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The Citric Acid Cycle02:36

The Citric Acid Cycle

The citric acid cycle, also known as the Krebs cycle or TCA cycle, consists of several energy-generating reactions that yield one ATP molecule, three NADH molecules, one FADH2 molecule, and two CO2 molecules.
Products of the Citric Acid Cycle00:53

Products of the Citric Acid Cycle

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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
PubMed
Summary
This summary is machine-generated.

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.

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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.