Properties of phosphoribulokinase of whole chloroplasts

The ability of intact spinach (Spinacia oleracea) chloroplast preparations to catalyze CO₂ fixation and photophosphorylation was examined. Under conditions optimal for CO₂ fixation, only poor photophosphorylation was observed. Conditions optimal for photophosphorylation were found to be highly inhibitory to the CO₂-fixing capacity of the intact chloroplast preparation.     

Carbon dioxide fixation in the light and in the dark by isolated spinach chloroplasts

Factors affecting CO₂ fixation in the spinach (Spinacia oleracea) chloroplast were investigated. Free magnesium ions are shown to be highly inhibitory for photosynthetic CO₂ fixation in isolated intact spinach chloroplasts. The pH optimum for CO₂ fixation is about 8.5 but is dependent upon the reaction medium. Conditions are defined under which chloroplasts illuminated in the absence of CO₂ accumulate ribulose 1,5-diphosphate, and fix CO₂ in a subsequent dark period when high magnesium ion concentrations are provided. The regulation of photosynthetic CO₂ assimilation by these factors is discussed.     

Isolation, Characterization, and Partial Purification of a Reduced Nicotinamide Adenine Dinucleotide Phosphate-dependent Dihydroxyacetone Reductase from the Halophilic Alga Dunaliella parva

An NADP⁺-dependent dihydroxyacetone reductase, which catalyzes specifically the reduction of dihydroxyacetone to glycerol, has been isolated from the halophilic alga Dunaliella parva. The enzyme has been purified about 220-fold. It has a molecular weight of about 65,000 and is highly specific for NADPH. The pH optima for dihydroxyacetone reduction and for glycerol oxidation are 7.5 and 9.2, respectively. The enzyme has a very narrow substrate specificity and will not catalyze the reduction of glyceraldehyde or dihydroxyacetone phosphate. It is suggested that this enzyme functions physiologically as a dihydroxyacetone reductase in the path of glycerol synthesis and accumulation in Dunaliella.     

Salt-Induced Metabolic Changes in Dunaliella salina

An increase of medium NaCl concentration induces Dunaliella cells to evolve O₂ photosynthetically even in the absence of CO₂. This NaCl-induced O₂ evolution may reflect the induced conversion of reserve carbohydrate to glycerol. The quantum yield for the NaCl-induced O₂ evolution, in the absence of CO₂, is 1.5-fold higher than that obtained for CO₂ fixation. Since the synthesis of glycerol from reserve carbohydrate in the absence of CO₂ requires only 0.5 ATP/NADPH, whereas photosynthesis requires at least 1.3 ATP/NADPH, it is concluded that the ATP/2e⁻ ratio coupled to NADP reduction in Dunaliella is lower than required for CO₂ fixation.     

Effect of temperature on glycerol retention in the halotolerant algae dunaliella and asteromonas

Algae of the genera Dunaliella and Asteromonas can maintain extremely high concentration gradients (>10⁴) of glycerol between the intracellular space and the medium. This unique ability is highly temperature-dependent. Treating the algae for several minutes at temperatures exceeding 60 C causes complete release of all the internally held glycerol; 50% release occurs around 50 C, but essentially none is released below 40 C. A similar behavior was observed in several species of Dunaliella, and one of Asteromonas and is independent of the salt concentration of the medium. The underlying mechanism may involve a temperature-dependent conformational transition of a component of the cellular membrane which is essential for glycerol impermeability.     

Desaspidin: a nonspecific uncoupler of photophosphorylation

The inhibitory effect of desaspidin is shown to depend on the oxidation-reduction state of the system investigated. The more reducing the conditions, the more desaspidin-inhibited the system and vice versa.