A Cytochrome P450 Enzyme Catalyses Oxetane Ring Formation in Paclitaxel Biosynthesis

Oxetane synthase (TmCYP1), a novel cytochrome P450 enzyme from Taxus × media cell cultures, has been functionally characterized to efficiently catalyse the formation of the oxetane ring in tetracyclic taxoids. Transient expression of TmCYP1 in Nicotiana benthamiana using 2α,5α,7β,9α,10β,13α-hexaacetoxytaxa-4(20),11(12)-diene (1) as a substrate led to the production of a major oxetane derivative, 1β-dehydroxybaccatin IV (1a), and a minor 4β,20-epoxide derivative, baccatin I (1b). However, feeding the substrate decinnamoyltaxinine J (2), a 5-deacetylated derivative of 1, yielded only 5α-deacetylbaccatin I (2b), a 4β,20-epoxide. A possible reaction mechanism was proposed on the basis of substrate-feeding, 2H and 18O isotope labelling experiments, and density functional theory calculations. This reaction could be an intramolecular oxidation-acetoxyl rearrangement and the construction of the oxetane ring may occur through a concerted process; however, the 4β,20-epoxide might be a shunt product. In this process, the C5-O-acetyl group in substrate is crucial for the oxetane ring formation but not for the 4(20)-epoxy ring formation by TmCYP1. These findings provide a better understanding of the enzymatic formation of the oxetane ring in paclitaxel biosynthesis.

Preparation of 18O-labelled azaspiracids for accurate quantitation using liquid chromatography-mass spectrometry

Azaspiracids (AZAs) are a group of polyether marine algal toxins known to accumulate in shellfish, posing a risk to human health and the seafood industry. Analysis of AZAs is typically performed using LC-MS, which can suffer from matrix effects that significantly impact the accuracy of measurement results. While the use of isotopic internal standards is an effective approach to correct for these effects, isotopically labelled standards for AZAs are not currently available. In this study, 18O-labelled AZA1, AZA2, and AZA3 were prepared by reaction with H218O under acidic conditions, and the reaction kinetics and sites of incorporation were studied using LC-HRMS/MS aided by mathematical analysis of their isotope patterns. Analysis of the isotopic incorporation in AZA1 and AZA3 indicated the presence of four exchangeable oxygen atoms. Excessive isomerization occurred during preparation of 18O-labelled AZA2, suggesting a role for the 8-methyl group in the thermodynamic stability of AZAs. Neutralized mixtures of 18O-labelled AZA1 and AZA3 were found to maintain their isotopic and isomeric integrities when stored at -20 °C and were used to develop an isotope-dilution LC-MS method which was applied to reference materials of shellfish matrices containing AZAs, demonstrating high accuracy and excellent reproducibility. Preparation of isotopically labelled compounds using the isotopic exchange method, combined with the kinetic analysis, offers a feasible way to obtain isotopically labelled internal standards for a wide variety of biomolecules to support reliable quantitation.

Malondialdehyde trapping by food phenolics

The reactions between malondialdehyde and 2,5-dimethylresorcinol, orcinol, olivetol, and alkylresocinols were studied in an attempt to investigate both if this lipid oxidation product is trapped by phenolics analogously to other reactive carbonyls and to elucidate the chemical structures of the produced adducts. After being formed, malondialdehyde is both partially fractionated to acetaldehyde and oligomerized into dimers and trimers. All these compounds react with phenolics producing three main kinds of derivatives: 5(or 7)-alkyl-7(or 5)-hydroxy-4-methyl-4H-chromene-3-carbaldehydes, 7-alkyl-9-hydroxy-6H-2,6-methanobenzo[d][1,3]dioxocine-5-carbaldehydes, and 4-(3-formylphenyl)-7-hydroxy-4H-chromene-3-carbaldehydes. A total of twenty-four adducts were isolated by semipreparative high-performance liquid chromatography (HPLC) and characterized by mono- and bi-dimensional nuclear magnetic resonance (NMR) spectroscopy and mass spectrometry (MS). Reaction pathways to explain the formation of all these compounds are proposed. Obtained results show that phenolics can trap malondialdehyde producing stable derivatives. The function(s) that such derivatives can play in foods remain(s) to be elucidated.

Effect of deuteration on a phosphatidylcholine lipid monolayer structure: New insights from vibrational sum-frequency generation spectroscopy

We used vibrational sum-frequency generation (VSFG) spectroscopy to elucidate the possible effect of various levels of isotopic substitution (H/D) on the properties of the DPPC monolayer by probing DPPC/D₂O interface. We found that deuteration of the choline group has a great impact on monolayer properties, while monolayers with deuterated alkyl chains do not exhibit any differences under our experimental conditions. In addition, deuteration of the choline group strongly affected the hydration of the phosphate group. We showed by probing symmetric stretching vibration of phosphate group that denser packing only slightly reduced the hydration of DPPC-d13 and DPPC-d75 monolayers. Moreover, addition of calcium ions, which generally cause a marked dehydration of the lipid monolayer, had no effect on lipid monolayers with deuterated choline group. We proposed that one way to explain this experimental finding could be deuteration induced changes in the structure of lipid's choline group, resulting in a well-hydrated but Ca²⁺ ion blocking structure. These results have important implications for various spectroscopic techniques, which commonly use deuteration of phospholipids to circumvent overlapping between vibrational bands.

A new concept for the production of 11C-labelled radiotracers

Background

The GMP-compliant production of radiopharmaceuticals has been performed using disposable units (cassettes) with a dedicated synthesis module. To expand this “plug ‘n’ synthesize” principle to a broader scope of modules we developed a pressure controlled setup that offers an alternative to the usual stepper motor controlled rotary valves. The new concept was successfully applied to the synthesis of N-methyl-[¹¹C]choline, L-S-methyl-[¹¹C]methionine and [¹¹C]acetate.

Results

The target gas purification of cyclotron produced [¹¹C]CO₂ and subsequent conversion to [¹¹C]MeI was carried out on a TRACERlab Fx C Pro module. The labelling reactions were controlled with a TRACERlab Fx FE module. With the presented modular principle we were able to produce N-methyl-[¹¹C]choline and L-S-methyl-[¹¹C]methionine by loading a reaction loop with neat N,N'-dimethylaminoethanol (DMAE) or an ethanol/water mixture of NaOH and L-homocysteine (L-HC), respectively and a subsequent reaction with [¹¹C]MeI. After 18 min N-methyl-[¹¹C]choline was isolated with 52% decay corrected yield and a radiochemical purity of > 99%. For L-S-methyl-[¹¹C]methionine the total reaction time was 19 min reaction, yielding 25% of pure product (> 97%). The reactor design was used as an exemplary model for the technically challenging [¹¹C]acetate synthesis. The disposable unit was filled with 1 mL MeMgCl (0.75 M) in tetrahydrofuran (THF) bevore [¹¹C]CO₂ was passed through. After complete release of [¹¹C]CO₂ the reaction mixture was quenched with water and guided through a series of ion exchangers (H⁺, Ag⁺ and OH⁻). The product was retained on a strong anion exchanger, washed with water and finally extracted with saline. The product mixture was acidified and degassed to separate excess [¹¹C]CO₂ before dispensing. Under these conditions the total reaction time was 18 ± 2 min and pure [¹¹C]acetate (n = 10) was isolated with a decay corrected yield of 51 ± 5%.

Conclusion

Herein, we described a novel single use unit for the synthesis of carbon-11 labelled tracers for preclinical and clinical applications of N-methyl-[¹¹C]choline, L-S-methyl-[¹¹C]methionine and [¹¹C]acetate.

Supplementary Information

The online version contains supplementary material available at 10.1186/s41181-022-00159-y.

Synthesis of natural/13C-enriched d-tagatose from natural/13C-enriched d-fructose

A concise, easily scalable synthesis of a rare ketohexose, d-tagatose, was developed, that is compatible with the preparation of d-[UL-¹³C₆]tagatose. Epimerization of the widely available and inexpensive ketohexose d-fructose at the C-4 position via an oxidation/reduction (Dess-Martin periodinane/NaBH₄) was a key step in the synthesis. Overall, fully protected natural d-tagatose (3.21 g) was prepared from d-fructose (9 g) on a 50 mmol scale in 23% overall yield, after five steps and two chromatographic purifications. d-[UL-¹³C₆]Tagatose (92 mg) was prepared from d-[UL-¹³C₆]fructose (465 mg, 2.5 mmol) in 16% overall yield after six steps and four chromatographic purifications.

Nitrene-functionalized ruthenium nanoparticles: Spectral evidence for the conjugated ruthenium-nitrene π bonds and the impact on the catalytic activity

In this manuscript, ruthenium (Ru) nanoparticles functionalized with nitrene ligands through the ruthenium-nitrene (RuN) π bonds are explored. By synthesizing the nitrene ligands with and without ¹⁵N-labelling, RuN π bonds on Ru nanoparticles are evidenced by experimental and theoretically calculated FTIR spectra. The coordination of nitrene ligands on Ru nanoparticles surface, the interfacial charge delocalization and the impact of nitrene ligands on the catalytic performance of Ru nanoparticles are further characterized by magic-angle spinning solid-state carbon nuclear magnetic resonance spectroscopy (¹³C NMR) of ¹³CO-adsorbed Ru nanoparticles, photoluminescence and the hydrogenation of styrene.

18O labeling on Ser45 but not on Ser35 supports the cooperative phosphorylation mechanism on tarantula thick filament activation

Thick filaments from some striated muscles are regulated by phosphorylation of myosin regulatory light chains (RLCs). A tarantula thick filament quasi-atomic model achieved by cryo-electron microscopy has advanced our understanding on how this regulation occurs. In native thick filaments, an asymmetric intramolecular interaction between the actin-binding region of one myosin head ("blocked") and the converter region of the other head ("free") switches both heads off, establishing the myosin interacting-heads motif (IHM). This structural finding, together with motility assays, sequence analysis, and mass spectrometry (MS) observations have suggested a cooperative phosphorylation activation (CPA) mechanism for thick filament activation. In the CPA mechanism, some myosin free heads are phosphorylated constitutively in Ser35 by protein kinase C (PKC) and -under Ca²⁺ control - others (free or blocked) heads temporally on Ser45 by myosin light chain kinase (MLCK), in a way that explains both force development and post-tetanic potentiation in tarantula striated muscle. We tested this model using MS to verify if Ca²⁺-activation phosphorylates de novo un-phosphorylated Ser35 heads. For this purpose, we standardized an approach based on ¹⁸O isotopic ATP labeling to accurately detect by MS-MS the RLC phosphorylation under Ca²⁺-activation. MS spectra showed de novo¹⁸O incorporation only on Ser45 but not on Ser35. As the constitutive Ser35 phosphorylation cannot be dephosphorylated, this result suggests that the number of RLCs on free heads with constitutively phosphorylated Ser35 does remain constant on Ca²⁺-activation supporting that the myosin has a basal activation and force modulation or potentiation is controlled by MLCK Ser45 phosphorylation.

Can stable isotope mass spectrometry replace ‎radiolabelled approaches in metabolic studies?

Metabolic pathways and the key regulatory points thereof can be deduced using isotopically labelled substrates. One prerequisite is the accurate measurement of the labeling pattern of targeted metabolites. The subsequent estimation of metabolic fluxes following incubation in radiolabelled substrates has been extensively used. Radiolabelling is a sensitive approach and allows determination of total label uptake since the total radiolabel content is easy to detect. However, the incubation of cells, tissues or the whole plant in a stable isotope enriched environment and the use of either mass spectrometry or nuclear magnetic resonance techniques to determine label incorporation within specific metabolites offers the possibility to readily obtain metabolic information with higher resolution. It additionally also offers an important complement to other post-genomic strategies such as metabolite profiling providing insights into the regulation of the metabolic network and thus allowing a more thorough description of plant cellular function. Thus, although safety concerns mean that stable isotope feeding is generally preferred, the techniques are in truth highly complementary and application of both approaches in tandem currently probably provides the best route towards a comprehensive understanding of plant cellular metabolism.

Soluble expression, purification and functional characterisation of carboxypeptidase G2 and its individual domains

Due to its applications in the treatment of cancer and autoimmune diseases, the 42 kDa zinc-dependent metalloenzyme carboxypeptidase G2 (CPG2) is of great therapeutic interest. An X-ray crystal structure of unliganded CPG2 reported in 1997 revealed the domain architecture and informed early rational drug design efforts, however further efforts at co-crystallization of CPG2 with ligands, substrates or inhibitors have not been reported. Thus key features of CPG2 such as the location of the active site, the presence of additional ligand-binding sites, stability, oligomeric state, and the molecular basis of activity remain largely unknown, with the current working understanding of CPG2 activity based primarily on computational modelling. To facilitate renewed efforts in CPG2 structural biology, we report the first high-yield (250 mg L(-1)) recombinant expression (and purification) of soluble and active CPG2 using the Escherichia coli expression system. We used this protocol to produce full-length enzyme, as well as protein fragments corresponding to the individual catalytic and dimerization domains, and the activity and stability of each construct was characterised. We adapted our protocol to allow for uniform incorporation of NMR labels ((13)C, (15)N and (2)H) and present preliminary solution-state NMR spectra of high quality. Taken together, our results offer a route for production and solution-state characterization that supports renewed effort in CPG2 structural biology as well as design of significantly truncated CPG2 proteins, which retain activity while yielding (potentially) improved immunogenicity.

Synthesis of nonadeuterated 1α,25-dihydroxyvitamin D2

An efficient convergent synthesis of nonadeuterated 1α,25-dihydroxyvitamin D2 (1) by Pd(0)-catalyzed coupling between the boronate ester (upper fragment) and the enol triflate (A-ring fragment) is described. This article is part of a Special Issue entitled '16th Vitamin D Workshop'.