the enzyme UDP-glucose-hexose-1-phosphate uridyltransferase (EC 2.7.7.12) in this tissue rules out catabolism by the classical Leloir pathway. The incorporation of carbon from UDPglc into Glc-1-P as opposed to sucrose may be regulated by the activities of the uridyltransferases. Pyrophosphate, in the same concentration range, inhibits UDP-gal formation (K_i=0.58±0.10 mM) and stimulates Glc-1-P formation. The ratio of units of pyrophosphatase to units of Gal-1-P uridyltransferase was higher in peduncles from growing fruit than from unpollinated fruit. Modulation of carbon partitioning through a uridyltransferase pathway may be a factor controlling growth of the cucumber fruit.Abbreviations Gal-1-P Galactose-1-phosphate - Glc-1-P glucose-1-phosphate - UDPgal uridine diphosphate galactose - UDPglc uridine diphosphate glucose Paper No. 6908 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh. 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