Abstract
1. All enzymes tested: malate dehydrogenase (MDH), glutamate dehydrogenase (GDH), glucose-6-phosphate dehydrogenase (G-6-PDH), and 6-phosphogluconate dehydrogenase (6-P-GDH)_behaved rhythmically during a period of continuous white light interrupted by darkness. All showed approximately the same frequency of 12-15 h. 2. The enzymes of the tricarboxylic-acid cycle (MDH and GDH) are in phase with each other but are out of phase with those of the oxidative pentose-phosphate pathway (G-6-PDH and 6-P-GDH) which are in turn in phase with each other. 3. GDH appears to be activated by the addition of Hoagland's solution which leads to an overt rhythm 24 h prior to darkness. The rhythms of MDH, G-6-PDH and 6-P-GDH cannot be demonstrated prior to the onset of darkness due to an inhibition of the MDH and pentose-phosphate cycle enzymes by light. 4. The control of the frequency and phase of these rhythms are discussed in relation to a positive correlation of the rhythms in enzyme activity presented here and the rhythms of the pyridine nucleotides presented elsewhere.
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