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Uptake of l-Ascorbate by Intact Spinach Chloroplasts
1Lehrstuhl für Pflanzenphysiologie, Universität Bayreuth, D-8580 Bayreuth, West Germany.
Plant Physiology
|September 1, 1983
Summary
Spinach chloroplasts efficiently transport ascorbate into the stroma via facilitated diffusion. This uptake mechanism exhibits Michaelis-Menten kinetics and is specific for the ascorbate anion.
Area of Science:
- Plant Physiology
- Biochemistry
- Molecular Transport
Background:
- Ascorbate (vitamin C) is a crucial antioxidant in plants, protecting against oxidative stress.
- Understanding ascorbate transport mechanisms in chloroplasts is vital for plant health and productivity.
Purpose of the Study:
- To investigate the mechanism and kinetics of ascorbate uptake by intact spinach chloroplasts.
- To determine the characteristics of the ascorbate transport system across the chloroplast envelope.
Main Methods:
- Utilized silicone oil filtering technique to study L-[1-(14)C]ascorbate uptake in isolated spinach chloroplasts.
- Analyzed uptake rates, substrate saturation kinetics (Michaelis-Menten), pH optimum, and energy requirements.
- Investigated the effect of inhibitors and preloaded substrates on ascorbate transport.
Main Results:
- Observed high ascorbate transport rates (>100 µmol/mg chlorophyll/h) into the stroma.
- Determined a K(m) of 18-40 mM, indicating substrate saturation kinetics.
- Identified a pH optimum of 7.0-7.5 and suggested monovalent ascorbate anion transport.
- Didehydroascorbate acted as a competitive inhibitor, and sulfhydryl reagents inhibited uptake.
Conclusions:
- Ascorbate uptake into spinach chloroplasts is mediated by a specific translocator via facilitated diffusion.
- The transport system is characterized by Michaelis-Menten kinetics and anion specificity.
- Findings provide insights into the regulation of ascorbate homeostasis within plant cells.