Transport of inorganic phosphate and C3- and C6-sugar phosphates across the envelope membranes of potato tuber amyloplasts
1995
Schott, K. | Borchert, S. | Muller-Rober, B. | Heldt, H.W.
Amyloplasts have been isolated from tubers of potato plants (Solanum tuberosum. cv. Desiree). As it is difficult to isolate amyloplasts that have a high starch content, we used transformed plants in which the content of starch was reduced. This was achieved by decreasing the activity of ADP-glucose pyrophosphorylase by antisense techniques (Muller-Rober et al., 1992. EMBO. 11. 1229-1238). In the isolated plastics the activity of glutamine-oxoglutarate-aminotransferase (glutamate synthase, EC 2.6.1.53) was dependent upon the intactness of the plastics. For the supply of redox equivalents the addition of glucose-6-phosphate (Glc6P) was required. Glucose-1-phosphate (Glc1P) did not support glutamate synthesis. Plastids were treated with Triton X-100 and the solubilized proteins reconstituted into liposomes. Transport measurements with these liposomes revealed that inorganic phosphate (Pi), dihydroxyacetone phosphate (DHAP), 3-phosphoglycerate and Glc6P are transported in a counter-exchange mode. Transport of phosphoenolpyruvate was low and Glc1P was virtually not transported in exchange for Pi. Kinetic constants were determined for the Pi/Pi and Glc6P/Pi counter exchanges. For comparison, proteins of mitochondria from potato tubers and pea leaves were reconstituted into liposomes. As expected, the Pi/Pi exchange across the mitochondrial membrane was not affected by DHAP and Glc6P. Kinetic constants of the Pi/Pi counter exchange were determined for potato tuber mitochondria.
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