THE PHOTOSYNTHETIC RHYTHM OF ACETABULARIA CRENULATA. II. MEASUREMENTS OF PHOTOASSIMILATION OF CARBON DIOXIDE AND THE ACTIVITIES OF ENZYMES OF THE REDUCTIVE PENTOSE CYCLE

¹ Tyco Laboratories, Inc., Waltham, Massachusetts

1. The photosynthetic rhythm of Acetabularia crenulata affects both light (quantum yield) and dark reactions in a parallel manner.

2. No significant difference was found between the activity of RuDP carboxylase in the extracts of samples taken at the middle of the light and dark periods nor was any difference detected in the affinity of this enzyme to CO₂. The activity of RuDP carboxylase in the cell extracts was sufficient to account for the observed rates of photoassimilation of CO₂ at saturating light intensities.

3. The activities of eight other enzymes of the reductive pentose phosphate cycle were also shown not to differ to a significant extent in extracts of cells taken at the middle of the light and dark periods. Five of these enzymes (phosphoglycerate kinase, glyceraldehyde-3-P dehydrogenase, triose isornerase, R-5-P isomerase and Ru-5-P kinase) had activities considerably above those required for the observed light-saturated rate of CO₂ assimilation while the activities of aldolase transketolase and transaldolase were of the same order of magnitude as the observed rates of photoassimilation.