Optimization of the allylic oxidation in the synthesis of 7-keto-delta5-steroidal substrates

A variety of delta5-steroids were converted into alpha, beta-unsaturated 7-ketones using a modification of the already known method of t-butyl hydroperoxide in the presence of copper iodide in acetonitrile. The same alteration was applied to another oxidative procedure, which had never been used before on steroidal substrates. The same oxidative agent was used in the presence of copper iodide, and tetra-n-butylammonium bromide was used as a phase-transfer catalyst in a two-phase system of water/methylene chloride. It was found that the allylic oxidation proceeded more efficiently when t-butyl hydroperoxide was added to the reaction mixture in portions. The initial addition of the total amount of oxidant or its dropwise addition afforded low yields. This observation contributes to the investigation of the reaction mechanism, and high-yield conversions of steroidal 5,6-enes into the corresponding conjugated 7-ones in short reaction times are reported.     

Oxidation of melatonin by AAPH-derived peroxyl radicals: Evidence of a pro-oxidant effect of melatonin

Background

Melatonin is well-established as a powerful reducing agent of oxidant generated in the cell medium. We aimed to investigate how readily melatonin is oxidized by peroxyl radicals ROO⋅ generated by the thermolysis of 2,2′-azobis(2-amidinopropane) hydrochloride (AAPH) and the role of glutathione (GSH) during the reaction course.

Methods

Chromatographic, mass spectroscopy, and UV–visible spectrometric techniques were used to study the oxidation of melatonin by ROO⋅ or horseradish peroxidase (HRP)/H₂O₂. Our focus was the characterization of products and the study of features of the reaction.

Results

We found that N¹-acetyl-N²-formyl-5-methoxykynuramine (AFMK) and a monohydroxylated derivative of melatonin were the main products of the reaction between melatonin and ROO⋅. Higher pH or saturation of the medium with molecular oxygen increased the yield of AFMK but did not affect the reaction rate. Melatonin increased the depletion of intracellular GSH mediated by AAPH. Using the HRP/H₂O₂ as the oxidant system, the addition of melatonin promoted the oxidation of GSH to GSSG.

Conclusions

These results show, for the first time, that melatonin radical is able to oxidize GSH.

General significance

We propose that this new property of melatonin could explain or be related to the recently reported pro-oxidant activities of melatonin.     

Competition between ammonia derived from internal glutamine hydrolysis and hydroxylamine present in the solution for incorporation into UTP as catalysed by Lactococcus lactis CTP synthase

CTP synthase catalyses the reaction: glutamine+UTP+ATP --> glutamate+CTP+ADP+P(i). The reaction is greatly stimulated by the allosteric binding of GTP. In addition to glutamine that is hydrolysed by the enzyme to ammonia and glutamate, CTP synthase will also utilise external sources of amino donors such as NH(4)Cl. This reaction is no longer dependent on allosteric activation by GTP. Hydroxylamine is also a substrate for Lactococcus lactis CTP synthase and results in the formation of N4-OH CTP. This product has the feature that it absorbs at 300nm where CTP absorption was shown to be greatly reduced and enabled the determination of N4-OH CTP formation in the presence of CTP synthesis derived from glutamine hydrolysis. Differences in initial rates determined for the hydroxylamine dependent reaction at 291nm in the presence and absence of glutamine and GTP were ascribed to simultaneous CTP and N4-OH CTP synthesis in the presence of these compounds. A characterisation of the apparent inhibition by GTP and glutamine of N4-OH CTP synthesis determined at 300nm showed that glutamine dependent CTP synthesis occurs at a rate of about 60% of that in the absence of hydroxylamine. GTP dependent inhibition of the ammonium chloride dependent reaction of L. lactis CTP synthase by the glutamine analog glutamate gamma-semialdehyde showed a partial inhibition with a maximum inhibition of about 60%. These results are interpreted in terms of a "half of the sites" mechanism for glutamine hydrolysis on CTP synthase.     

Purification and characterization of dermatan sulfate from the skin of the eel,Anguilla japonica

Glycosaminoglycans were isolated from the eel skin (Anguilla japonica) by actinase and endonuclease digestions, followed by a beta-elimination reaction and DEAE-Sephacel chromatography. Dermatan sulfate was the major glycosaminoglycan in the eel skin with 88% of the total uronic acid. The content of the IdoA2Salpha1-->4GalNAc4S sequence in eel skin, which shows anticoagulant activity through binding to heparin cofactor II, was two times higher than that of dermatan sulfate from porcine skin. The anti-IIa activity of eel skin dermatan sulfate was determined to be 2.4 units/mg, whereas dermatan sulfate from porcine skin shows 23.2 units/mg. The average molecular weight of dermatan sulfate was determined by gel chromatography on a TSKgel G3000SWXL column as 14 kDa. Based on 1H NMR spectroscopy, the presence of 3-sulfated and/or 2,3-sulfated IdoA residues was suggested. The reason why highly sulfated dermatan sulfate does not show anticoagulant activity is discussed. In addition to dermatan sulfate, the eel skin contained a small amount of keratan sulfate, which was identified by keratanase treatment.     

When are metal ion-dependent hydroxyl and alkoxyl radical adducts of 5,5-dimethyl-1-pyrroline N-oxide artifacts?

The formation of the 5,5-dimethyl-1-pyrroline N-oxide (DMPO)/.OH adduct of the spin trap DMPO has been reported to occur through nucleophilic addition of water in the presence of aqueous ferric chloride (K. Makino, T. Hagiwara, A. Hagi, M. Nishi, and A. Murakami, 1990, Biochem. Biophys. Res. Commun. 172, 1073-1080). Due to the serious implications of these findings with respect to many spin trapping studies, the suitability of DMPO as a hydroxyl radical spin trap was studied in typical Fenton systems. Using 17O-enriched water, we show conclusively that nucleophilic addition of water occurs at the nitrone carbon (or C-2 position) of DMPO in the presence of either Fe or Cu ions. Furthermore, our results demonstrate that this nucleophilic reaction is a major pathway to the DMPO/.OH adduct, even during the reaction of Fe(II) or Cu(I) with hydrogen peroxide. Primary alkoxyl adducts of DMPO also form in aqueous solution through nucleophilic addition in the presence of both Fe(III) and Cu(II). Attempts to obtain secondary and tertiary alkoxyl adducts by this mechanism were unsuccessful, possibly due to steric effects. When the reaction is carried out in various buffers, however, or in the presence of metal ion chelators, nucleophilic addition to DMPO from Fe(III) is effectively suppressed. Chelators also suppress the reaction with Cu(II). Hence, under most common experimental conditions in biochemical free radical research, nucleophilic addition to DMPO should not be of major concern.     

Interaction of bovine serum amine oxidase with the polyamine oxidase inactivator MDL 72527

MDL 72527 was considered a selective inhibitor of FAD-dependent polyamine oxidases. In the present communication, we demonstrate that MDL 72527 inactivates bovine serum amine oxidase, a copper-containing, TPQ-enzyme, time-dependently at 25 degrees C. In striking contrast, the enzyme remained active after incubation with excessive MDL 72527 at 37 degrees C, even after 70 h of incubation. Inactivation of BSAO with MDL 72527 at 25 degrees C did not involve the cofactor, as was shown by spectroscopy and by reaction with phenylhydrazine. Docking of MDL 72527 is difficult, owing to its size and two lipophilic moieties, and it has been shown that minor changes in reaction rate of substrates cause major changes in K(m) and k(cat)/K(m). We hypothesise that subtle conformational changes between 25 and 37 degrees C impair MDL 72527 from productive binding and prevent the nucleophilic group from reacting with the double bond system.     

Heterogeneity of the interflavanyl bond in proanthocyanidins from natural sources lacking C-4 (C-ring) deoxy flavonoid nucleophiles

The proanthocyanidin pool in the floral kingdom usually involves the presence of carbon-carbon bonds linking predominantly flavan-3-ol constituent moieties. Such an ensemble of flavan-3-ol units originates via electrophilic aromatic substitution of flavan-4-yl carbocations (or their equivalents) derived from flavan-4-ols and/or flavan-3,4-diols and the nucleophilic centers of the m-oxygenated A-rings of flavan-3-ol nucleophiles. In the absence of these potent flavan-3-ol nucleophiles with their aptitude for the formation of carbon-carbon bonds, alternative centers emerge as participants in interflavanyl bond formation. Such a phenomenon is demonstrated for the distribution of various profisetinidin-, prorobinetinidin-, proguibourtinidin-, promelacacinidin- and proteracacinidin-type pro- and leuco-anthocyanidins in several southern hemisphere heartwood species.     

Reaction of phosphorylated and O-glycosylated peptides by chemically targeted identification at ambient temperature.

Conditions for carrying out chemically targeted identification of peptides containing phosphorylated or glycosylated serine residues have been investigated. Ba(OH)2 was used at ambient temperature to catalyze the beta-elimination reaction at 25 degrees C. Nucleophilic addition of 2-aminoethanethiol was performed in both parallel and tandem experiments. The method was demonstrated by the reaction of beta-casein tryptic digest phosphopeptides and an O-glycosylated peptide. Contrary to an earlier report by others, the glycopeptide was found to react with essentially the same kinetics as phosphopeptides. Conversion of four phosphoserines in residues 15, 17, 18, and 19 from bovine beta-casein N-terminal tryptic phosphopeptides were followed by monitoring the time course of the addition reaction. The chemistry proceeded rapidly at room temperature with a half-reaction time of 15 min. No side-reaction products were observed; however, care was taken to minimize all counter ions that either precipitate barium or neutralize the base. Digestion of the converted peptides with lysine endopeptidase identified all five phosphoserines in the beta-casein tryptic digest. Alternatively, preincubation with base followed by nucleophilic addition of the thiol was found to work satisfactorily. The use of the water-soluble hydrochloride of 2-aminoethanethiol allowed beta-elimination, nucleophilic addition, and desalting to be carried out on a micro C18 reverse phase pipette tip.     

Formation Mechanism of the Free Radical Product and Its Precursor by the Reaction of Dehydro-L-ascorbic Acid with Amino Acid

During the formation of radical A (2) and its precursor (tris(2-deoxy-2-L-ascorbyl)amine, 1) by the reaction of dehydroascorbic acid (DHA) with amino acid, ascorbic acid (AsA) and the reduced red pigment (3) were newly identified, in addition to scorbamic acid (SCA) and the red pigment (4), as intermediate products. The addition of AsA to the DHA-amino acid reaction, as well as to the DHA-SCA reaction, greatly increased the formation of 3 and 1. The reaction of AsA with 4 gave rapidly 3, followed by the gradual production of 1. From these results, a reaction pathway is proposed that 3 formed by the reduction of 4 with AsA is a key intermediate and its condensation with DHA followed by reduction with AsA might produce 2 and 1.     

N-Alkylation of tripodal iron(III) imidazolate complexes: Reactivity and structure

The N-alkylation of iron(III) complexes of the tripodal imidazolate complexes derived from the Schiff base condensation of tris(2-aminoethyl)amine (tren) with three molar equivalents of 2-imidazolecarboxaldehyde (2ImH), 4-imidazolecarboxaldehyde (4ImH) or 4-methyl-5-imidazolecarboxaldehyde (5-Me4ImH) was investigated. While each complex possesses three nucleophilic imidazolate nitrogen atoms, only the complex derived from 2-imidazolecarboxaldehyde, Fetren(2Im)₃, was completely alkylated under the ambient conditions used in this work. Using methyl iodide as the alkylating agent, a correlation between spin state of the product and degree of methylation was observed. Low spin iron complexes were more nucleophilic than high spin systems. The structure reactivity relationship was exploited in the reaction of Fetren(2Im)₃ with methyl iodide and allyl iodide to give [Fetren(N-Me2Im)₃]²⁺ and [Fetren(N-allyl2Im)₃]²⁺. The products are iron(II) due to reduction of the iron(III) by iodide ion which builds up in the reaction mixture as the alkylation reaction proceeds. These complexes were characterized by a number of methods including EA, IR, ES-MS, Mössbauer spectroscopy, magnetic susceptibility and X-ray diffraction.     

Heme-induced Trypanosoma cruzi proliferation is mediated by CaM kinase II

Trypanosoma cruzi, the etiologic agent of Chagas disease, is transmitted through triatomine vectors during their blood-meal on vertebrate hosts. These hematophagous insects usually ingest approximately 10 mM of heme bound to hemoglobin in a single meal. Blood forms of the parasite are transformed into epimastigotes in the crop which initiates a few hours after parasite ingestion. In a previous work, we investigated the role of heme in parasite cell proliferation and showed that the addition of heme significantly increased parasite proliferation in a dose-dependent manner [1]. To investigate whether the heme effect is mediated by protein kinase signalling pathways, parasite proliferation was evaluated in the presence of several protein kinase (PK) inhibitors. We found that only KN-93, a classical inhibitor of calcium-calmodulin-dependent kinases (CaMKs), blocked heme-induced cell proliferation. KN-92, an inactive analogue of KN-93, was not able to block this effect. A T. cruzi CaMKII homologue is most likely the main enzyme involved in this process since parasite proliferation was also blocked when Myr-AIP, an inhibitory peptide for mammalian CaMKII, was included in the cell proliferation assay. Moreover, CaMK activity increased in parasite cells with the addition of heme as shown by immunological and biochemical assays. In conclusion, the present results are the first strong indications that CaMKII is involved in the heme-induced cell signalling pathway that mediates parasite proliferation.     

Korormicin insensitivity in Vibrio alginolyticus is correlated with a single point mutation of Gly-140 in the NqrB subunit of the Na(+)-translocating NADH-quinone reductase

Na(+)-translocating NADH-quinone reductase (NQR) from the marine Vibrio alginolyticus is strongly inhibited by a new antibiotic korormicin. Korormicin specifically inhibits the Na(+)-dependent reaction of the NQR complex and acts as a purely non-competitive inhibitor for Q-1 with the inhibitor constant of 82 pM. Korormicin-resistant mutants were isolated from V. alginolyticus and the NQR complex was purified from a mutant KR2. Similar to 2-n-heptyl-4-hydroxyquinoline N-oxide (HQNO), korormicin acted as a purely noncompetitive inhibitor to the NQR complex from the mutant KR2, but the inhibitor constant increased to 8 microM, which is 10(5)-fold higher than that of the wild-type NQR complex. The inhibitor constant of HQNO, however, was only slightly affected by the acquisition of korormicin resistance. The spontaneous mutation was caused by a single mutation of G-422 to T-422 in the nucleotide sequence of the nqrB gene, which resulted in the conversion of Gly-140 to Val-140. Thus, Gly-140 seems to play an important role for the binding of korormicin to the NqrB subunit. The fact that korormicin is a purely noncompetitive inhibitor for Q-1 strongly supports the presence of one of Q-1 binding sites in the NqrB subunit, which also has a covalently bound FMN at Thr-235.     

Effects of L-proline and post-plating temperature treatment on Maize (Zea mays L.) anther culture

Summary Comparison of different post-plating temperature regimes with a control treatment (27° C) revealed that a short-term cold (8/14°C:2/2 days or 14°C:4 days) as well as a heat treatment (30°C:14 days) increased the production of embryro-like-structures (ELS) from cultured maize anthers. The beneficial effects of short-term cold treatments were magnified 2–3 times when L-proline (PROL) was added to the induction medium (125–500 mg/L). In the best treatment (14°C:4 days, 125 mg/L L-proline) one genotype produced 143.5 ELS/100 anthers. Anthers subjected to high temperature (30°C:4 days, 30°C:7 days, 30°C:14 days) generally showed a lower response than did cold treated anthers, although genotypic differences were observed. Regeneration frequency did not appear to be affected by the presence of L-proline in the induction medium.Abbreviations ELS Embryo-like-structures - PROL L-proline     

Heterogeneous catalytic reduction of anthropogenic pollutant, 4-nitrophenol by silver-bionanocomposite using Cylindrocladium floridanum

In the present investigation, the silver-bionanocomposite with fcc structured Ag-nanocrystals was synthesized using the fungus, Cylindrocladium floridanum through a novel, environmentally benign biological process. Silver-bionanocomposite was systematically characterized by UV-Vis spectroscopy, XRD, SEM, EDX, and TEM techniques. TEM analysis of mycelia confirmed the presence of silver nanoparticles (AgNPs) on the outer surface of the cell wall and inner of cytoplasmic membrane of the fungus, when cultured in aqueous solution of AgNO3 at 30 °C for a period of 7 days in static condition. Additionally, it was observed that bionanocomposite with AgNPs functions as an efficient heterogeneous catalyst in the degradation of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP), in the presence of reducing agent, sodium borohydride which was reflected by UV-Vis spectra of the catalytic reaction kinetics. This is the first report of the silver-bionanocomposite using fungus, Cy. floridanum, heterogeneously catalyzing the reduction of a toxic pollutant, 4-NP to 4-AP.     

Study on the reaction mechanism and the static injection chemiluminescence method for detection of acetaminophen

Acetaminophen, also called paracetamol, is found in Tylenol, Excedrin and other products as over–the‐counter medicines. In this study, acetaminophen as a luminol signal enhancer was used in the chemiluminescence (CL) substrate solution of horseradish peroxidase (HRP) for the first time. The use of acetaminophen in the luminol–HRP–H₂O₂ system affected not only the intensity of the obtained signal, but also its kinetics. It was shown that acetaminophen was to be a potent enhancer of the luminol–HRP–H₂O₂ system. A putative enhancement mechanism for the luminol–H₂O₂–HRP–acetaminophen system is presented. The resonance of the nucleophilic amide group and the benzene ring of acetaminophen structure have a great effect on O‐H bond dissociation energy of the phenol group and therefore on phenoxyl radical stabilization. These radicals act as mediators between HRP and luminol in an electron transfer reaction that generates luminol radicals and subsequently light emission, in which the intensity of CL is enhanced in the presence of acetaminophen. In addition, a simple method was developed to detect acetaminophen by static injection CL based on the enhanced CL system of luminol–H₂O₂–HRP by acetaminophen. Experimental conditions, such as pH and concentrations of substrates, have been examined and optimized. The proposed method exhibited good performance, the linear range was from 0.30 to 7.5 mM, the relative standard deviation was 1.86% (n = 10), limit of detection was 0.16 mM and recovery was 99 ± 4%. Copyright © 2013 John Wiley & Sons, Ltd.     

Radiosynthesis of three [11C]ureido-substituted benzenesulfonamides as PET probes for carbonic anhydrase IX in tumors

Three ureido-substituted benzenesulfonamides 1a-c have been developed as potent inhibitors for carbonic anhydrase IX, which is overexpressed in hypoxic tumors. In this study, we labeled these unsymmetrical ureas 1a-c using [(11)C]phosgene ([(11)C]COCl(2)) as a labeling agent with the expectation that [(11)C]1a-c could become promising positron tomography probes for imaging carbonic anhydrase IX in tumors. The strategy for radiosynthesis of [(11)C]1a-c was to react hydrochloride of anilines 2a-c with [(11)C]COCl(2) to give isocyanate [(11)C]4a-c, followed by a reaction with 4-aminobenzenesulfonamide (3).     

Aromatic N-hydroxyguanidines as new reduction cosubstrates for dopamine beta-hydroxylase

Conversion of neurotransmitter dopamine into norepinephrine is catalyzed by dopamine beta-hydroxylase (DbH). The reaction requires the presence of both molecular oxygen and a reducing cosubstrate, the assumed physiological cosubstrate being ascorbic acid. We have investigated the ability of a new family of molecules, N-aryl-N'-hydroxyguanidines, to serve as cosubstrates for DbH. N-(4-Methoxyphenyl)-N'-hydroxyguanidine proved to be an efficient reducing agent for DbH. The complete N-hydroxyguanidine moiety was required for activity, as any modification of this function resulted in non-cosubstrate compounds. Moreover, analysis of the products formed from N-(4-methoxyphenyl)-N'-hydroxyguanidine showed that the main oxidation product was a nitrosoimine. Modification of the aromatic para-substituent evidenced an influence of its electronic properties on the catalytic activity whereas steric factors seemed less important. In addition, changing the methoxy-substituent from the para- to the ortho-position led to an inactive compound. Our results demonstrate that N-aryl-N'-hydroxyguanidines are new efficient reducing cosubstrates for DbH and prove that specific interactions with the reducing cosubstrate do take place at the active site of the enzyme.     

Synthesis, the crystal structure, and high-resolution NMR spectroscopy of methyl 4-O-acetyl-3-azido-2,3,6-trideoxy-6-iodo-alpha-D-arabino-hexopyranoside

Selective tosylation followed by acetylation of methyl 3-azido-2,3-dideoxy-alpha-D-arabino-hexopyranoside (1) in pyridine at room temperature affords a mixture of methyl 4-O-acetyl-3-azido-2,3-dideoxy-6-di-O-p-tolylsulfonyl-alpha-D-arabino-hexopyranoside (4) and methyl 3-azido-2,3-dideoxy-4,6-di-O-p-tolylsulfonyl-alpha-D-arabino-hexopyranoside (3). Compound 4 undergoes nucleophilic displacement with sodium iodide in acetic anhydride to give methyl 4-O-acetyl-3-azido-2,3,6-trideoxy-6-iodo-alpha-D-arabino-hexopyranoside (7), whose crystal structure and (1H) and (13)C NMR data are reported. This compound adopts the 4C(1) conformation.     

QM/MM investigation on the catalytic mechanism of Bacteroides thetaiotaomicron α-glucosidase BtGH97a

Bacteroides thetaiotaomicron α-glucosidase BtGH97a is an inverting enzyme. In this paper, the hydrolysis mechanism of p-nitro-phenyl α-d-glucopyranoside (pNP-Glc) catalyzed by BtGH97a was firstly studied by using quantum mechanical/molecular mechanical (QM/MM) approach. Two possible reaction pathways were considered. In the first pathway, a water molecule deprotonated by a nucleophilic base (here E439 or E508) attacks firstly on the anomeric carbon of pNP-Glc, then a proton from an acid residue (E532) attacks on the glycosidic oxygen to finish the hydrolysis reaction (named as nucleophilic attack-first pathway). In the second pathway, the proton from E532 attacks firstly on the glycosidic oxygen, then the water deprotonated by the nucleophilic base attacks on the anomeric carbon of pNP-Glc (named as proton attack-first pathway). Our calculation results indicate that the nucleophilic attack-first pathway is favorable in energy, in which the nucleophilic attack process is the rate-determining step with an energy barrier of 15.4 kcal/mol in the case of residue E508 as nucleophilic base. In this rate-determining step, the deprotonation of water and the attack on the anomeric carbon are concerted. In the proton attack-first pathway, the proton attack on the glycosidic oxygen is the rate-determining step, and the energy barrier is 24.1 kcal/mol. We conclude that the hydrolysis mechanism would follow nucleophilic attack-first pathway.     

Noninvasive evaluation of liver metabolism by 2H and 13C NMR isotopomer analysis of human urine

Mammalian liver disposes of acetaminophen and other ingested xenobiotics by forming soluble glucuronides that are subsequently removed via renal filtration. When given in combination with the stable isotopes 2H and 13C, the glucuronide of acetaminophen isolated from urine provides a convenient "chemical biopsy" for evaluating intermediary metabolism in the liver. Here, we describe isolation and purification of urinary acetaminophen glucuronide and its conversion to monoacetone glucose (MAG). Subsequent 2H and 13C NMR analysis of MAG from normal volunteers after ingestion of 2H2O and [U-13C3]propionate allowed a noninvasive profiling of hepatic gluconeogenic pathways. The method should find use in metabolic studies of infants and other populations where blood sampling is either limited or problematic.