Abstract
In thalli of the aquatic liverwort, Riccia fluitans, 3-O-methyl-D-glucose (3-OMG) is not metabolized. Intracellular compartmentation, accumulation and transmembrane fluxes of 3-OMG have been determined by compartmental analysis. A novel set of equations has been derived to extend this method to non-steady-state conditions of constant but unequal unidirectional fluxes. Efflux kinetics with 3-OMG and L-glucose revealed two intracellular flux compartments, presumably cytoplasm and vacuole; an additional quickly exchanging compartment (half-time approx. 1 min) has been assigned to the apoplast. With 1 mM 3-OMG given externally, cytoplasmic 3-OMG concentration (c c) attains a quasi-steady state of about 10 mM lasting for >100 h, whereas the presumed vacuolar concentration (c v) rises steadily, but does not reach flux equilibrium even after two weeks (c v=46 mM). After 24 h incubation with 0.03 mM 3-OMG, c c=1 mM approx., and c v=3 mM approx., thus indicating accumulation by active hexose transport at both the plasmalemma and tonoplast. External D-glucose, but no D-mannitol, competitively inhibits 3-OMG uptake (cis-inhibition) and stimulates 3-OMG efflux at the plasmalemma by a factor up to 2.5. This trans-stimulation saturates half-maximally at 1.5 mM D-glucose. It clearly indicates a hexose carrier in the plasmalemma with one substrate-binding site for D-glocose and 3-OMG, alternately exposed to the cytoplasmic and outside compartment. The extent of the measured trans-stimulation can only be explained, if in the transport cycle the translocation of the empty substrate-binding site across the plasmalemma is rate-limiting.
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Abbreviations
- A :
-
initial tracer content (cpm); ϕ=unidirectional flux (nmol s-1 g DW-1)
- K :
-
efflux rate constant (s-1)
- 3-OMG:
-
3-O-methyl-D-glucose
- Q :
-
chemical content (μmol g DW-1)
- Q * :
-
tracer content (cpm)
- s :
-
specific activity (s=Q *Q-1)
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Peter Gogarten, J., Bentrup, FW. Fluxes and compartmentation of 3-O-methyl-D-glucose in Riccia fluitans . Planta 159, 423–431 (1983). https://doi.org/10.1007/BF00392078
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DOI: https://doi.org/10.1007/BF00392078