Light and Temperature Regulation of Early Morning Crassulacean Acid Metabolism in Opuntia erinacea var Columbiana (Griffiths) L. Benson

During the early morning period, light and temperature exert distinctively different influences on the gas exchange patterns of the Crassulacean acid metabolism plant Opuntia erinacea through their effects on acid metabolism. An initial decrease in CO₂ uptake was triggered by illumination and was apparently due to a decreased CO₂ diffusion gradient through light-mediated decarboxylation of malate. In contrast, the morning burst of CO₂ uptake occurred at high temperature presumably in response to increases in both stomatal conductance and the CO₂ diffusion gradient, resulting from the temperature-regulated fixation of endogenous CO₂, primarily into malate. Subsequent stomatal closure, apparently due to elevated levels of internal CO₂ through rapid decarboxylation of malate at high temperature, was primarily responsible for the final termination of early morning Crassulacean acid metabolism.     

The Involvement of Aspartate and Glutamate in the Decarboxylation of Malate by Isolated Bundle Sheath Chloroplasts from Zea mays

Aspartate or glutamate stimulated the rate of light-dependent malate decarboxylation by isolated Zea mays bundle sheath chloroplasts. Stimulation involved a decrease in the apparent K_m (malate) and an increased maximum velocity of decarboxylation. In the presence of glutamate other dicarboxylates (succinate, fumarate) competitively inhibited malate decarboxylation by intact chloroplasts with respect to malate with an apparent K_i of about 6 millimolar. For comparison the K_i for inhibition of nicotinamide adenine dinucleotide phosphate-malic enzyme from freshly lysed chloroplasts by these dicarboxylates was 15 millimolar. A range of compounds structurally related to aspartate stimulated malate decarboxylation by intact chloroplasts. K_a values for stimulation at 5 millimolar malate were 1.7, 5, and 10 millimolar for l-glutamate, l-aspartate, and β-methyl-dl-aspartate, respectively. Certain compounds, notably cysteic acid, which stimulated malate decarboxylation by intact chloroplasts inhibited malate decarboxylation by nicotinamide adenine dinucleotide phosphate-malic enzyme obtained from lysed chloroplasts and assayed under comparable conditions. It was concluded that aspartate, glutamate, and related compounds affect the transport of malate into the intact chloroplasts and that malate translocation does not take place on the general dicarboxylate translocator previously reported for higher plant chloroplasts.     

In Vivo Measurement of Indole-3-acetic Acid Decarboxylation in Aging Coleus Petiole Sections

The concentration of indoleacetic acid (IAA) in plant tissues is regulated, in part, by its rate of decarboxylation. However, the commonly used in vitro assays for IAA oxidase may not accurately reflect total in vivo decarboxylation rates. A method for measuring in vivo decarboxylation was utilized in which ¹⁴CO₂ is collected following uptake of [1-¹⁴C]IAA by excised tissue sections. After a 30-minute equilibration period, the evolution of ¹⁴CO₂ was found to follow an approximately linear course with respect to both time and tissue weight.

Decarboxylation rates were measured by this method in petiole sections of the Princeton clone of Coleus blumei Benth. Both the ¹⁴CO₂ evolved per milligram tissue and the percent of [1-¹⁴C]IAA uptake decarboxylated were highest in sections from the youngest petioles tested, and declined in the older tissue. Thin layer chromatography of acetonitrile extracts from the [1-¹⁴C]IAA-treated petioles showed a decreasing amount of free IAA and an increase at the retardation factor of indoleacetylaspartate in the older sections. The decreased decarboxylation rates in the older petioles may be attributable to a generally lower metabolic rate and increased protection of the IAA by conjugation.

     

Mitochondria as a site of C4 acid decarboxylation in C4-pathway photosynthesis.

One group of C₄, species utilize a NAD-malic enzyme to decarboxylate C₄ acids. This enzyme, together with a major isoenzyme of aspartate aminotransferase and a NAD-malate dehydrogenase, is localized in the mitochondria of the bundle sheath cells and the following pathway for C₄, acid decarboxylation has been proposed: aspartate → oxaloacetate → malate → CO₂ + pyruvate. The present study reports that mitochondria isolated from the bundle sheath cells of one of these species, Atriplex spongiosa, are capable of decarboxylating C₄, acids at rates between 5 and 8 μmol/min/mg chlorophyll. For maximum decarboxylating activities, these particles required aspartate, 2-oxoglutarate and phosphate as well as malate; in the absence of any one of these compounds, activity was reduced to 0.3–0.8 μmol/min/mg chlorophyll. Rates for C₄ acid decarboxylation were much greater than the respiratory activities of these particles, including the capacity to form citrate or to oxidize malate, succinate, pyruvate or 2-oxoglutarate (0.03–0.6 μmol/min/mg chlorophyll). A comparison of mitochondria prepared from leaves of various C₄, and C₃, species showed that only the mitochondria from the bundle sheath cells of plants with high NAD-malic enzyme have capacities for rapid C₄ acid decarboxylation. The effects of a variety of experimental conditions on C₄ acid decarboxylating activities are also reported. The role of these mitochondria in C₄ photosynthesis is discussed.     

Stereochemistry of the decarboxylation of l-ornithine with ornithine decarboxylase from mouse kidney

Using highly purified ornithine decarboxylase isolated from androgen-treated mice, [1R-²H]putrescine was generated by the decarboxylation of l-ornithine in D₂O, and [1S-²H]putrescine was generated from [2-²H]ornithine by carrying out the decarboxylation in H₂O. Chirality of the putrescines was then determined from the 200-MHz ¹H NMR spectra of their bis-camphanamides in the presence of Eu(fod)₃. These results demonstrated that decarboxylation had taken place with retention of configuration.     

Altered glycine decarboxylation inhibition in isonicotinic Acid hydrazide-resistant mutant callus lines and in regenerated plants and seed progeny

Isonicotinic acid hydrazide (INH), an inhibitor of the photorespiratory pathway blocking the conversion of glycine to serine and CO₂, has been used as a selective agent to obtain INH-resistant tobacco (Nicotiana tabacum) callus cells. Of 22 cell lines that were INH-resistant, none were different from wild-type cells in their ability to take up [³H]INH or to oxidize INH to isonicotinic acid. In 7 of the 22 cell lines, INH resistance was associated with decreased inhibition of NAD-dependent glycine decarboxylation activity in isolated mitochondrial preparations. In the cell line that was most extensively investigated (I 24), this biochemical phenotype (exhibiting a 3-fold higher K_i with INH) was observed in leaf mitochondria of regenerated plants and of plants produced from them by self-fertilization. After crosses between resistant and sensitive plants, the decreased inhibition of glycine decarboxylation was observed among F₂ and backcross progeny only in those plants previously identified as INH-resistant by callus growth tests. In contrast, in siblings identified as INH-sensitive, glycine decarboxylation was inhibited by INH at the wild-type level. This demonstration of the transfer of an altered enzyme property from callus to regenerated plants and through seed progeny fulfills an important requirement for the use of somatic cell genetics to produce biochemical mutants of higher plants.     

Decarboxylation of Sorbic Acid by Spoilage Yeasts Is Associated with the PAD1 Gene

The spoilage yeast Saccharomyces cerevisiae degraded the food preservative sorbic acid (2,4-hexadienoic acid) to a volatile hydrocarbon, identified by gas chromatography mass spectrometry as 1,3-pentadiene. The gene responsible was identified as PAD1, previously associated with the decarboxylation of the aromatic carboxylic acids cinnamic acid, ferulic acid, and coumaric acid to styrene, 4-vinylguaiacol, and 4-vinylphenol, respectively. The loss of PAD1 resulted in the simultaneous loss of decarboxylation activity against both sorbic and cinnamic acids. Pad1p is therefore an unusual decarboxylase capable of accepting both aromatic and aliphatic carboxylic acids as substrates. All members of the Saccharomyces genus (sensu stricto) were found to decarboxylate both sorbic and cinnamic acids. PAD1 homologues and decarboxylation activity were found also in Candida albicans, Candida dubliniensis, Debaryomyces hansenii, and Pichia anomala. The decarboxylation of sorbic acid was assessed as a possible mechanism of resistance in spoilage yeasts. The decarboxylation of either sorbic or cinnamic acid was not detected for Zygosaccharomyces, Kazachstania (Saccharomyces sensu lato), Zygotorulaspora, or Torulaspora, the genera containing the most notorious spoilage yeasts. Scatter plots showed no correlation between the extent of sorbic acid decarboxylation and resistance to sorbic acid in spoilage yeasts. Inhibitory concentrations of sorbic acid were almost identical for S. cerevisiae wild-type and Δpad1 strains. We concluded that Pad1p-mediated sorbic acid decarboxylation did not constitute a significant mechanism of resistance to weak-acid preservatives by spoilage yeasts, even if the decarboxylation contributed to spoilage through the generation of unpleasant odors.     

CO(2) Assimilation and Malate Decarboxylation by Isolated Bundle Sheath Chloroplasts from Zea mays

Conditions for optimal CO₂ fixation and malate decarboxylation by isolated bundle sheath chloroplasts from Zea mays were examined. The relative rates of these processes varied according to the photosynthetic carbon reduction cycle intermediate provided. Highest rates of malate decarboxylation, measured as pyruvate formation, were seen in the presence of 3-phosphoglycerate, while carbon fixation was highest in the presence of dihydroxyacetone phosphate; only low rates were measured with added ribose-5-phosphate. Chloroplasts exhibited a distinct phosphate requirement and this was optimal at a level of 2 millimolar inorganic phosphate in the presence of 2.5 millimolar 3-phosphoglycerate, dihydroxyacetone phosphate, or ribose-5-phosphate. Malate decarboxylation and CO₂ fixation were stimulated by additions of AMP, ADP, or ATP with half-maximal stimulation occurring at external adenylate concentrations of about 0.15 millimolar. High concentrations (>1 millimolar) of AMP were inhibitory. Aspartate included in the incubation medium stimulated malate decarboxylation and CO₂ assimilation. In the presence of aspartate, the apparent Michaelis constant (malate) for malate decarboxylation to pyruvate by chloroplasts decreased from 6 to 0.67 millimolar while the calculated V_max for this process increased from 1.3 to 3.3 micromoles per milligram chlorophyll. Aspartate itself was not metabolized. It was concluded that the processes mediating the transport of phosphate, 3-phosphoglycerate, and dihydroxyacetone phosphate transport on the one hand, and also of malate might differ from those previously described for chloroplasts from C₃ plants.     

Stimulation of the oxidative decarboxylation of indole-3-acetic acid in citrus tissues by ethylene

Ethylene has been shown to stimulate the degradation of indole-3-acetic acid (IAA) in citrus leaf tissues via the oxidative decarboxylation pathway, resulting in the accumulation of indole-3-carboxylic acid (ICA). Preliminary data indicated that ethylene stimulates only the first step of this pathway, i.e. the decarboxylation of IAA which leads to the formation of indole-3-methanol. The effect of ethylene seems to be a specific one since 2,5-norbornadiene, an ethylene action inhibitor, significantly inhibited the stimulation of IAA decarboxylation by ethylene. It has long been suggested that peroxidase or a specific form of the peroxidase complex (`IAA oxidase') catalyse this step. However, we did not observe a clear effect of ethylene on the peroxidase system. An alternative possibility, that the stimulatory effect of ethylene on IAA catabolism results from increased formation of hydrogen peroxide (H₂O₂), a co-factor for peroxidase activity, was verified by direct measurements of H₂O₂ in the tissues or by assaying the activity of gluthathione reductase, which has been shown to be induced by oxygen species. This possibility is further supported by the observations showing that IAA decarboxylation in control tissues was enhanced to the level detected in ethylene-treated tissues by application of H₂O₂.     

Modulation of Rubisco Activity during the Diurnal Phases of the Crassulacean Acid Metabolism Plant Kalanchoë daigremontiana

The regulation of Rubisco activity was investigated under high, constant photosynthetic photon flux density during the diurnal phases of Crassulacean acid metabolism in Kalanchoë daigremontiana Hamet et Perr. During phase I, a significant period of nocturnal, C₄-mediated CO₂ fixation was observed, with the generated malic acid being decarboxylated the following day (phase III). Two periods of daytime atmospheric CO₂ fixation occurred at the beginning (phase II, C₄–C₃ carboxylation) and end (phase IV, C₃–C₄ carboxylation) of the day. During the 1st h of the photoperiod, when phosphoenolpyruvate carboxylase was still active, the highest rates of atmospheric CO₂ uptake were observed, coincident with the lowest rates of electron transport and minimal Rubisco activity. Over the next 1 to 2 h of phase II, carbamylation increased rapidly during an initial period of decarboxylation. Maximal carbamylation (70%–80%) was reached 2 h into phase III and was maintained under conditions of elevated CO₂ resulting from malic acid decarboxylation. Initial and total Rubisco activity increased throughout phase III, with maximal activity achieved 9 h into the photoperiod at the beginning of phase IV, as atmospheric CO₂ uptake recommenced. We suggest that the increased enzyme activity supports assimilation under CO₂-limited conditions at the start of phase IV. The data indicate that Rubisco activity is modulated in-line with intracellular CO₂ supply during the daytime phases of Crassulacean acid metabolism.     

Determination of uronic acid in uronides by application of a new microgram-scale isotope dilution method for carbon dioxide.

Conditions studied earlier by Tracey [(1948) Biochem. J.43, 185] are used for acid decarboxylation in sealed tubes of uronide samples supplemented with 6-¹⁴C-labeled uronic acid. The specific activity of the CO₂ evolved is measured as the ratio of radioactivity to area of the CO₂ peak obtained in a gas chromatogram. By appropriate standardization, samples containing some 60 nmol of uronic acid can be analyzed with reproducibility and apparent accuracy of about ±2% (mean deviation). The techniques developed for uronic acid analysis should apply with minor modification to any problem requiring accurate measurement of CO₂ in small amounts.     

Partial structure and properties of the ferredoxin from Rhodymenia palmata

A ferredoxin of MW 11 000 was isolated from the marine alga Rhodymenia palmata (Palmaria palmata). In its oxidised form the ferredoxin had absorption maxima at 276, sh 281, 328, 423 and 465 nm, and contained a single [2Fe-2S] cluster. The midpoint potential of the ferredoxin was ?400 mV and it effectively mediated electron transport in NADP⁺-photoreduction by higher plant chloroplasts, and pyruvate decarboxylation by the phosphoroclastic system of an anacrobic bacterium. The amino acid composition was Lys₃, His₁, Arg₁, Asx₁₂, Thr₉, Ser₈, Glx₁₃, Pro₄, Gly₈, Ala₇, Cys₅, Val₈, Ile₄, Leu₉, Tyr₄, Phe₂; tryptophan and methionine were absent from the molecule. The N-terminal amino acid region consisting of ca half the total amino acid sequence was determined using an automatic sequencer.     

Photorespiration-deficient Mutants of Arabidopsis thaliana Lacking Mitochondrial Serine Transhydroxymethylase Activity

Three allelic mutants of Arabidopsis thaliana which lack mitochondrial serine transhydroxymethylase activity due to a recessive nuclear mutation have been characterized. The mutants were shown to be deficient both in glycine decarboxylation and in the conversion of glycine to serine. Glycine accumulated as an end product of photosynthesis in the mutants, largely at the expense of serine, starch, and sucrose formation. The mutants photorespired CO₂ at low rates in the light, but this evolution of photorespiratory CO₂ was abolished by provision of exogenous NH₃. Exogenous NH₃ was required by the mutants for continued synthesis of glycine under photorespiratory conditions. These and related results with wild-type Arabidopsis suggested that glycine decarboxylation is the sole site of photorespiratory CO₂ release in wild-type plants but that depletion of the amino donors required for glyoxylate amination may lead to CO₂ release from direct decarboxylation of glyoxylate. Photosynthetic CO₂ fixation was inhibited in the mutants under atmospheric conditions which promote photorespiration but could be partially restored by exogenous NH₃. The magnitude of the NH₃ stimulation of photosynthesis indicated that the increase was due to the suppression of glyoxylate decarboxylation. The normal growth of the mutants under nonphotorespiratory atmospheric conditions indicates that mitochondrial serine transhydroxymethylase is not required in C₃ plants for any function unrelated to photorespiration.     

On the decarboxylation of α-keto acids by isolated chloroplasts

The mechanism of the decarboxylation of α-keto acids by isolated chloroplasts has been studied with the aid of superoxide dismutase and catalase. Using photosynthetic and enzymatic systems, which are known to catalyze peroxidic oxidations, we have been able to demonstrate that both the superoxide free radical ion and H₂O₂ are necessary for maximal rates of decarboxylation. In isolated chloroplasts, an auto-oxidizable electron acceptor as well as an electron donor for Photosystem I are absolute requirements for the decarboxylation. H₂O₂ seems to be the primary oxidant in the decarboxylation of pyruvate or glyoxylate by isolated chloroplasts. A secondary rate of decarboxylation is superimposed on the primary one, mediated by superoxide free radical ion. Mn²⁺ stimulates the decarboxylation probably via intermediarily-formed Mn³⁺ in a reaction, which is neither inhibited by catalase nor by superoxide dismutase. A decarboxylation of pyruvate or glyoxylate by isolated chloroplasts in the presence of NADP⁺ is initiated, as soon as the available NADP⁺ is fully reduced. In this case, the open-chain electron transport seems to switch from NADP⁺ to oxygen as the terminal electron acceptor.     

Transfer RNA-Peroxidase Interaction: INHIBITION OF INDOLE-3-ACETIC ACID OXIDATION

Transfer RNA from wheat germ, yeast, and Escherichia coli inhibited the indoleacetic acid (IAA)-induced spectral change in horseradish peroxidase (EC 1.11.1.7) and the decarboxylation of IAA. The inhibition was limited to a delay after which the increase in A₄₂₇ and the decarboxylation of IAA resumed at the same rate as in the control; the duration of the inhibition was dependent on, but not proportional to, the concentration of tRNA. Alkaline hydrolysis destroyed the inhibitory activity of tRNA. The inhibition was completely abolished when the tRNA was added 30 seconds after IAA. Thus, the tRNA appears not to react with the enzyme intermediates formed during the reaction with IAA. The inhibition by tRNA was rapidly reversed by H₂O₂ or additional IAA, but not by 2,4-dichlorophenol. Results suggest that the tRNA interferes with the initial reaction between IAA and the heme moiety of free peroxidase, thus preventing the formation of highly active enzyme intermediates essential for IAA degradation.     

Malate decarboxylation in isolated mitochondria from the crassulacean acid metabolism plant Sedum praealtum

Mitochondria isolated from the Crassulacean acid metabolism plant Sedum praealtum were demonstrated to decarboxylate added malate at basal rates of 30–50 μmol mg^?1 original chlorophyll h^?1. The basal rate could be stimulated markedly by the addition of ADP, oxaloacetic acid, an uncoupler of oxidative phosphorylation, or NAD, with maximum rates of 70–100 μmol mg^?1 original chlorophyll h^?1 observed. These observed rates were high enough to account for a large proportion of the estimated rate of malate decarboxylation in vivo. The major products of malate oxidation by the mitochondria in most cases were found to be pyruvate and CO₂, indicating that malate oxidation in these mitochondria proceeds mainly through NAD malic enzyme rather than NAD malate dehydrogenase. Under conditions employed little of the pyruvate formed was further oxidized, suggesting a fate other than oxidation (conversion to starch) for this pyruvate. Malate decarboxylation by mitochondria and by partially purified NAD malic enzyme was markedly inhibited by NaHCO₃. A possible physiological role is suggested for this inhibition as a feedback control on the enzyme.     

Bacterial Decarboxylation of o-Phthalic Acids

The decarboxylation of phthalic acids was studied with Bacillus sp. strain FO, a marine mixed culture ON-7, and Pseudomonas testosteroni. The mixed culture ON-7, when grown anaerobically on phthalate but incubated aerobically with chloramphenicol, quantitatively converted phthalic acid to benzoic acid. Substituted phthalic acids were also decarboxylated: 4,5-dihydroxyphthalic acid to protocatechuic acid; 4-hydroxyphthalic and 4-chlorophthalic acids to 3-hydroxybenzoic and 3-chlorobenzoic acids, respectively; and 3-fluorophthalic acid to 2-and 3-fluorobenzoic acids. Bacillus sp. strain FO gave similar results except that 4,5-dihydroxyphthalic acid was not metabolized, and both 3- and 4-hydroxybenzoic acids were produced from 4-hydroxyphthalic acid. P. testosteroni decarboxylated 4-hydroxyphthalate (to 3-hydroxybenzoate) and 4,5-dihydroxyphthalate but not phthalic acid and halogenated phthalates. Thus, P. testosteroni and the mixed culture ON-7 possessed 4,5-dihydroxyphthalic acid decarboxylase, previously described in P. testosteroni, that metabolized 4,5-dihydroxyphthalic acid and specifically decarboxylated 4-hydroxyphthalic acid to 3-hydroxybenzoic acid. The mixed culture ON-7 and Bacillus sp. strain FO also possessed a novel decarboxylase that metabolized phthalic acid and halogenated phthalates, but not 4,5-dihydroxyphthalate, and randomly decarboxylated 4-hydroxyphthalic acid. The decarboxylation of phthalic acid is suggested to involve an initial reduction to 1,2-dihydrophthalic acid followed by oxidative decarboxylation to benzoic acid.     

Structure and Biosynthesis of Cuticular Lipids: Hydroxylation of Palmitic Acid and Decarboxylation of C(28), C(30), and C(32) Acids in Vicia faba Flowers

The structure and composition of the cutin monomers from the flower petals of Vicia faba were determined by hydrogenolysis (LiAlH₄) or deuterolysis (LiAlD₄) followed by thin layer chromatography and combined gas-liquid chromatography and mass spectrometry. The major components were 10, 16-dihydroxyhexadecanoic acid (79.8%), 9, 16-dihydroxyhexadecanoic acid (4.2%), 16-hydroxyhexadecanoic acid (4.2%), 18-hydroxyoctadecanoic acid (1.6%), and hexadecanoic acid (2.4%). These results show that flower petal cutin is very similar to leaf cutin of V. faba. Developing petals readily incorporated exogenous [1-¹⁴C]palmitic acid into cutin. Direct conversion of the exogeneous acid into 16-hydroxyhexadecanoic acid, 10, 16-dihydroxy-, and 9, 16-dihydroxyhexadecanoic acid was demonstrated by radio gas-liquid chromatography of their chemical degradation products. About 1% of the exogenous [1-¹⁴C]palmitic acid was incorporated into C₂₇, C₂₉, and C₃₁n-alkanes, which were identified by combined gas-liquid chromatography and mass spectrometry as the major components of the hydrocarbons of V. faba flowers. The radioactivity distribution among these three alkanes (C₂₇, 15%; C₂₉, 48%; C₃₁, 38%) was similar to the per cent composition of the alkanes (C₂₇, 12%; C₂₉, 43%; C₃₁, 44%). [1-¹⁴C]Stearic acid was also incorporated into C₂₇, C₂₉, and C₃₁n-alkanes in good yield (3%). Trichloroacetate, which has been postulated to be an inhibitor of fatty acid elongation, inhibited the conversion of [1-¹⁴C]stearic acid to alkanes, and the inhibition was greatest for the longer alkanes. Developing flower petals also incorporated exogenous C₂₈, C₃₀, and C₃₂ acids into alkanes in 0.5% to 5% yields. [G-³H]n-octacosanoic acid (C₂₈) was incorporated into C₂₇, C₂₉, and C₃₁n-alkanes. [G-³H]n-triacontanoic acid (C₃₀) was incorporated mainly into C₂₉ and C₃₁ alkanes, whereas [9, 10, 11-³H]n-dotriacontanoic acid (C₃₂) was converted mainly to C₃₁ alkane. Trichloroacetate inhibited the conversion of the exogenous acids into alkanes with carbon chains longer than the exogenous acid, and at the same time increased the amount of the direct decarboxylation product formed. These results clearly demonstrate direct decarboxylation as well as elongation and decarboxylation of exogenous fatty acids, and thus constitute the most direct evidence thus far obtained for an elongation-decarboxylation mechanism for the biosynthesis of alkanes.     

The importance of transamination and decarboxylation in phenylalanine metabolism in vivo in the rat

The extent of hydroxylation, transamination, and decarboxylation in the metabolism of excess phenylalanine in vivo has been examined by measuring the amount of radioactivity from [¹⁴C]phenylalanine that is converted to ¹⁴CO₂ and urinary metabolites. Transamination and direct decarboxylation represent only 6% of total phenylalanine metabolism. The major aromatic metabolites in the urine after phenylalanine loading are phenylacetylglycine, phenylacetic acid, phenylpyruvate, and phenylalanine. A small but significant portion (1.5%) of phenylalanine is degraded to nonaromatic compounds. The maximum phenylalanine oxidation in vivo is approximately $\text{75\%}\text{24}$ h at saturating concentrations of phenylalanine; thus, the major route of degradation of phenylalanine in the rat, even when intake is high, is via formation of acetoacetic acid and fumaric acid.     

Quantum Requirement for Photosynthesis in Sedum praealtum during Two Phases of Crassulacean Acid Metabolism

The quantum requirement (QR) for photosynthesis in Sedum praealtum, a Crassulacean acid metabolism plant, was compared with that of wheat, a C₃ plant, and maize, a C₄ plant, at 30 C. During the deacidification phase in S. praealtum, approximately 16 moles quanta were absorbed per mole malate consumed. This is equivalent to 16 moles quanta per mole CO₂ fixed, assuming 1 mole CO₂ is assimilated per mole malate decarboxylated. This QR for Crassulacean acid metabolism is similar to that of the C₃ or C₄ plant under atmospheric conditions, even though there are considerable differences in the biochemistry of photosynthesis. During late-afternoon C₃-like fixation of atmospheric CO₂ in S. praealtum, the QR was relatively high with values of 41 under 21% O₂ and 19 under 2% O₂. During the deacidification phase in S. praealtum, the relatively low QR can be accounted for by the repression of photorespiration and saturation of photosynthesis from the elevated CO₂ concentration in the leaves during malate decarboxylation.