¹³C Nuclear Magnetic Resonance Studies of Crassulacean Acid Metabolism in Intact Leaves of Kalanchoë tubiflora

Abstract

'3C nuclear magnetic resonance spectroscopy of intact leaves of Kalanchoe tubiflora was used to observe Crassulacean acid metabolism in vivo.13C signals from C4 of malate were observed after overnight exposure of leaves to '3CO2. Illumination of the labeled leaves resulted in a gradual decrease in the malate signals. After a period of darkness in normal air, '3C signals were detected in all four carbons of malate in the previously labeled leaves. The '3C nuclear magnetic resonance spectrum of malate in solution was pH dependent, which allowed an estimation ofthe vacuolar pH from the whole leaf spectrum. The pH was 4.0 following a 14-hour dark period, but rose to greater than 6.0 after 6 hours of illumination.The application of NMR2 spectroscopy to intact tissue has been a major innovation in the study of cellular metabolism. 'H, 13C, '5N, and 31P NMR of intact tissue and cell suspensions has provided information on pH, metabolite levels and compartmentation, reaction rates, and pathway fluxes within living cells (for reviews, see 4,16,22). Studies of plant tissue by NMR have included 3P NMR of root tips (6,7,17,18) to the dark incubation period. About 15 leaves (5 g total fresh weight) were placed in two 10-ml beakers with the petioles submerged in water. The beakers were then placed in a I-L vacuum flask that contained 100 mg Ba'3CO3 (90% isotope enrichment, Stohler Isotope Chemicals) in a third 10-ml beaker. The air in the flask was replaced with C02-free air by evacuating the flask and then equilibrating the pressure in the flask with air that was drawn through a 20-cm column of Ascarite. Five ml of 50% lactic acid was then added to the Ba'3CO3 with a syringe to generate the '3CO2. The flask was sealed and placed in the dark in a 15°C incubation chamber. After overnight (14 h) incubation, the beakers with the leaves were removed from the flask and placed in the light (320 ME/m2'. s) in a 30°C incubation chamber.Leaf samples (-1 g) were removed at various times for '3C NMR analysis. The leaves were placed in 10-mm NMR tubes and submerged in 2H20. A 0.5-mm capillary containing acetone was inserted to provide a reference peak. Proton noise-decoupled '3C NMR at 22.5 MHz was performed on a JEOL FX9OQ FT-NMR spectrometer at 25C. A 36°pulse angle and a 3.82-s repetition rate were used to maximize signal-to-noise from the slowly relaxing carboxyl atoms (T, _ 7.5 s). Natural abundance 13C NMR spectra of malic acid were recorded using 0.5 M malic acid in H20 and titrated with KOH. A coaxial insert containing 2H20 provided a lock signal in these measurements. All chemical shifts were relative to tetramethylsilane and referenced to the acetone methyl signals (30.6 ,g/g).Following NMR spectroscopy of the intact tissue, the leaves were frozen. Later, an aqueous extract ofthe leaves was prepared following the method of Martin et al.(1 1). The extracts were titrated to pH 7.0 with 10 mN KOH and then lyophilized.RESULTS AND DISCUSSION Figure la shows the natural abundance '3C NMR spectrum of malic acid and the reson...

Estimation of Ammonium Ion Distribution between Cytoplasm and Vacuole Using Nuclear Magnetic Resonance Spectroscopy

“…13), because of the insensitivity of 31Pi chemical shifts at pH values below 6 (20). 13C NMR spectra of '3C-labeled malate were used to obtain sensitive measurements of vacuolar pH (1,18,26).…”

Malate Metabolism in the Dark After 13CO2 Fixation in the Crassulacean Plant Kalanchoë tubiflora

“…NMR is a useful technique to study ['3C]malate metabolism in plants because the 4-'3C and 1-'3C carbonyl groups of malate are spectrally well separated (2,13,16) and it has been established that only singly labeled malate is formed in the dark during CAM (3,13). Thus, the sum of the 4-C and 1-C resonances represent the total pool of labeled malate.…”

Observation of Cytoplasmic and Vacuolar Malate in Maize Root Tips by 13C-NMR Spectroscopy

“…ID). The chemical shifts of the carboxylic acid groups of malate are relatively insensitive to pHs above -6.5 (26). Cytoplasm occupies most of the extravacuolar space in root tips (25) and is near neutral while the vacuole is acidic (22 (Fig.…”

“…Peaks 1 and 2 in Figure 1D are assigned to Cl and C4 of malate, respectively, because (a) the chemical shifts match those of pure malate over a wide pH range (cf. 15,26); (b) the intensities of peaks 1 and 2 are selectively reduced when malic enzyme and NADP are added to the extract (Fig. 2); (c) under the experimental conditions described in Figure 1B, root tips accumulate several Asmol of Chemical Shift, ppm malate per g fresh weight (data presented below)-quantities of "3C-malate that are readily detectable by high resolution '3C-NMR.…”

“…No signals were detected from malate pools experiencing near-neutral pHs, approximating the cytoplasmic pH of plant cells (22). This result was attributed to the small proportion of the cells occupied by cytoplasm in leaves of CAM plants (26). The cells that make up root apices contain a large proportion of cytoplasm (25) and so are particularly suited for studies of primary metabolism.…”