Glucose control of sucrose synthase in the maize scutellum

The in vivo amounts of UDPG, UTP, UDP and UMP, metabolites known to influence the activity of sucrose phosphate synthase (SPS) and sucrose synthase (SS), were measured throughout 5 hr incubations of scutellum slices in fructose or water, i.e. under conditions of sucrose synthesis or breakdown. Cytosolic concentrations were estimated assuming that these metabolites were confined to the cytosol. Within the estimated in vivo concentration ranges, UDPG, UTP and UDP had little effect on the in vitro SS activity, but glucose (100 mM) inhibited SS in the synthesis direction by 63–70% and in the breakdown direction by 86–93%. Glucose inhibition of SS was considerably less when saturating levels of substrates were used. Sucrose did not inhibit SS. It is concluded that during germination the glucose produced from starch breakdown in the maize endosperm enters the scutellum and inhibits SS, preventing a futile cycle and limiting SS participation in sucrose synthesis.