Die Saccharase im Stoffwechsel der Kartoffelknolle: IV. Zum Regulationsmechanismus der Saccharasesynthese

Abstract

Invertase in metabolism of potato tuber IV. Regulation of synthesis of invertase On the basis of experiments described previously it is supposed that synthesis of invertase in potato tuber is regulated by its substrate, Elevated sucrose levels in the cytoplasm induces enzyme synthesis. The produced hexoses are sequestered in storage compartments abroad from metabolic active centres (Hardy 1968). The substrate regulation of invertase synthesis in the growing tuber corresponds to the modell of an „einfaches offenes System“ described by Bertalanffy (Fig. 1). Above the regulation by substrate, synthesis of invertase is subject to product feedback repression by hexoses. As soon as the storage pool of cells is saturated with hexoses the enzyme synthesis is retarded by repression. This theoretical thesises are supported with examples. Changes in sugar relations observed in tubers during development, during storage and sprouting, and in mother tubers are discussed. The higher invertase activity and hexoses content in the basical than in the apical end of the tuber, for instance, was led back to an excess of translocated sucrose near the stolons during tuber growth. Similar, the dependence of hexoses content of tubers from latitude (north-south) it were grown is explained by the higher rate of translocation of carbohydrates from haulm to the tubers in the northern grown potatoes (Burton a. Wilson 1967). The relative high rate of importation of sucrose results in high invertase activity and hexoses content. The accumulation of sucrose in cold stored tubers also induces the production of invertase (Pressey a. Shaw 1966), and according to that hexoses content increases. After some time hexoses are reaching a constant level - the storage pool is saturated - and in spite of continual low temperatures the enzyme synthesis is retarded (Fig. 3). The pattern of enzyme activity in mother tubers shows a similar course like that in cold stored tubers, although in consequence of translocation no sucrose can be detected, Apparently, in this case the rate of enzyme synthesis increases because the hexoses are not effective as repressor. The capacity of vacuoles for hexoses continually increases on account of the continual exportation of nearly all soluble substances. Only when the velocity of translocation decreases, the hexoses reach the concentration of saturation and invertase synthesis is retarded by repression (Fig. 4).