ANALYSIS OF CARBOHYDRATE TRANSPORT ACROSS THE ENVELOPE OF ISOLATED CAULIFLOWER-BUD AMYLOPLASTS

Autor(en): MOHLMANN, T
BATZ, O
MAASS, U
NEUHAUS, HE
Stichwörter: Biochemistry & Molecular Biology; BIOSYNTHESIS; CAPACITIES; ENDOSPERM; GLUCOSE-6-PHOSPHATE; PLASTIDS; STARCH SYNTHESIS; TRANSLOCATOR; TRIOSE PHOSPHATES
Erscheinungsdatum: 1995
Herausgeber: PORTLAND PRESS
Journal: BIOCHEMICAL JOURNAL
Volumen: 307
Ausgabe: 2
Startseite: 521
Seitenende: 526
Zusammenfassung: 
Using isolated amyloplasts from cauliflower buds, we have characterized the interaction and transport of various carbohydrates across the envelope membrane of a heterotrophic plastid. According to our results, glucose 6-phosphate (Glc6P) and glucose 1-phosphate (Glc1 P) do not share the same transport protein for uptake into cauliflower-bud amyloplasts. Glc6P-dependent starch synthesis is strongly inhibited in the presence of dihydroxyacetone phosphate (DHAP) or 4,4'-di-isothiocyano-2,2'-stilbenedisulphonic acid (DIDS), whereas Glc1P-dependent starch synthesis is hardly affected by these compounds. Analysis of the Glc6P uptake into proteoliposomes reconstituted from the envelope proteins of cauliflower-bud amyloplasts indicate that Glc6P is taken up in a counter-exchange mode with P-i, DHAP or Glc6P, whereas Glc1P does not act as a counter-exchange substrate. P-i is a strong competitive inhibitor of Glc6P uptake (K-i 0.8 mM) into proteoliposomes, whereas Glc1P does not significantly inhibit Glc6P transport. Beside a hexose-phosphate translocator, these amyloplasts possess an envelope protein mediating the transport of glucose across the membrane. This translocator exhibits an apparent K-m for glucose of 2.2 mM and is inhibited by low concentrations of phloretin, known to be a specific inhibitor of glucose-transport proteins. Maltose inhibits the uptake of glucose (K-i 2.3 mM), indicating that both carbohydrates share the same translocator.
ISSN: 02646021

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