The Effect of Glyoxylate on Photosynthesis and Photorespiration by Isolated Soybean Mesophyll Cells

Abstract

Incubating isolated soybean leaf mesophyO ceOs with glyoxylate increased the rates of CO2 fixation by as much as 150%. In order to cause this stimulation, the glyoxylate must be presented to the cels before the NaHCO. (13,20). Photosynthesis was assayed in stoppered 10-ml Fernbach flasks in a reaction mixture that contained 0.3 M sorbitol, 50 mm Tris-HCl (pH 7.8), 2 mm NaNO3, 2 mm EDTA, 1 mm MnCl2, 1 mM MgCl2, 0.5 mm K2HPO4, and 2 mm DTT (added fresh daily).The isolated soybean leaf cells, containing 5-30 jig Chl and suspended in the same solution, were generally added to previously equilibrated flasks in a 25 C waterbath and illuminated for 15 s to 20 min in the absence of added NaHCO3. Next the NaH'4CO3 was added and the illumination continued for an additional 5 min before the reaction was stopped by adding 0.1 ml 3 N HCI. The amount of acid-stable radioactivity was determined by liquid scintillation counting of 0.1-or 0.2-ml aliquots of the acidified reaction mixture.All reactions were in equilibrium with air unless otherwise indicated. In some experiments the reaction vessels were equilibrated with C02-free gases by blowing the gas vigorously against the liquid surface for 10 min before the cells were added. The activity of the RuBP carboxylase in the cells was assayed by first making the membrane more permeable with toluene (7) and then adding the carboxylation substrates RuBP and NaH14CO3. The cells were preilluminated and assayed in the same solution that was used in the photosynthetic studies except that the MgC12 concentration was increased to 25 mm (1). After the cells were illuminated at 25 C for 5 min, 25-,lI samples of the cell mixture were transferred to the reaction flasks. Thirty s later the reaction was terminated by adding 0.1 ml 3 N HCI. The reaction flasks contained 50 pl of a 4:1 (v/v) ethanol to toluene solution, 0.35 mM RuBP, the indicated amount of NaH'4CO3, (2)(3)(4)uCi/,umol)

Effects of Glyoxylate on Photosynthesis by Intact Chloroplasts

“…Glyoxylate inhibited photosynthesis in intact chloroplasts, as observed by Oliver and Zelitch (15). The inhibition was not specific for glyoxylate, since salts of other weak acids such as lactate, glycolate, acetate, and formate also inhibited photosynthesis.…”

“…The known mechanism for modifying the flux of carbon between the cycles is by changing the availability of the gaseous substrates CO2 and 02-It has been speculated that intermediates of the oxidative photosynthetic carbon cycle may regulate the relative magnitudes of the oxidative and reductive path- ' ways by a feedback mechanism (15,16). Oliver and Zelitch (16) reported that glyoxylate was a potent inhibitor of CO2 fixation in isolated spinach chloroplasts, but that glyoxylate did not inhibit ribulose-P2 carboxylase activity.…”

Effects of Aminoacetonitrile on Net Photosynthesis, Ribulose-1,5-Bisphosphate Levels, and Glycolate Pathway Intermediates

“…A recent study (5) showed that 10 mm glyoxylate caused a reversible inhibition of the RuBP carboxylaseoxygenase; however the inhibition occurred only with the fully activated enzyme in the absence of effectors which may not be the actual state of the enzyme in vivo (21). Higher concentrations of glyoxylate have been reported to inhibit photosynthesis by chloroplasts (20), but to stimulate net photosynthesis in tobacco leaf discs (20) and isolated soybean mesophyll cells (18). In these experiments too, the order of addition of CO2 and glyoxylate was critical.…”

Glyoxylate Induced Changes in the Carbon and Nitrogen Metabolism of the Cyanobacterium Anabaena cylindrica

“…tobacco (21,23), soybean cells (16,22), and spinach cells (17). An inhibition by glyoxylate ofthe synthesis or metabolism of glycolate is suggested, which prevents a drain of RuBP2 from the Calvin cycle.…”