REGIOSELECTIVE SYNTHESIS OF 1,3,5-13C3 AND 2,4-13C2-LABELED 2-DEOXYRIBONOLACTONES

The synthesis of 1,3,5-¹³C₃- and 2,4-¹³C₂-labeled 5-O-bromobenzyl-2-deoxyribonolactones 2, precursors to ¹³C-enriched nucleoside phosphoramidites for solid-phase synthesis of DNA oligonucletides, is described. An equimolar combination of these two multiply labeled lactones affords a “population-labeled” mixture of isotopomers which exhibits an approximately 50-fold increase in the sensitivity of ¹³C-NMR compared to natural abundance measurements. The ¹³C-¹³C 2-bond and 4-bond coupling constants are reported for the lactones; all are < 2 Hz, confirming that this labeling scheme should be especially useful for NMR-relaxation measurements.     

Tetrapyrrole biosynthesis in Anacystis nidulans; incorporation of [1-13C]-, [2-13C]-, [1,2-13C]- and [2-13C, 2-2H3]acetate

Labelling experiments with [2-¹³C]- and [1,2-¹³C]acetate showed that both photopigments of Anacystis nidulans, chlorophyll a and phycocyanobilin, share a common biosynthetic pathway from glutamate. The fate of deuterium during these biosynthetic events was studied using [2-¹³C, 2-²H₃]acetate as a precursor and determining the labelling pattern by ¹³C NMR spectroscopy with simultaneous [¹H, ²H]-broadband decoupling. The loss of ²H (ca 20%) from the precursor occurred at an early stage during the tricarboxylic acid cycle. After formation of glutamate there was no further loss of ²H in the assembly of the cyclic tetrapyrrole intermediates or during decarboxylation and modification of the side-chains. Thus the labelling data support a divergence in the pathway to cyclic and linear tetrapyrroles after protoporphyrin IX.     

Multiply 13C-substituted monosaccharides: synthesis of D-(1,5,6-13C3)glucose and D-(2,5,6-13C3)glucose.

D-(1,5,6-13C3)Glucose (7) has been synthesized by a six-step chemical method. D-(1,2-13C2)Mannose (1) was converted to methyl D-(1,2-13C2)mannopyranosides (2), and 2 was oxidized with Pt-C and O2 to give methyl D-(1,2-13C2)mannopyranuronides (3). After purification by anion-exchange chromatography, 3 was hydrolyzed to give D-(1,2-13C2)mannuronic acid (4), and 4 was converted to D-(5,6-13C2)mannonic acid (5) with NaBH4. Ruff degradation of 5 gave D-(4,5-13C2)arabinose (6), and 6 was converted to D-(1,5,6-13C3)glucose (7) and D-(1,5,6-13C3)mannose (8) by cyanohydrin reduction. D-(2,5,6-13C3)Glucose (9) was prepared from 8 by molybdate-catalyzed epimerization.     

1,2,3,4-13C] testosterone and [1,2,3,4-13C] estradiol

The preparation of [1,2,3,4-13C] testosterone and of [1,2,3,4-13C] estradiol by total synthesis is described. The 13C labels are introduced by alkylating intermediate 1 with [1,2,3,4-13C]l-iodo-3,3-ethylenedioxybutane (2) to obtain intermediate 10. Hydrolysis of the ketal function, cyclization, aromatization and removal of protective groups gave [1,2,3,4-13C] estradiol. Labeled testosterone was prepared by methylating intermediate 10 and by subsequent treatment with acid. The labeled steroids can be used as tracers for in vivo metabolic studies and as internal standards for the development of definitive gc-ms quantitative methods.     

Incorporation and utilization of [3-13C]lactate and [1,2-13C]acetate by rat skeletal muscle.

Skeletal muscle can utilize many different substrates, and traditional methodologies allow only indirect discrimination between oxidative and nonoxidative uptake of substrate, possibly with contamination by metabolism of other internal organs. Our goal was to apply 1H- and 13C-nuclear magnetic resonance spectroscopy to monitor the patterns of [3-13C]lactate and [1,2-13C]acetate (model of simple carbohydrates and fats, respectively) utilization in resting vs. contracting muscle extracts of the isolated perfused rat hindquarter. Total metabolite concentrations were measured by using NADH-linked fluorometric assays. Fractional oxidation of [3-13C]lactate was unchanged by contraction despite vascular endogenous lactate accumulation. Although label accumulated in several citric acid cycle (CAC) intermediates, contraction did not increase the concentration of CAC intermediates in any muscle extracts. We conclude that 1) the isolated rat hindquarter is a viable, well-controlled model for measuring skeletal muscle 13C-labeled substrate utilization; 2) lactate is readily oxidized even during contractile activity; 3) entry and exit from the CAC, via oxidative and nonoxidative pathways, is a component of normal muscle metabolism and function; and 4) there are possible differences between gastrocnemius and soleus muscles in utilization of nonoxidative pathways.     

Hydrogen turnover and subcellular compartmentation of hepatic [2-(13)C]glutamate and [3-(13)C]aspartate as detected by (13)C NMR.

(13)C NMR monitored the dynamics of exchange from specific hydrogens of hepatic [2-(13)C]glutamate and [3-(13)C]aspartate with deuterons from intracellular heavy water providing information on alpha-ketoglutarate/glutamate exchange and subcellular compartmentation. Mouse livers were perfused with [3-(13)C]alanine in buffer containing or not 50% (2)H(2)O for increasing periods of time (1 min < t < 30 min). Liver extracts prepared at the end of the perfusions were analyzed by high resolution (13)C NMR (150.13 MHz) with (1)H decoupling only and with simultaneous (1)H and (2)H decoupling. (13)C-(2)H couplings and (2)H-induced isotopic shifts observed in the glutamate C2 resonance, allowed to estimate the apparent rate constants (forward, reverse; min(-1)) for (i) the reversible exchange of [2-(13)C]glutamate H2 as catalyzed mainly by aspartate aminotransferase (0.32, 0.56), (ii) the reversible exchange of [2-(13)C]glutamate H3(proS) as catalyzed by NAD(P) isocitrate dehydrogenase (0.1, 0.05), and (iii) the irreversible exchanges of glutamate H3(proR) and H3(proS) as catalyzed by the sequential activities of mitochondrial aconitase and NAD isocitrate dehydrogenase of the tricarboxylic acid cycle (0.035), respectively. A similar approach allowed to determine the rates of (1)H-(2)H exchange for the H2 (0.4, 0.5) or H3(proR) (0.3, 0.2) or the H2 and H3(proS) hydrogens (0.20, 0.23) of [3-(13)C]aspartate isotopomers. The ubiquitous subcellular localization of (1)H-(2)H exchange enzymes and the exclusive mitochondrial localization of pyruvate carboxylase and the tricarboxylic acid cycle resulted in distinctive kinetics of deuteration in the H2 and either or both H3 hydrogens of [2-(13)C]glutamate and [3-(13)C]aspartate, allowing to follow glutamate and aspartate trafficking through cytosol and mitochondria.     

Interaction of two prolamins with 1-13C oleic acid by 13C NMR

In this paper, we analyzed the interaction of Z19 prolamin from a BR451 maize variety and pennisetin from a BRS1501 pearl millet variety with 1-(13)C-enriched oleic acid (OA) by (13)C NMR in solution. In both proteins, we identified the presence of free fatty acids by NMR in solid state and solution. The interactions were analyzed at the protein/OA molar ratios of 1:1 and 1:4. In the Z19/OA 1:1 mixture in 70% ethanol and 30% D(2)O, the chemical shift of OA C1 was 182.9 ppm, about 3 ppm above that of the pure OA in the same solvent. In contrast, upon addition of OA to the pennisetin (1:1), the chemical-shift value slightly decreased by less than 1 ppm. The chemical-shift titration curve of OA C1 in an apparent pH range of 5.5-7.3 shifted by approximately 0.3 pH units toward higher pH values in the pennisetin/OA 1:1 complex relative to the pure OA. The results obtained for the pennisetin/OA 1:4 mixture were similar to the complexes at a 1:1 molar ratio. A significant difference was observed between the 1:1 and 1:4 curves for Z19. The titration curve for Z19/OA 1:1 suggested specific binding at the sites with electrostatic interaction.     

Generation of C3- and C2-deuterated l-lactic acid by human erythrocytes exposed to d-[1-13C]glucose,d-[2-13c]glucose and d-[6-13C]glucose in the presence of D2O

• 1.1. The generation of C₂- and C₃-deuterated l-lactate was monitored by ¹³C NMR in human erythrocytes exposed to d-[1-¹³glucose, d-[2-¹³C]glucose or d-te-¹³C]glucose and incubated in a medium prepared in D₂O.
• 2.2. The results suggested that the deuteration of the C₁ of d-fructose 6-phosphate in the phosphoglucoisomerase reaction, the deuteration of the C₁ of d-glyceraldehyde-3-phosphate in the sequence of reactions catalyzed by triose phosphate isomerase and aldolase and the deuteration of the C₃ of pyruvate in the reaction catalyzed by pyruvate kinase were all lower than expected from equilibration with D₂O.
• 3.3. Moreover, about 40% of the molecules of pyruvate generated by glycolysis apparently underwent deuteration on their C₃ during interconversion of the 2-keto acid and l-alanine in the reaction catalyzed by glutamate-pyruvate transaminase.
• 4.4. The occurrence of the latter process was also documented in cells exposed to exogenous [3-¹³C]pyruvate.
• 5.5. This methodological approach is proposed to provide a new tool to assess in intact cells the extent of back-and-forth interconversion of selected metabolic intermediates.

     

Mathematical modelling for the generation of L-[3-2H,3-13C]lactic acid isotopomers by erythrocytes exposed to either D-[1-13C]glucose or D-[6-13C]glucose in the presence of2H2O

Cytosolic purine nucleoside phosphorylase (PNPase) is a well known, and described enzyme which exists in a variety of organisms, both procaryotic and eucaryotic. More recently this enzyme was found in bovine liver mitochondria. The mitochondrial purine nucleoside phosphorylase was purified 63 fold and has a molecular weight of 48–60 kD. From Lineweaver-Burk plots apparent K_m's of 23M for inosine, 42 M for deoxyinosine, 40 M for phosphate, 2 M for hypoxanthine, and 163 M for ribose-1-phosphate were calculated. Both 8-aminoguanosine (K_i=0.5 M) and araG (K_i=381 M) are inhibitors of the enzyme. The protein's isoelectric point (pI) was calculated at a pH of 4.2. Preliminary immunological work showed no cross-reactivity between epitopes on the mitochondrial protein and those on PNPase from human erythrocytes. The apparent K_m's calculated for the mitochondrial enzyme are,with the exception of that using hypoxanthine, within the range commonly associated with K_m's from the cytosolic species. The mitochondrial enzyme's molecular weight and pI are less than normally described. The enzyme's isolation from mitochondria, together with several unique characteristics, suggest that it is a separate protein from that found in the cytosol.     

Synthesis of 3,4-13C2 steroids

A-ring enollactones $\text{1a}$, $\text{1b}$ or $\text{9}$ derived from 4-cholesten-3-one, testosterone benzoate or 3-oxo-4-estren-17β-yl benzoate were condensed with [1,2-¹³C₂]acetyl chloride to give intermediates $\text{2a}$, $\text{2b}$ or $\text{10}$. $\text{2a}$ and $\text{2b}$ were cyclized by acid or base to give 3,4-¹³C₂-labeled 4-cholesten-3-one and testosterone, respectively. [3,4-¹³C₂]4-Cholesten-3-one was converted via reduction of its trimethylsilyl enol ether to [3,4-¹³C₂]cholesterol. Acetyl enollactone $\text{10}$ was cyclized in acetic acid to [3,4-¹³C₂]3-oxo-4-estren-17β-yl benzoate followed by aromatization and hydrolysis to produce [3,4-¹³C₂]estradiol-17β. Alternatively, cyclization of $\text{10}$ with base afforded [3,4-¹³C₂]3-oxo-4-estren-17β-ol directly, which was then oxidized and aromatized to yield [3,4-¹³C₂]estrone. Ozonolysis of progesterone, conversion to the diketal ester $\text{16}$ and acylation followed by acid hydrolysis furnished [3,4-¹³C₂]progesterone.     

A (13)C NMR study on [3-(13)C]-, [1-(13)C]Ala-, or [1-(13)C]Val-labeled transmembrane peptides of bacteriorhodopsin in lipid bilayers: insertion, rigid-body motions, and local conformational fluctuations at ambient temperature

We have recorded (13)C NMR spectra of selectively [3-(13)C]Ala-, [1-(13)C]Ala-, or [1-(13)C]Val-labeled synthetic transmembrane peptides of bacteriorhodopsin (bR) and enzymatically cleaved C-2 fragment in the solid and dimyristoylphosphatidylcholine bilayer. It turned out that these transmembrane peptides either in hexafluoroisopropanol or cast from it take an ordinary alpha-helix (alpha(I)-helix) irrespective of their amino acid sequences with reference to the conformation-dependent (13)C chemical shifts of (Ala)(n) taking the alpha-helix form. These transmembrane peptides are not always static in the lipid bilayer as in the solid state but undergo rigid-body motions with various frequencies as estimated from suppressed peaks either by fast isotropic or large-amplitude motions (>10(8) Hz) or intermediate frequencies (10(5) or 10(3) Hz). Further, (13)C chemical shifts of the [3-(13)C]Ala-labeled peptides in the bilayer were displaced downfield by 0.3-1.1 ppm depending upon amino acid sequence with respect to those in the solid state, which were explained in terms of local conformational fluctuation (10(2) Hz) deviated from the torsion angles (alpha(II)-helix) from those of standard alpha-helix, under anisotropic environment in lipid bilayer, in addition to the above-mentioned rigid-body motions. The carbonyl (13)C peaks, on the other hand, are not sensitively displaced by such local anisotropic fluctuations, because they are more sensitive to the manner of hydrogen-bond interactions. The amino acid sequences of these peptides inserted within the bilayer were not always the same as those of intact bR, causing disposition of the transmembrane alpha-helical segment from that of intact bR. Finally, we confirmed that the (13)C NMR peak positions of the random coil form are located at the boundary between the alpha-helix and a turned structure in loop regions.     

13C NMR studies on [4-13C] cholesterol incorporated in sonicated phosphatidylcholine vesicles

1. 90.5 MHz 13C NMR linewidth measurements were performed on mixed sonicated [4-13C] cholesterol/phosphatidylcholine vesicles of different fatty acid composition. 2. From the Dy3+ -induced shift of the C4 resonance of cholesterol it suggested that this part of the molecule is localized in the ester bond region of the bilayer. 3. The local motion of the cholesterol ring system is restricted and independent of fatty acid composition. 4. At cholesterol concentrations below 30 mol percent the ring system becomes more immobilised when the fatty acids of the phosphatidylcholine molecules enter the gel state.     

Metabolism of [2-13C]succinate in renal cells determined by 13C NMR

[2-13C]Succinate has been used to examine the metabolic carbon flux from the Krebs cycle in rat renal proximal convoluted tubular (PCT) cells under physiological and pathophysiological conditions. Therefore, we developed a mathematical model that enabled us to determine the metabolic fluxes of the Krebs cycle. A mathematical model for the calculation of flux from [2-13C]succinate was used to determine fluxes in rat PCT cells during chronic acidosis in the presence and absence of 0.1 mM angiotensin II. The relative carbon efflux via glutamate dehydrogenase in rat renal PCT cells increases during chronic acidosis from 0.27 to 0.39, whereas this carbon flux is not affected by the presence of peptide hormone angiotensin II in the incubation medium. The fraction of intermediate 13C-labelled oxaloacetate transformed into the phosphoenolpyruvate and aspartate pools increases significantly from 0.41 to 0.57 in the case of chronic acidosis. The carbon efflux is not affected by angiotensin II. The 13C-NMR data also show that the carbon efflux through phosphoenolpyruvate carboxykinase increases from 0.35 to 0.56 in rat renal PCT cells derived from chronic acidotic animals, as well as in the presence of angiotensin II. The present results indicate that angiotensin II affects only the flux through phosphoenolcarboxykinase, whereas chronic acidosis increases the flux through phosphoenolpyruvate carboxykinase as well as the gluconeogenic flux.