The biosynthesis of uridine diphosphate glucose and sucrose in sugar beet leaf

Abstract

1. 1. The mechanisms for biosynthesis of UDPG and sucrose in sugarbeet leaf and leaf homogenate have been investigated by providing radioactive G-1-P with various cofactors and identifying the radioactive products after short time intervals. When radioactive G-1-P is fed to an excised leaf, sucrose equally labeled in both moieties is formed. When UTP is fed with G-1-P, the yield of radioactive sucrose is reduced, and a relatively large proportion of the radioactive glucose is converted to UDPG. 2. 2. The leaf homogenate converts radioactive G-1-P to radioactive glucose and fructose. A mixture of radioactive G-1-P and UTP is converted to radioactive UDPG, and UDP plus ATP can replace UTP. A method for preparation and purification of radioactive UDPG is outlined. 3. 3. Radioactive sucrose, labeled in the glucose moiety, is biosynthesized by leaf homogenates during 15 min. incubation when radioactive G-1-P plus UTP and F-6-P are added in appropriate concentrations. There is no reaction when UTP is omitted, and fructose or F-1,6-P cannot replace F-6-P. However, UDPG does replace G-1-P plus UTP for sucrose synthesis. Traces of radioactive sucrose phosphate are found in those systems which form sucrose. 4. 4. These data show that sucrose is biosynthesized in the sugar-beet leaf as the product of a sequence of reactions, in which UDPG, formed from G-1-P and UTP, reacts with F-6-P to give sucrose phosphate. The sucrose phosphate is rapidly dephosphorylated, and UTP is regenerated from UDP by ATP.