Biosynthesis of sucrose phosphate with sugar cane leaf chloroplasts.

Abstract

Leloir and his collaborators (7, 11, 12) and others (1, 3, 6, 9, 13, 15, 16) have shown that synthesis of sucrose in extracts of a number of plants can occur by 2 different enzymic reactions: A) UDP3-Dglucose + D-fructose sucrose + UDP; B) UDPD-gltucose + D-fructose 6-P = sucrose-P + UDP. In reaction (A) D-fructose serves as the glucosyl acceptor and the reaction is freely reversible. In reaction (B) in which D-fructose 6-P is the acceptor, the sucrose-P formed is hydrolyzed by a phosphatase in a reaction which is practically irreversible, producing free sucrose. It is of interest to note that small amounts of sucrose-P have been detected among label-d photosynthetic products in plants (2, 5). In a previous communication Frydman and Hassid (8) showed that extracts of sugarcane leaves contained an enzyme which produced sucrose from UDPD-glucose and D-fructose, but neither sucrose nor sucrose-P was formed when D-fructose wvas reDlaced bv D-fructose 6-P in the reaction mixture. This indicated that only the enzyme which catalyzed reaction (A) was present in the extract. However, when chloroplasts isolated from the sugarcane leaves were used -as the enzyme source, sucrose was formed from UDP-D-glucose and D-fructose 6-P as well as from UDP-D-glucose and D-fructose. Since sucrose-P was not detected in the incubation mixture containing UDP-D-glucose and D-fructose 6-P, it could not be decided whether the sucrose was formed by enzymic removal of P from sucrose-P or by glucosylation of D-fructose produced by the action of a phosphatase onl D-fructose 6-P. Bird and Stocking (4) have demonstrated the synthesis of suicrose from UDP-D-glucose and D-fructose or D-fructose 6-P by chloroplasts isolated in a nonaqueous medium; but they were also unable to detect the formation of sucrose-P. Evidence is presented in this paper that the chloroplasts of sugarcane leaves and spinach leaves contain, in addition to the enzyme that forms sucrose from UDP-D-glucose and D-fructose, the enzyme that catalyzes the formation of sucrose-P from UDP-Dglucose and D-fructose 6-P.