The effect of d-mannose on sucrose storage in the corn scutellum: Evidence for two sucrose transpsort mechanisms

Abstract

Evidence is presented that tissue slices of the corn scutellum contain two distinct mechanisms for the storage of sucrose. The first mechanism is responsible for the storage of exogenously supplied sucrose and is strongly inhibited by the presence of d-mannose in the sucrose-containing bathing medium. The second mechanism is responsible for the transport of sucrose (newly synthesized from hexose) from the synthesis compartment of the cell into storage. This mechanism is not inhibited by mannose. Although the intercompartmental storage of newly synthesized sucrose is unaffected by the presence of mannose, the synthesis of the disaccharide from exogenously supplied fructose is strongly inhibited when mannose is present in the bathing medium. In addition, treatment of tissue slices with mannose resulted in a drastic reduction in the ATP level of the tissue. This reduction in ATP was not accompanied by an equivalent increase in ADP or AMP. It is suggested that the effect of mannose in inhibiting the storage of exogenous sucrose and in inhibiting the synthesis of sucrose from exogenous fructose arises from ATP deficiency. This deficiency is presumably the result of mannose and mannose-6-phosphate inhibitions of glycolysis at the level of hexokinase and phosphoglucose isomerase, respectively, and/or the trapping of inorganic phosphate in a pool of non-metabolizable mannose-6-phosphate. Possible mechanisms for sucrose storage are discussed.