Reversible unidirectional inhibition of sucrose synthase activity by disulfides.

Sucrose synthase (UDPglucose:D-fructose 2-alpha-D-glucosyltransferase, EC2.4.1.13), which catalyzes the synthesis and cleavage of sucrose, exhibits differences in some properties between the two reactions. When enzyme previously incubated with oxidized glutathione or oxidized thioredoxin was used, sucrose cleavage was inhibited whereas sucrose synthesis proceeded at a normal rate. Sucrose cleavage activity could be restored by incubation with dithiothreitol or reduced glutathione. The thioredoxin effect was influenced by the presence of cleavage reaction substrates--i.e., sucrose and UDP. Thioredoxin action was rather slow compared with the catalytic reaction. These findings may have important implications for understanding the metabolic role of sucrose synthase and oxidized thioredoxin. Theoretically, the fact that an enzyme catalyzing a reversible reaction is inhibited in one direction only suggests that a modification in the enzyme affinities for its substrates must have occurred.