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1.
In one group of C 4 species, including Chloris gayana, C 4 acids are decarboxylated via phosphoenolpyruvate carboxykinase to give phosphoenolpyruvate as the initial C 3 product. This paper presents an analysis of the kinetics of labeling of various photosynthetic intermediates in Chloris gayana leaves exposed to 14CO 2, and the pool sizes of these intermediates, primarily to provide information about the subsequent metabolism of phosphoenolpyruvate. Saturation labeling of the C-4 of aspartate and malate, and the C-1 of 3-phosphoglycerate, indicated photosynthetically active pools of 0.45, 0.22, and 0.95 μol/mg chlorophyll, respectively. For aspartate and 3-phosphoglycerate, the total leaf pools and the photosynthetic pools were of similar size, but the total pool of malate was about 100 times larger than the photosynthetically active pool. From the relative rates of labeling of phosphoenolpyruvate, pyruvate, alanine, and C-1, C-2 plus C-3 of aspartate, during steady-state 14CO 2 assimilation, relative pool sizes were calculated to be about 10:11:78:100, respectively. Pulse/chase labeling of leaves provided estimates of relative photosynthetic pool sizes in the ratio of about 6:15:90:100, respectively, where aspartate is arbitrarily assigned a value of 100 in both cases. Notably, labeling of alanine was consistent with its derivation from the C-1, C-2 plus C-3 carbons of aspartate, and the alanine pool was at least eight times larger than the phosphoenolpyruvate pool that showed similar labeling kinetics. Results were consistent with the view that at least most of the phosphoenolpyruvate produced by C 4 acid decarboxylation is metabolized via alanine. 相似文献
2.
3,4-Dichloropropionanilide- 14C (propanil) labeled in either the C-1 or C-3 carbon atoms of the propionic acid moiety was applied to the roots of pea ( Pisum sativum L.) and rice ( Oryza sativa L.) plants in nutrient solution (0.1 m m-0.28 m m). Radioactivity was detected throughout the treated plants, but the greatest labeling was found in the roots. None of the products that contained aniline were radioactive, suggesting that the plants split the propionic acid moiety from propanil. The fate of the propionate moiety of propanil was determined by recovery of 14CO 2 from plants exposed to propanil- 14C. The time-course of the 14CO 2 production demonstrated that the intact propionic acid was cleaved from the propanil and subsequently catabolized by the β-oxidation catabolic sequence. The appearance of radioactivity in the shoots was attributed to the incorporation of products of propionate metabolism. Both the susceptible pea plants and the tolerant rice plants converted a high percentage of the administered propanil- 14C to 14CO 2. 相似文献
3.
The metabolism of fixed 14CO 2 and the utilization of the C-4 carboxyl of malate and aspartate were examined during photosynthetic induction in Flaveria trinervia, a C 4 dicot of the NADP-malic enzyme subgroup. Pulse/chase experiments indicated that both malate and aspartate appeared to function directly in the C 4 cycle at all times during the induction period (examined after 30 seconds, 5 minutes and 20 minutes illumination). However, the rate of loss of 14C-label from the C-4 position of malate plus aspartate was relatively slow after 30 seconds of illumination, compared to treatments after 5 or 20 minutes of illumination. Similarly, the appearance of label in other photosynthetic products (e.g. 3-phosphoglycerate, sugar phosphates, alanine) during the chase periods was generally slower after only 30 seconds of leaf illumination, compared to that after 5 of 20 minutes illumination. This may be due to the lower rate of photosynthesis after 30 seconds illumination. The appearance of label in carbons 1→3 of each C 4 acid during the chase periods was relatively slow after either 30 seconds or 5 minutes illumination, while there was a relatively rapid accumulation of label in carbons 1→3 of both C 4 acids after 20 minutes illumination. Thus, while the turnover rate of the 14C-4 label in both C 4 acids increased only during the first 5 minutes of the induction period, only later during induction is there an increased rate of appearance of label in other carbon atoms of the C 4 acids. The implied source of 14C for labeling of the 1→3 positions of the C 4 acids is an apparent carbon flux from 3-phosphoglycerate of the reductive pentose phosphate pathway to phosphoenolpyruvate of the C 4 cycle. 相似文献
4.
Chlorobium limicola was grown on a mineral salts medium with CO 2 as the main carbon source supplemented with specifically labeled 14C propionate and the incorporation of 14C into alanine ( intracellular pyruvate), aspartate ( oxaloacetate), and glutamate ( -ketoglutarate) was studied in long term labeling experiments. During growth in presence of propionate 30% of the cell carbon were derived from propionate and 70% from CO 2. Propionate was not oxidized to CO 2.All three amino acids were found to be labeled. The labeling patterns indicate that propionate was assimilated via propionyl CoA, methylmalonyl CoA and succinyl CoA. When 1- 14C propionate was the labeled precursor no radioactivity was found in the carboxyl group(s) of alanine, aspartate and glutamate, excluding the incorporation of propionate into the amino acids via succinate oxidation to fumarate. With 1- 14C propionate preferentially aspartate (C-3) and glutamate (C-2) became labeled, with 2- 14C propionate alanine (C-3) and glutamate (C-4). These findings indicate that propionate was incorporated into the amino acids via succinyl CoA, -ketoglutarate, isocitrate, and citrate, followed by a si-type cleavage of citrate to oxaloacetate and acetyl CoA (or acetate). Similar experiments with U- 14C acetate confirm these conclusions. Thus, all reactions of the proposed reductive tricarboxylic acid cycle could be demonstrated in autotrophically growing cells. 相似文献
5.
Production of 6-ethyl-5-hydroxy-2,7-dimethoxy-1,4-naphthoquinone was obtained by growth of Hendersonula toruloidea on Czapek-Dox broth supplemented with malt extract. Stationary cultures were grown at 28°C for 21–22 days yielding about 6 mg of metabolite per 700 ml of culture fluid. The best incorporations of isotopic tracers were obtained by addition at the 20th day of growth, followed by harvest 24–48 hr later. With [2- 14C]acetate, incorporation values were in the range of 0.1–0.3% with dilution values from 2000 to 5900. With [1- 14C]propionate, incorporations were much lower (0.04%) and dilutions much higher (120,000). Activity from [ ]methionine was incorporated only into the OCH 3 groups (incorporation values, 0.5–0.7%). Nuclear magnetic resonance studies confirmed that propionate was not a precursor. Using [1,2- 13C]acetate, substantial enrichments were obtained at all carbon atoms except those of the OCH 3 groups. The following pairs of carbon atoms were shown to be derived from acetate units: C-1 + 2, C-3 + 4, C-5 + 10, C-6 + 7, C-8 + 9, C-11 + 12. The biosynthetic pathway is clearly that of acetate plus polymalonate. Experiments with [2- 13C 2H 3]acetate suggested that the “starter” acetate unit was located at positions C-12 + 11. 相似文献
6.
Variable pH 13C NMR and 1H NMR spectroscopic studies of the β-cyclodextrin (β-CD) in alkaline aqueous solutions revealed that β-CD does not deprotonate at pH < 12.0. Further increase in solution pH results in the deprotonation of OH-groups adjacent to C-2 and C-3 carbon atoms of β-CD glucopyranose units, whereas the deprotonation of OH-groups adjacent to C-6 carbon atoms is expressed less markedly. The p Ka values for β-CD OH-groups adjacent to C-2 and C-3 carbon atoms are rather close, p Ka1,2 being 13.5 ± 0.2 (22.5 °C). 相似文献
7.
Evidence is presented to support the hypothesis that serine, rather than 3-phosphoglycerate of the Calvin cycle, is a precursor of the tricarboxylic acid cycle during photosynthesis by the higher plant, Vicia faba. Identification of the serine intermediate is based upon a unique C 1 > C 2 > C 3 isotope distribution for that metabolite following the fixation of 14CO 2. This labeling pattern, while incompatible with an origin either in the Calvin cycle or the glycolate pathway, satisfies a critical criterion for the 3-carbon precursor of the anomalously labeled organic acids. The predominant carboxyl carbon atom labeling of serine reflects either a mixing of two pools of that metabolite, ie., C 1 = C 2 > C 3 and C 1 > C 2 = C 3, or a higher order of complexity in its synthesis. An anomalous C 1 = C 2 > C 3 < C 4 distribution for aspartate, however, suggests an origin by the carboxylation of a 3-carbon intermediate related to serine which has a C 1 = C 2 > C 3 distribution. The latter distribution has been proposed for the serine intermediate of the postulated formate pathway. This pathway is described by the generalized metabolic sequence: CO 2 → formate → serine → organic acids. Corresponding carbon atom distributions for citrate (C 1 > C 2), aspartate (C 2 > C 3), and serine (C 2 > C 3) belie a precursor-product relationship with alanine (C 2 = C 3), which is a molecular parameter of the Calvin cycle product, 3-phosphoglycerate. 相似文献
8.
When ribulose-1,5-bisphosphate carboxylase is assayed under N 2 using [ 3H]ribulose 1,5-bisphosphate and 14CO 2, [ 3H]3-phosphoglycerate and [ 14C]3-phosphoglycerate are produced in nonstoichiometric amounts in a ratio which approaches 7 at low concentrations of CO 2 (2 micromolar) assuming a 1:1 ratio at Vmax (280 micromolar). The log of the molar ratio varies as a linear function of log[CO 2]. Nonstoichiometry could be explained by CO 2 contaminatio of the reactants or tritium contamination of the products. However, the magnitude of CO 2 contamination required (18 ± 4 micromolar) is far in excess of controlled CO 2 (<0.1 micromolar), and the required tritium contaminant would have to vary from 30 to 85% of the purified 3-phosphoglycerate at the 58 and 2 micromolar CO 2 assay levels, respectively. This contrasts with detectable tritium contamination which is only 1 to 4% and correctable. Nonstoichiometry is evident using either 1 or 5 labeled [ 3H]ribulose 1,5-bisphosphate. When 3-phosphoglycerate is reisolated as glycerate the 3H/ 14C ratio remains unchanged. 相似文献
9.
The products of short time photosynthesis and of enhanced dark 14CO 2 fixation (illumination in helium prior to addition of 14CO 2 in dark) by Chlorella pyrenoidosa and Anacystis nidulans were compared. Glycerate 3-phosphate, phosphoenolpyruvate, alanine, and aspartate accounted for the bulk of the 14C assimilated during enhanced dark fixation while hexose and pentose phosphates accounted for the largest fraction of isotope assimilated during photosynthesis. During the enhanced dark fixation period, glycerate 3-phosphate is carboxyl labeled and glucose 6-phosphate is predominantly labeled in carbon atom 4 with lesser amounts in the upper half of the C 6 chain and traces in carbon atoms 5 and 6. Tracer spread throughout all the carbon atoms of photosynthetically synthesized glycerate 3-phosphate and glucose 6-phosphate. During the enhanced dark fixation period, there was a slow formation of sugar phosphates which subsequently continued at 5 times the initial rate long after the cessation of 14CO 2 uptake. To explain the kinetics of changes in the labelling patterns and in the limited formation of the sugar phosphates during enhanced dark CO 2 fixation, the suggestion is made that most of the reductant mediating these effects did not have its origin in the preillumination phase. It is concluded that a complete photosynthetic carbon reduction cycle operates to a limited extent, if at all, in the dark period subsequent to preillumination. 相似文献
10.
The Gram positive anaerobe Acetobacterium woodii is able to grow autotrophically with a mixture of H 2 and CO 2 as the energy and carbon source. The question, by which pathway CO 2 is assimilated, was studied using long term isotope labeling.Autotrophically growing cultures produced acetate parallel to cell proliferation, and, when U-[ 14C]acetate was present as tracer, incorporated radioactivity into all cell fractions. The specific radioactivity and the label positions were determined for those representative cell compounds which biosynthetically originated directly from acetyl CoA (N-acetyl groups), pyruvate (alanine), oxaloacetate (aspartate), -ketoglutarate (glutamate), and hexosephosphates (glucosamine). Per mol compound the same amount of labeled acetate was incorporated into N-acetyl groups, alanine (C-2, C-3), aspartate (C-2, C-3), and twice the amount into glutamate (C-2, C-3, C-4, C-5) and into glucosamine. Consequently, the unlabeled carbon atoms of the C 3–C 6 compounds must have been derived from CO 2 by carboxylation subsequent to acetyl CoA synthesis. When 0.2 mM 2-[ 14C]pyruvate was added to autotrophically growing cultures, also a substantial amount of radioactivity was incorporated. Two important differences in comparison to the acetate experiment were observed: The N-acetyl groups were almost unlabeled and glutamate contained the same specific radioactivity as alanine or aspartate.These data showed that acetyl CoA is the central intermediate for biosynthesis and excluded the operation of the Calvin cycle in A. woodii. The results were consistent with the operation of a different autotrophic CO 2 fixation pathway in which CO 2 is converted into acetyl CoA by total synthesis via methyltetrahydrofolate; acetyl CoA is then further reductively carboxylated to pyruvate. 相似文献
11.
1. Isolated perfused goat udders supplied with glucose, acetate and amino acids were infused for several hours with NaH 14CO 3. 2. Lactose, milk-fat fatty acids and glycerol had very little radioactivity. The specific radioactivity (counts./min./mg. of C) of milk citrate was 9–16% that of the carbon dioxide in the perfusion fluid and 19% that estimated for tissue carbon dioxide. The specific radioactivity of tissue citrate resembled that of milk citrate. 3. The radioactivity in citrate was predominantly in C-6, suggesting some carboxylation of α-oxoglutarate in addition to carboxylation of C 3 compounds. 4. [1- 14C]Glutamate was infused in a similar experiment, and milk citrate radioactivity was predominantly in C-1+C-5. 5. The results are discussed in relation to the contribution of glucose and acetate carbon to citrate. The implications of the carboxylation of α-oxoglutarate are considered. 相似文献
12.
Production of 6-ethyl-5-hydroxy-2,7-dimethoxy-1,4-naphthoquinone was obtained by growth of Hendersonula toruloidea on Czapek-Dox broth supplemented with malt extract. Stationary cultures were grown at 28°C for 21–22 days yielding about 6 mg of metabolite per 700 ml of culture fluid. The best incorporations of isotopic tracers were obtained by addition at the 20th day of growth, followed by harvest 24–48 hr later. With [2- 14C]acetate, incorporation values were in the range of 0.1–0.3% with dilution values from 2000 to 5900. With [1- 14C]propionate, incorporations were much lower (0.04%) and dilutions much higher (120,000). Activity from [ 14CH3]methionine was incorporated only into the OCH 3 groups (incorporation values, 0.5–0.7%). Nuclear magnetic resonance studies confirmed that propionate was not a precursor. Using [1,2- 13C]acetate, substantial enrichments were obtained at all carbon atoms except those of the OCH 3 groups. The following pairs of carbon atoms were shown to be derived from acetate units: C-1 + 2, C-3 + 4, C-5 + 10, C-6 + 7, C-8 + 9, C-11 + 12. The biosynthetic pathway is clearly that of acetate plus polymalonate. Experiments with [2- 13C 2H 3]acetate suggested that the “starter” acetate unit was located at positions C-12 + 11. 相似文献
13.
Mass spectrometric techniques were used to trace the incorporation of [ 18O]oxygen into metabolites of the photorespiratory pathway. Glycolate, glycine, and serine extracted from leaves of the C 3 plants, Spinacia oleracea L., Atriplex hastata, and Helianthus annuus which had been exposed to [ 18O]oxygen at the CO 2 compensation point were heavily labeled with 18O. In each case one, and only one of the carboxyl oxygens was labeled. The abundance of 18O in this oxygen of glycolate reached 50 to 70% of that of the oxygen provided after only 5 to 10 seconds exposure to [ 18O]oxygen. Glycine and serine attained the same final enrichment after 40 and 180 seconds, respectively. This confirms that glycine and serine are synthesized from glycolate. The labeling of photorespiratory intermediates in intact leaves reached a mean of 59% of that of the oxygen provided in the feedings. This indicates that at least 59% of the glycolate photorespired is synthesized with the fixation of molecular oxygen. This estimate is certainly conservative owing to the dilution of labeled oxygen at the site of glycolate synthesis by photosynthetic oxygen. We examined the yield of 18O in glycolate synthesized in vitro by isolated intact spinach chloroplasts in a system which permitted direct sampling of the isotopic composition of the oxygen at the site of synthesis. The isotopic enrichment of glycolate from such experiments was 90 to 95% of that of the oxygen present during the incubation. The carboxyl oxygens of 3-phosphoglycerate also became labeled with 18O in 20- and 40-minute feedings with [18O]oxygen to intact leaves at the CO2 compensation point. Control experiments indicated that this label was probably due to direct synthesis of 3-phosphoglycerate from glycolate during photorespiration. The mean enrichment of 3-phosphoglycerate was 14 ± 4% of that of glycine or serine, its precursors of the photorespiratory pathway, in 10 separate feeding experiments. It is argued that this constant dilution of label indicates a constant stoichiometric balance between photorespiratory and photosynthetic sources of 3-phosphoglycerate at the CO2 compensation point. Oxygen uptake sufficient to account for about half of the rate of 18O fixation into glycine in the intact leaves was observed with intact spinach chloroplasts. Oxygen uptake and production by intact leaves at the CO2 compensation point indicate about 1.9 oxygen exchanged per glycolate photorespired. The fixation of molecular oxygen into glycolate plus the peroxisomal oxidation of glycolate to glyoxylate and the mitochondrial conversion of glycine to serine can account for up to 1.75 oxygen taken up per glycolate. These studies provide new evidence which supports the current formulation of the pathway of photorespiration and its relation to photosynthetic metabolism. The experiments described also suggest new approaches using stable isotope techniques to study the rate of photorespiration and the balance between photorespiration and photosynthesis in vivo. 相似文献
14.
One-carbon metabolic transformations associated with cell carbon synthesis and methanogenesis were analyzed by long- and short-term 14CH 3OH or 14CO 2 incorporation studies during growth and by cell suspensions. 14CH 3OH and 14CO 2 were equivalently incorporated into the major cellular components (i.e., lipids, proteins, and nucleic acids) during growth on H 2-CO 2-methanol. 14CH 3OH was selectively incorporated into the C-3 of alanine with decreased amounts fixed in the C-1 and C-2 positions, whereas 14CO 2 was selectively incorporated into the C 1 moiety with decreasing amounts assimilated into the C-2 and C-3 atoms. Notably, 14CH 4 and [3- 14C]alanine synthesized from 14CH 3OH during growth shared a common specific activity distinct from that of CO 2 or methanol. Cell suspensions synthesized acetate and alanine from 14CO 2. The addition of iodopropane inhibited acetate synthesis but did not decrease the amount of 14CH 3OH or 14CO 2 fixed into one-carbon carriers (i.e., methyl coenzyme M or carboxydihydromethanopterin). Carboxydihydromethanopterin was only labeled from 14CH 3OH in the absence of hydrogen. Cell extracts catalyzed the synthesis of acetate from 14CO (~1 nmol/min per mg of protein) and an isotopic exchange between CO 2 or CO and the C-1 of pyruvate. Acetate synthesis from 14CO was stimulated by methyl B 12 but not by methyl tetrahydrofolate or methyl coenzyme M. Methyl coenzyme M and coenzyme M were inhibitory to acetate synthesis. Cell extracts contained high levels of phosphotransacetylase (>6 μmol/min per mg of protein) and acetate kinase (>0.14 μmol/min per mg of protein). It was not possible to distinguish between acetate and acetyl coenzyme A as the immediate product of two-carbon synthesis with the methods employed. 相似文献
15.
Mass spectral analysis of T-2 toxin formed during the growth of Fusarium sporotrichioides (ATCC 24043) in the presence of H 218O showed incorporation of up to three 18O atoms per toxin molecule. The carbonyl oxygens of the acetates at C-4 and C-15 and of the isovalerate at C-8 were derived from H 2O. Toxin formed in the presence of 18O molecular oxygen incorporated up to six 18O atoms per toxin molecule. The overall incorporation was 78 and 92% of toxin molecules labeled for H 218O and 18O 2 labeled samples, respectively. The oxygens of position 1, the 12,13-epoxide, and the hydroxyl groups at C-3, C-4, C-8, and C-15 were all derived from molecular oxygen. 相似文献
16.
[5,6- 14C, 13C 2]Nicotinic acid was prepared from [ 14C, 13C]methyl iodide via nitromethane, 2-nitroacetaldehyde oxime, 3-nitroquinoline, 3-aminoquinoline, and quinoline in 20% overall yield. Administration of this material to Nicotiana tabacum and N. glauca afforded labeled anabasine, anatabine, nicotine, and nornicotine. Qualitative and quantitative incorporation (0.07–4.5% specific incorporation) was determined by radioactive assay and by examination of the 13C NMR spectra of these alkaloids. Satellites due to spin-spin coupling of the incorporated contiguous 13C atoms were observed at the resonances due to C-5 and C-6 in anabasine, nicotine, and nornicotine. In anatabine, satellites were found at C-5, C-6, C-5′, and C-6′. 相似文献
17.
The chlorophyll-based specific activity of cytochrome oxidase and three exclusively mitochondrial enzymes of the tricarboxylic acid cycle showed little variation between leaves of C 3 and C 4 plants or between mesophyll and bundle sheath cells of Atriplex spongiosa and Sorghum bicolor. However, a large, light-dependent transfer of label from intermediates of the tricarboxylic acid cycle to photosynthetic products was a feature of leaves of C 4 plants. This light-dependent transfer of label was barely detectable in leaves of C 3 plants and in leaves of F 1 and F 3 hybrids of Atriplex rosea (C 4) and Atriplex patula spp hastata (C 3). The light-dependent transfer of label to photosynthetic products in leaves of C 4 plants was inhibited by the tricarboxylic acid cycle inhibitors malonate and fluoroacetate. The requirement for continued tricarboxylic acid cycle activity was also indicated in experiments with specifically labeled succinate- 14C. These experiments, together with the distribution of 14C in glucose prepared from sucrose- 14C formed during the metabolism of succinate-2,3- 14C, confirmed that the photosynthetic metabolism of malate and aspartate derived from the tricarboxylic acid cycle, and not the refixation of respiratory CO 2, was the main path of carbon from the cycle to photosynthesis. 相似文献
18.
Desulfovibrio vulgaris (Marburg) was grown on hydrogen plus sulfate as sole energy source and acetate plus CO 2 as the sole carbon sources. The incorporation of U- 14C acetate into alanine, aspartate, glutamate, and ribose was studied. The labelling data show that alanine is synthesized from one acetate (C-2 + C-3) and one CO 2 (C-1), aspartate from one acetate (C-2 + C-3) and two CO 2 (C-1 + C-4), glutamate from two acetate (C-1–C-4) and one CO 2 (C-5), and ribose from 1.8 acetate and 1.4 CO 2. These findings indicate that in Desulfovibrio vulgaris (Marburg) pyruvate is formed via reductive carboxylation of acetyl-CoA, oxaloacetate via carboxylation of pyruvate or phosphoenol pyruvate, and -ketoglutarate from oxaloacetate plus acetyl-CoA via citrate and isocitrate. Since C-5 of glutamate is derived from CO 2, citrate must have been formed via a (R)-citrate synthase rather than a(S)-citrate synthase. The synthesis of ribose from 1.8 mol of acetate and 1.4 mol of CO 2 excludes the operation of the Calvin cycle in this chemolithotrophically growing bacterium. 相似文献
19.
In mechanistic studies by isotope-exchange tecniques of the conversion of D-fructose and D-glucose into 2-(hydroxyacetyl)furan, it was shown that both sugars are converted in acidified, tritiated water into the furan containing essentially no carbon-bound tritium. As the hydroxymethyl carbon atom of the furan corresponds to C-1 of the hexose, this result suggests that one of the hydrogen atoms in this group, when it is produced from D-glucose, must arise intramolecularly. This hypothesis was verified by synthesizing D-glucose-2- 3H and converting it into the furan in acidified water. The 2-(hydroxyacetyl)furan obtained was labeled exclusively on the hydroxymethyl carbon atom, thus showing that intramolecular hydrogen-transfer occurs, during the conversion, from C-2 of D-glucose to the carbon atom corresponding to C-1. The specific activities of the product and reactant permitted calculation of the tritium isotope-effect ( kh/kt4.4) for the reaction. The precise step for the transfer from C-2 of the aldose to the carbon atom corresponding to C-1 was found to be during the isomerization of D-glucose to D-fructose, as evidenced by the conversion of D-glucose- 2- 3H into D-fructose- 1- 3H in acidified water. 相似文献
20.
The administration of nicotinic-[2- 14C] acid to Dioscorea hispida plants afforded radioactive dioscorine (1.9% absolute incorporation) and a systematic degradation of the alkaloid indicated that essentially all the activity was located at C-3. Dioscorine derived from nicotinic-[5,6- 14C, 13C 2] acid was also labelled. Its proton noise decoupled 13C NMR spectrum contained satellites at C-1 and C-7 due to spin-spin coupling of contiguous 13C atoms arising from direct incorporation of the labelled nicotinic acid. A biosynthetic scheme representing a novel utilization of nicotinic acid is proposed. 相似文献
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