An Oxygenase-Independent Cholesterol Catabolic Pathway Operates under Oxic Conditions

Cholesterol is one of the most ubiquitous compounds in nature. The 9,10-seco-pathway for the aerobic degradation of cholesterol was established thirty years ago. This pathway is characterized by the extensive use of oxygen and oxygenases for substrate activation and ring fission. The classical pathway was the only catabolic pathway adopted by all studies on cholesterol-degrading bacteria. Sterolibacterium denitrificans can degrade cholesterol regardless of the presence of oxygen. Here, we aerobically grew the model organism with ¹³C-labeled cholesterol, and substrate consumption and intermediate production were monitored over time. Based on the detected ¹³C-labeled intermediates, this study proposes an alternative cholesterol catabolic pathway. This alternative pathway differs from the classical 9,10-seco-pathway in numerous important aspects. First, substrate activation proceeds through anaerobic C-25 hydroxylation and subsequent isomerization to form 26-hydroxycholest-4-en-3-one. Second, after the side chain degradation, the resulting androgen intermediate is activated by adding water to the C-1/C-2 double bond. Third, the cleavage of the core ring structure starts at the A-ring via a hydrolytic mechanism. The ¹⁸O-incorporation experiments confirmed that water is the sole oxygen donor in this catabolic pathway.     

Substrate Uptake and Subcellular Compartmentation of Anoxic Cholesterol Catabolism in Sterolibacterium denitrificans

Cholesterol catabolism by actinobacteria has been extensively studied. In contrast, the uptake and catabolism of cholesterol by Gram-negative species are poorly understood. Here, we investigated microbial cholesterol catabolism at the subcellular level. ¹³C metabolomic analysis revealed that anaerobically grown Sterolibacterium denitrificans, a β-proteobacterium, adopts an oxygenase-independent pathway to degrade cholesterol. S. denitrificans cells did not produce biosurfactants upon growth on cholesterol and exhibited high cell surface hydrophobicity. Moreover, S. denitrificans did not produce extracellular catabolic enzymes to transform cholesterol. Accordingly, S. denitrificans accessed cholesterol by direction adhesion. Cholesterol is imported through the outer membrane via a putative FadL-like transport system, which is induced by neutral sterols. The outer membrane steroid transporter is able to selectively import various C₂₇ sterols into the periplasm. S. denitrificans spheroplasts exhibited a significantly higher efficiency in cholest-4-en-3-one-26-oic acid uptake than in cholesterol uptake. We separated S. denitrificans proteins into four fractions, namely the outer membrane, periplasm, inner membrane, and cytoplasm, and we observed the individual catabolic reactions within them. Our data indicated that, in the periplasm, various periplasmic and peripheral membrane enzymes transform cholesterol into cholest-4-en-3-one-26-oic acid. The C₂₇ acidic steroid is then transported into the cytoplasm, in which side-chain degradation and the subsequent sterane cleavage occur. This study sheds light into microbial cholesterol metabolism under anoxic conditions.     

The 7α-hydroxysteroid dehydratase Hsh2 is essential for anaerobic degradation of the steroid skeleton of 7α-hydroxyl bile salts in the novel denitrifying bacterium Azoarcus sp. strain Aa7

Bile salts are steroid compounds from the digestive tract of vertebrates and enter the environment via defecation. Many aerobic bile-salt degrading bacteria are known but no bacteria that completely degrade bile salts under anoxic conditions have been isolated so far. In this study, the facultatively anaerobic Betaproteobacterium Azoarcus sp. strain Aa7 was isolated that grew with bile salts as sole carbon source under anoxic conditions with nitrate as electron acceptor. Phenotypic and genomic characterization revealed that strain Aa7 used the 2,3-seco pathway for the degradation of bile salts as found in other denitrifying steroid-degrading bacteria such as Sterolibacterium denitrificans. Under oxic conditions strain Aa7 used the 9,10-seco pathway as found in, for example, Pseudomonas stutzeri Chol1. Metabolite analysis during anaerobic growth indicated a reductive dehydroxylation of 7α-hydroxyl bile salts. Deletion of the gene hsh2 _Aa7 encoding a 7-hydroxysteroid dehydratase led to strongly impaired growth with cholate and chenodeoxycholate but not with deoxycholate lacking a hydroxyl group at C7. The hsh2 _Aa7 deletion mutant degraded cholate and chenodeoxycholate to the corresponding C₁₉-androstadienediones only while no phenotype change was observed during aerobic degradation of cholate. These results showed that removal of the 7α-hydroxyl group was essential for cleavage of the steroid skeleton under anoxic conditions.     

Three new 18,19-seco-ursane glycosides from Elsholtzia bodinieri

Chromatographic separation of an extract of the aerial part of Elsholtzia bodinieri resulted in the isolation of three new 18,19-seco-ursane glycosides, bodiniosides E-G (1–3). Their structures were elucidated as 2α,12β,23-trihydroxy-3-(β-d-glucopyranosyl)-19-oxo-18,19-seco-urs-13(18)-en-28-O-β-d-glucopyranosyl ester (1), 3-β-d-glucopyranosyl-19-β-d-glucopyranosyl-12β,21-dihydroxy-18,19-seco-urs-13(18)-en-28-oic acid (2), and 2α,12β,21-trihydroxy-3-β-d-glucopyranosyl-19-β-d-glucopyranosyl-18,19-seco-urs-13(18)-en-28-oic acid (3), respectively, by extensive NMR techniques, including 1D- and 2D-NMR experiments, as well as comparing with spectral data with those of the known analogues.     

Sesquiterpenoids and triterpenoids from Abies holophylla and their bioactivities

Six previously unreported and 11 known terpenoids were isolated from Abies holophylla. The structures of the six compounds were established as two unusual bisabolane sesquiterpenoids, three nortriterpenoids, and one 3,4-seco-triterpenoid based on the detailed analysis of their 1D and 2D NMR spectroscopic data. In addition, electronic circular dichroism (ECD) calculations and molecular orbital (MO) analysis were used to assign the absolute configuration of one bisabolane sesquiterpenoid, abiesesquine A. Abiesesquine A showed the strongest inhibitory effects against LPS-induced nitric oxide (NO) production in RAW264.7 macrophages with an IC₅₀ value of 113.1 μM. Lanosta-7,9(11),24-trien-26-oic acid showed potent cytotoxic activity against COLO-205, LOVO, and QGY-7703 tumor cells with IC₅₀ values of 0.9, 4.2, and 2.0 μM, respectively. (23R,25R)-3,4-seco-9βH-Lanosta-4(28),7-dien-26,23-olid-3-oic acid, exhibited a significant antiproliferation effect against A549 cells (IC₅₀ = 14.7 μM).     

The immunotropic activity of lupane and oleanane 2,3-<Emphasis Type="Italic">seco</Emphasis>-triterpenoids

The immunotropic effect of the 2,3-seco derivatives of allobetulon, betulonic acid, and the methyl ester of betulonic acid were studied. It was found that the highest activity is shown by compounds with an oleanane fragment. The presence of a free C28-carboxyl group enhances the activity of lupane 2,3-seco derivatives. A significant contribution to the development of the immune response is introduced by a functional group at the C3 atom in the A ring—the C3-carboxy derivatives intensify the processes of antibody production and alter the number and ratio of leukocyte forms. It is shown that 2,3-seco-1-cyano-19β,28-epoxy-18α-olean-3-oic acid stimulates humoral immunity with a positive influence on hematopoiesis.     

Kaurene derivatives from Alepidea amatynsia

Alepidea amatynsia afforded several known diterpenes, ent-16-kauren-19-oic acid, its 9(11)-dehydro derivative, ent-16-kauren-12-on-19-oic acid, wedelia seco-kaurenolide and a further seco-diterpene. The structure of the latter was established by ¹H NMR spectroscopy.     

Investigation of Beckett–Casy model 1: Synthesis of novel 16,17-seco-naltrexone derivatives and their pharmacology

Novel 16,17-seco-naltrexone derivatives 3 were synthesized using a 16–17 bond cleavage reaction of naltrexone as the key reaction to examine the Beckett–Casy model. All the prepared 16,17-seco-naltrexone derivatives 3 showed lower affinities for opioid receptors than naltrexone. Although the results of binding assay seem to support the existence of a cavity in the model, further investigation using 15,16-nornaltrexone derivatives 26 will be needed to confirm the model.     

Cerebrospinal Fluid Steroidomics: Are Bioactive Bile Acids Present in Brain?

In this study we have profiled the free sterol content of cerebrospinal fluid by a combination of charge tagging and liquid chromatography-tandem mass spectrometry. Surprisingly, the most abundant cholesterol metabolites were found to be C₂₇ and C₂₄ intermediates of the bile acid biosynthetic pathways with structures corresponding to 7α-hydroxy-3-oxocholest-4-en-26-oic acid (7.170 ± 2.826 ng/ml, mean ± S.D., six subjects), 3β-hydroxycholest-5-en-26-oic acid (0.416 ± 0.193 ng/ml), 7α,x-dihydroxy-3-oxocholest-4-en-26-oic acid (1.330 ± 0.543 ng/ml), and 7α-hydroxy-3-oxochol-4-en-24-oic acid (0.172 ± 0.085 ng/ml), and the C₂₆ sterol 7α-hydroxy-26-norcholest-4-ene-3,x-dione (0.204 ± 0.083 ng/ml), where x is an oxygen atom either on the CD rings or more likely on the C-17 side chain. The ability of intermediates of the bile acid biosynthetic pathways to activate the liver X receptors (LXRs) and the farnesoid X receptor was also evaluated. The acidic cholesterol metabolites 3β-hydroxycholest-5-en-26-oic acid and 3β,7α-dihydroxycholest-5-en-26-oic acid were found to activate LXR in a luciferase assay, but the major metabolite identified in this study, i.e. 7α-hydroxy-3-oxocholest-4-en-26-oic acid, was not an LXR ligand. 7α-Hydroxy-3-oxocholest-4-en-26-oic acid is formed from 3β,7α-dihydroxycholest-5-en-26-oic acid in a reaction catalyzed by 3β-hydroxy-Δ⁵-C₂₇-steroid dehydrogenase (HSD3B7), which may thus represent a deactivation pathway of LXR ligands in brain. Significantly, LXR activation has been found to reduce the symptoms of Alzheimer disease (Fan, J., Donkin, J., and Wellington C. (2009) Biofactors 35, 239–248); thus, cholesterol metabolites may play an important role in the etiology of Alzheimer disease.     

Microbial Hydroxylation of (−)-α-Santonin by Aspergillus niger

The microbiological transformation of a sesquiterpene lactone, (−)-α-santonin was carried out by using Aspergillus niger MIL 5024 and MIL 5025. Strain MIL 5024 brings about the transformation of (−)-α-santonin (400 mg) to 11-hydroxy-(−)-α-santonin (P1) (50.4 mg), 3,6,9-trihydroxy-9,10-seco-selina-1,3,5(10)-trien-12-oic acid-12,6-lactone (P2) (22.4 mg), and 3,6-dihydroxy-9,10-seco-selina-1,3,5(10)-trien-9,12-dioic acid-12,6-lactone (P3) (27.1 mg), which were isolated and characterized by UV, IR, mass and ¹H-NMR spectroanalyses. All of these products are described for the first time. Results similar to those with MIL 5024 were also obtained from the transformation of (−)-α-santonin by the other isolate, strain MIL 5025.     

13C NMR chemical shifts of ovalifoliene and related compounds with the 2,3-seco-alloaromadendrane skeleton: Structure of (+)-9α-acetoxyovalifoliene,a plant growth inhibitor

¹³C NMR spectra of several 2,3-seco-alloaromadendrane-type acetyl hemiacetals were analysed and the structure of an additional plant growth-inhibitor was determined to be (+)-9α-acetoxyovalifoliene. The biological activity of the new compound is also described.     

Elevated Proton Leak of the Intermediate OH in Cytochrome c Oxidase

The kinetics of the formation and relaxation of transmembrane electric potential (Δψ) during the complete single turnover of CcO was studied in the bovine heart mitochondrial and the aa₃-type Paracoccus denitrificans enzymes incorporated into proteoliposome membrane. The real-time Δψ kinetics was followed by the direct electrometry technique. The prompt oxidation of CcO and formation of the activated, oxidized (O_H) state of the enzyme leaves the enzyme trapped in the open state that provides an internal leak for protons and thus facilitates dissipation of Δψ (τ^app ≤ 0.5-0.8 s). By contrast, when the enzyme in the O_H state is rapidly re-reduced by sequential electron delivery, Δψ dissipates much slower (τ^app > 3 s). In P. denitrificans CcO proteoliposomes the accelerated Δψ dissipation is slowed down by a mutational block of the proton conductance through the D-, but not K-channel. We concluded that in contrast to the other intermediates the O_H state of CcO is vulnerable to the elevated internal proton leak that proceeds via the D-channel.     

The synthesis of triterpenic amides on the basis of 2,3-<Emphasis Type="Italic">seco</Emphasis>-1-cyano-19β,28-epoxy-18α-oleane-3-oic acid

Novel 2,3-seco-triterpenic amides were prepared by the interaction of the chloride of 1-cyano-19β,28-epoxy-18α-oleane-3-oic acid with primary amines and synthetic and biogenic amino acids. A cytotoxic triterpenic conjugate with a residue of the ethyl ester of β-alanine was found among the synthesized nitrogen-containing derivatives. Treatment with this conjugate in a concentration of 100 μM resulted in the 45.5% survival of melanoma cells in the medium.     

Glycolysis in human erythrocytes containing elevated concentrations of 2,3-P2-glycerate

In studies on the mechanism of the inhibitory effect of 2,3-diphosphoglycerate on glycolysis in human erythrocytes, the following results were obtained:1) Glucose consumption and lactate production are reduced by 70 and 40% relative to normal erythrocytes in red blood cells containing five times the normal amount of 2,3,-P₂-glycerate (“high-diphosphoglycerate” cells) at an extracellular pH of 7.4. The marked dependency of glycolysis on the extracellular pH observed in normal erythrocytes is almost completely lost in the “high-diphosphoglycerate” cells.2) About 50% of the inhibition of glycolysis in “high-diphosphoglycerate” cells can be accounted for by the 2,3-P₂-glycerate-induced decrease of the red-cell pH. This fall of the red-cell pH which occurs as a consequence of the Donnan effect of the non-penetrating 2,3-P₂-glycerate anion leads to a reduction of the glycolytic rate due to the properties of the enzyme phosphofructokinvse.3) The remaining part of the inhibitory effect must be attributed to an inhibition by 2,3-P₂-glycerate of glycolytic enzymes. From measurements of glycolytic rates and of the concentrations of glycolytic intermediates in the absence and presence of methylene blue it is concluded that the hexokinase reaction is inhibited by an elevation of 2,3-P₂-glycerate concentration. A marked increase of 3-P-glycerate concentration in “high-diphosphoglycerate” cells suggests that also the enzyme pyruvate kinase is inhibited by 2,3-P₂-glycerate.4) The dependencies of net-change of 2,3-P₂-glycerate concentration on the red-cell pH are identical in normal and “high-diphosphoglycerate” cells indicating that the balance between formation and decomposition of 2,3-P₂-glycerate is the same in erythrocytes with normal and very high concentrations 2,3-P₂-glycerate.     

Équilibres conformationnels de glucides au niveau de liaisons σ SP-SP: Partie V. Dérivés 2,3,-O-isopropylidène de pyranosides hybrides sp en C-4. Comparaison avec leurs analogues saturés

The conformation in solution of derivatives of methyl hexopyranosides has been studied by n.m.r. The esters of methyl 2,3-O-isopropylidene-α-D-manno- and -talopyranosides as well as their 4-deoxy-4-C-methyl analog having a manno configuration exist mainly in a flattened (^4,0F) chair conformation (⁴C₁). The presence in the talo epimer of the 4-deoxy-4-C-methyl analog of the bulky methyl group on the endo side of the bicyclic system results in a skew form (³S₁). The methyl 4-deoxy-2,3-O-isopropylidene-4-C-methylene-α-D-lyxo-hexopyranosides monosubstituted at C-4′ adopt, in solution, a conformation close to ³S₁, whichever their configuration (cis or trans) at the double bond, as indicated by their allylic coupling constants.     

A C31-secodammarane-type triterpenic acid, 12-deoxy alnustic acid,from the female flowers of alnus pendula

A new C_31--secodammarane-type triterpenic acid, 12-deoxy alnustic acid, was isolated from the female flowers of Alnus pendula. Its structure was elucidated to be (20S)-20-hydroxy-24-methylene-3,4-secodainmar-4(28)-en-3-oic acid by a combination of chemical and spectroscopic methods.     

Mechanisms of Cre recombinase synaptic complex assembly and activation illuminated by Cryo-EM

Cre recombinase selectively recognizes DNA and prevents non-specific DNA cleavage through an orchestrated series of assembly intermediates. Cre recombines two loxP DNA sequences featuring a pair of palindromic recombinase binding elements and an asymmetric spacer region, by assembly of a tetrameric synaptic complex, cleavage of an opposing pair of strands, and formation of a Holliday junction intermediate. We used Cre and loxP variants to isolate the monomeric Cre-loxP (54 kDa), dimeric Cre₂-loxP (110 kDa), and tetrameric Cre₄-loxP₂ assembly intermediates, and determined their structures using cryo-EM to resolutions of 3.9, 4.5 and 3.2 Å, respectively. Progressive and asymmetric bending of the spacer region along the assembly pathway enables formation of increasingly intimate interfaces between Cre protomers and illuminates the structural bases of biased loxP strand cleavage order and half-the-sites activity. Application of 3D variability analysis to the tetramer data reveals constrained conformational sampling along the pathway between protomer activation and Holliday junction isomerization. These findings underscore the importance of protein and DNA flexibility in Cre-mediated site selection, controlled activation of alternating protomers, the basis for biased strand cleavage order, and recombination efficiency. Such considerations may advance development of site-specific recombinases for use in gene editing applications.     

Photosynthetic Characteristics of C(3)-C(4) Intermediate Flaveria Species : I. Leaf Anatomy, Photosynthetic Responses to O(2) and CO(2), and Activities of Key Enzymes in the C(3) and C(4) Pathways

Four species of the genus Flaveria, namely F. anomala, F. linearis, F. pubescens, and F. ramosissima, were identified as intermediate C₃-C₄ plants based on leaf anatomy, photosynthetic CO₂ compensation point, O₂ inhibition of photosynthesis, and activities of C₄ enzymes. F. anomala and F. ramosissima exhibit a distinct Kranz-like leaf anatomy, similar to that of the C₄ species F. trinervia, while the other C₃-C₄ intermediate Flaveria species possess a less differentiated Kranz-like leaf anatomy. Photosynthetic CO₂ compensation points of these intermediates at 30°C were very low relative to those of C₃ plants, ranging from 7 to 14 microliters per liter. In contrast to C₃ plants, net photosynthesis by the intermediates was not sensitive to O₂ concentrations below 5% and decreased relatively slowly with increasing O₂ concentration. Under similar conditions, the percentage inhibition of photosynthesis by 21% O₂ varied from 20% to 25% in the intermediates compared with 28% in Lycopersicon esculentum, a typical C₃ species. The inhibition of carboxylation efficiency by 21% O₂ varied from 17% for F. ramosissima to 46% for F. anomala and were intermediate between the C₄ (2% for F. trinervia) and C₃ (53% for L. esculentum) values. The intermediate Flaveria species, especially F. ramosissima, have substantial activities of the C₄ enzymes, phosphoenolpyruvate carboxylase, pyruvate, orthophosphate dikinase, NADP-malic enzyme, and NADP-malate dehydrogenase, indicating potential for C₄ photosynthesis. It appears that these Flaveria species may be true biochemical C₃-C₄ intermediates.     

New syntheses of 2′-C-methylnucleosides starting from d-glucose and d-ribose

Effective general methods have been developed for the synthesis of 2′-C-methylnucleosides starting from d-glucose and d-ribose. 3-O-benzyl-1,2-O-isopropylidene-3-C-methyl-α-d-allofuranose was prepared in 5 steps from d-glucose and converted into 1,2,3-tri-O-acetyl-2-C-methyl-5-O-p-methylbenzoyl-d-ribofuranose (5), the starting compound for nucleoside synthesis. Compound 5 was also synthesised from 2-C-hydroxymethyl-2,3-O-isopropylidene-5-O-trityl-d-ribofuranose, prepared in 3 steps from d-ribose. Condensation of 5 with the bis-trimethylsilyl derivatives of uracil, N⁴-benzoylcytosine, and N⁶-benzoyladenine in the presence of F₃CSO₃OSiMe₃ followed by removal of the protecting acyl groups yielded the corresponding 2′-C-methylnucleosides.     

Synthesis of 3,6-dideoxy-3-(methylamino)hexoses for G.L.C.-M.S. identification of Rhizobium lipopolysaccharide components

A direct synthetic route from methyl α-d-glucopyranoside to 3,6-dideoxy-3-(methylamino)hexoses having the d-gluco, d-galacto, and d-manno configurations has been developed. Methyl α-d-glucoside was converted into the 4,6- <O-benzylidene-2,3,-di-O-tosyl derivative, which has then transformed into the 4-O-benzyl-6-deoxy 2,3-ditosylate (5) by successive reductive cleavage of the acetal ring, iodination, and reduction. The intermediate 5 was readily converted into the allo 2,3-epoxide, which yielded the pivotal intermediate methyl 4-O-benzyl-3,6-dideoxy-3-(methylamino)-α-d-glucopyranoside (7) by cleavage of the oxirane ring with methylamine. The amino compound 7 can be directly converted into the derivatized galacto and manno derivatives for mass-spectrometric identification by selective inversion at C-4 and C-2, respectively, followed by hydrolysis, reduction, and acetylation.