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Isocitrate lyase in green leaves
H R Godavari1, S S Badour, E R Waygood
1Department of Botany, University of Manitoba, Winnipeg, Manitoba. R3T 2N2, Canada.
Plant Physiology
|May 1, 1973
Summary
Isocitrate lyase was found in spinach, wheat, and maize leaves. Spinach extracts contain heat-stable inhibitors that are removed by dark starvation or Sephadex filtration.
Area of Science:
- Biochemistry
- Plant Physiology
Background:
- Isocitrate lyase (EC 4.1.3.1) is a key enzyme in the glyoxylate cycle.
- Its presence and regulation in common crop plants are of significant physiological interest.
Purpose of the Study:
- To demonstrate the presence and characterize the activity of isocitrate lyase in spinach, wheat, and maize.
- To investigate endogenous inhibitors of isocitrate lyase in spinach.
Main Methods:
- Enzyme assays on crude dialyzed leaf extracts.
- Identification of reaction products using phenylhydrazone formation and isotopic labeling.
- Inhibition studies with various compounds and heat treatment.
- Gel filtration (Sephadex) for inhibitor removal.
- Determination of enzyme kinetics (pH optima, Km).
Main Results:
- Isocitrate lyase activity was detected in spinach, wheat, and maize leaves.
- Glyoxylate and succinate were identified as reaction products.
- Spinach extracts contained at least two thermostable, proteinaceous inhibitors of isocitrate lyase.
- Inhibitor levels decreased after dark starvation or Sephadex filtration.
- The enzyme exhibited double pH optima (7.4 and 8.0) and an apparent Km of 0.1 mM.
- Several compounds inhibited activity, while ribulose 1,5-diphosphate activated it.
Conclusions:
- Isocitrate lyase is present and active in the leaves of spinach, wheat, and maize.
- Spinach leaves possess endogenous, heat-stable inhibitors of isocitrate lyase, suggesting a regulatory mechanism.
- Enzyme activity is modulated by pH, substrate analogs, and specific activators/inhibitors.
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