A hybrid catalyst-triggered cascade reaction for cholesterol detection using a smartphone-based miniature fluorescent apparatus

A new hybrid biological-chemical catalyst, magnetic nanoparticles functionalized with cholesterol oxidase (Fe3O4/APTES/ChOx), was developed for cholesterol detection. In the presence of cholesterol, the enzyme produced H2O2, which facilitated the generation of fluorescent molecules from the fluorogenic substrate with the assistance of Fe3O4 nanoparticles. A smartphone camera with a miniature fluorescent apparatus was used to assess fluorescence emission. Then, a smartphone application was employed to translate the fluorescence intensity to the red, green, and blue (RGB) domain. The developed approach achieved excellent selectivity and acceptable performances while supporting an onsite analysis approach. The practical operational range spanned from 5 to 100 nM, with a detection limit of 0.85 nM. Fe3O4/APTES/ChOx was applied for up to four replicates of reuse and demonstrated stability for at least 30 days. The applicability of the method was evaluated in milk samples, and the results were in accordance with the reference method.

Highly sensitive electrochemical detection of cholesterol based on Au-Pt NPs/PAMAM-ZIF-67 nanomaterials

A cholesterol biosensor was constructed by bimetallic (Au and Pt) and poly(amidoamine)-zeolite imidazole framework (PAMAM-ZIF-67). First, PAMAM-ZIF-67 nanomaterial was immobilized onto the electrode, and then Au and Pt were modified on the electrode by the electro-deposition method. Subsequently, cholesterol oxidase (ChOx) and cholesterol esterase (ChEt) were fixed on the electrode. The stepwise modification procedures were recorded by impedance spectroscopy and voltammetry. The current response presented a linear relation to the logarithm of cholesterol content when content ranged between 0.00015 and 10.24 mM, and the minimum detection concentration reached 3 nM. The electrode was also used for the cholesterol assay in serum, which hinted at its potentially valuable in clinical diagnostics. An electrochemical biosensor based on gold nanoparticles, platinum nanoparticles, and polyamide-zeolitic imidazolate frameworks was developed for detection of cholesterol. First, polyamide-zeolitic imidazolate frameworks nanomaterial was fixed onto the electrode modified of mercaptopropionic acid by Au-S bond. Then, gold nanoparticles and platinum nanoparticles were electrodeposited on the above electrode. Subsequently, cholesterol oxidase and cholesterol esterase were co-immobilized on the surface of the modified electrode to fabricate the cholesterol biosensor. The biosensor has also been used for the measurement of cholesterol in human serum, which implied potential applications in biotechnology and clinical diagnostics.

Facile immobilization of cholesterol oxidase on Pt,Ru-C nanocomposite and ionic liquid-modified carbon paste electrode for an efficient amperometric free cholesterol biosensing

In present work, the enzyme cholesterol oxidase (ChOx) was immobilized by Nafion® (Naf) on Pt,Ru-C nanocomposite and an ionic liquid (IL)-modified carbon paste electrode (CPE) in order to create cholesterol biosensor (Naf/ChOx/Pt,Ru-C/IL-CPE). The prepared working electrodes were characterized using scanning electron microscopy-energy-dispersive spectrometry, while their electrochemical performance was evaluated using electrochemical impedance spectroscopic, cyclic voltammetric, and amperometric techniques. Excellent synergism between IL 1-allyl-3-methylimidazolium dicyanamide ([AMIM][DCA]), Pt,Ru-C, and ChOx, as modifiers of CPE, offers the most pronounced analytical performance for improved cholesterol amperometric determination in phosphate buffer solution pH 7.50 at a working potential of 0.60 V. Under optimized experimental conditions, a linear relationship between oxidation current and cholesterol concentration was found for the range from 0.31 to 2.46 µM, with an estimated detection limit of 0.13 µM and relative standard deviation (RSD) below 5.5%. The optimized amperometric method in combination with the developed Naf/ChOx/Pt,Ru-C/IL-CPE biosensor showed good repeatability and high selectivity towards cholesterol biosensing. The proposed biosensor was successfully applied to determine free cholesterol in a human blood serum sample via its enzymatic reaction product hydrogen peroxide despite the presence of possible interferences. The percentage recovery ranged from 99.08 to 102.81%, while RSD was below 2.0% for the unspiked as well as the spiked human blood serum sample. The obtained results indicated excellent accuracy and precision of the method, concluding that the developed biosensor can be a promising alternative to existing commercial cholesterol tests used in medical practice.

Heterologous Expression and Function of Cholesterol Oxidase: A Review

Cholesterol was first found in gallstones as an animal sterol; hence it is called cholesterol. Cholesterol oxidase is the chief enzyme in the process of cholesterol degradation. Its role is obtained by the coenzyme FAD, which catalyzes the isomerization and oxidation of cholesterol to produce cholesteric 4-ene-3-ketone and hydrogen peroxide at the same time. Recently, a great advance has been made in the discovery of the structure and function of cholesterol oxidase, and it has proven added value in clinical discovery, medical care, food and biopesticides development and other conditions. By recombinant DNA technology, we can insert the gene in the heterologous host. Heterologous expression (HE) is a successful methodology to produce enzymes for function studies and manufacturing applications, where Escherichia coli has been extensively used as a heterologous host because of its economical cultivation, rapid growth, and efficiency in offering exogenous genes. Heterologous expression of cholesterol oxidase has been considered for several microbial sources, such as Rhodococcus equi, Brevibacterium sp., Rhodococcus sp., Streptomyces coelicolor, Burkholderia cepacia ST-200, Chromobacterium, and Streptomyces spp. All related publications of numerous researchers and scholars were searched in ScienceDirect, Scopus, PubMed, and Google Scholar. In this article, the present situation and promotion of heterologous expression of cholesterol oxidase, the role of protease, and the perspective of its possible applications were reviewed.

Recombinant Extracellular Cholesterol Oxidase from Nocardioides simplex

Cholesterol oxidase is a highly demanded enzyme used in medicine, pharmacy, agriculture, chemistry, and biotechnology. It catalyzes oxidation of 3β-hydroxy-5-ene- to 3-keto-4-ene- steroids with the formation of hydrogen peroxide. Here, we expressed 6xHis-tagged mature form of the extracellular cholesterol oxidase (ChO) from the actinobacterium Nocardioides simplex VKM Ac-2033D (55.6 kDa) in Escherichia coli cells. The recombinant enzyme (ChO_Ns) was purified using affinity chromatography. ChO_Ns proved to be functional towards cholesterol, cholestanol, phytosterol, pregnenolone, and dehydroepiandrosterone. Its activity depended on the structure and length of the aliphatic side chain at C17 atom of the steroid nucleus and was lower with pregnenolone and dehydroepiandrosterone. The enzyme was active in a pH range of 5.25÷6.5 with the pH optimum at 6.0. Kinetic assays and storage stability tests demonstrated that the characteristics of ChO_Ns were generally comparable with or superior to those of commercial ChO from Streptomyces hygroscopicus (ChO_Sh). The results contribute to the knowledge on microbial ChOs and evidence that ChO from N. simplex VKM Ac-2033D is a promising agent for further applications.

On-Site Colorimetric Detection of Cholesterol Based on Polypyrrole Nanoparticles

Herein, we report a facile method for cholesterol detection by coupling the peroxidase-like activity of polypyrrole nanoparticles (PPy NPs) and cholesterol oxidase (ChOx). ChOx can catalyze the oxidation of cholesterol to produce H₂O₂. Subsequently, PPy NPs, as a nanozyme, induce the reaction between H₂O₂ and 3,3',5,5'-tetramethylbenzidine (TMB). Under optimal conditions, the increase is proportional to cholesterol with concentrations from 10 to 800 μM in absorbance of TMB at 652 nm. The linear range for cholesterol is 10-100 μM, with a detection limit of 3.5 μM. This reported method is successfully employed for detection of cholesterol in human serum. The recovery percentage is ranged within 96-106.9%. Furthermore, we designed a facile and simple portable assay kit using the proposed system, realizing the on-site semiquantitative and visual detection of cholesterol in human serum. The cholesterol content detected from the portable assay kit were closely matching those obtained results from solution-based assays, thereby holding great potential in clinical diagnosis and health management.

Cholesterol oxidase from Rhodococcus erythropolis with high specificity toward β-cholestanol and pytosterols

Two genes (choRI and choRII) encoding cholesterol oxidases belonging to the vanillyl-alcohol oxidase (VAO) family were cloned on the basis of putative cholesterol oxidase gene sequences in the genome sequence data of Rhodococcus erythropolis PR4. The genes corresponding to the mature enzymes were cloned in a pET vector and expressed in Escherichia coli. The two cholesterol oxidases produced from the recombinant E. coli were purified to examine their properties. The amino acid sequence of ChoRI showed significant similarity (57%) to that of ChoRII. ChoRII was more stable than ChoRI in terms of pH and thermal stability. The substrate specificities of these enzymes differed distinctively from one another. Interestingly, the activities of ChoRII toward β-cholestanol, β-sitosterol, and stigmasterol were 2.4-, 2.1-, and 1.7-fold higher, respectively, than those of cholesterol. No cholesterol oxidases with high activity toward these sterols have been reported so far. The cholesterol oxidation products from these two enzymes also differed. ChoRI and ChoRII oxidized cholesterol to form cholest-4-en-3-one and 6β-hydroperoxycholest-4-en-3-one, respectively.

Cholesterol oxidase immobilized inulin based nanocomposite as the sensing material for cholesterol in biological and food samples

In the present study, inulin based nanocomposite viz., TiO₂-MWCNT@Inulin was prepared by embedding Inulin (a biopolymer extracted from Allium sativum L.) with TiO₂ and MWCNTs. The morphology of the prepared nanocomposite was characterized by High Resolution transmission electron microscopy (HRTEM). Cholesterol oxidase (ChOx) enzyme was then immobilized into the nanocomposite and the immobilization was examined by UV-vis and FT-IR spectral studies. The ChOx immobilized nanocomposite was integrated into carbon paste (CP) matrix to prepare the working electrode for the sensing of cholesterol. Electrochemical characterization of the modified CP/TiO₂-MWCNT@Inulin/ChOx electrode was done by cyclic voltammetric (CV) and electrochemical impedance spectroscopic (EIS) studies. Differential pulse voltammetric (DPV) studies were carried out to determine the concentration of cholesterol at the interface of the newly fabricated electrode. The fabricated electrode demonstrated a linear range from 83 μM to 14.28 mM, low limit of detection (35 μM), good sensitivity (21.26 μA mM^-1  cm^-2), low K_m (0.49 mM), high stability (120 days) and good selectivity. The presence of Inulin biopolymer played a vital role in attaching ChOx enzyme firmly to the nanocomposite thereby enhancing the stability and electron transfer efficiency of the electrode. The analysis of product that was formed within the electrochemical cell during the electrochemical oxidation of cholesterol was performed by using sodium nitroprusside. This resulted in a deep purple coloured solution which suggested the electrochemical conversion of cholesterol to cholestenone. The practical applicability of the fabricated electrode was also assessed by the determination of cholesterol in spiked blood serum and milk samples.

Coupling Two Sequential Biocatalysts with Close Proximity into Metal-Organic Frameworks for Enhanced Cascade Catalysis

The encapsulation of multiple enzyme/nanoenzyme systems within mental-organic frameworks (MOFs) shows great promise for a myriad of practical applications. Herein, two sequential biocatalysts, oxidase and hemin, were coupled together with close proximity using a bifunctional polymer, poly(1-vinylimidazole) (PVI), and encapsulated into MOFs. As a demonstration of the power of such a protocol, glucose oxidase&PVI-hemin encapsulated in ZIF-8 showed significant enhancement of bioactivity for a cascade reaction compared to its counterpart without PVI. For the colorimetric assay of glucose, it showed a low limit of detection of 0.4 μM (S/N = 3), high selectivity, and excellent stability. Because there are numerous biocatalysts that can readily be coupled and encapsulated into MOFs, a myriad of interesting properties can be simply realized by encapsulating different sequential biocatalysts.

In situ encapsulation of horseradish peroxidase in zeolitic imidazolate framework-8 enables catalyzing luminol reaction under near-neutral conditions for sensitive chemiluminescence determination of cholesterol

HRP@ZIF-8 nanocomposite was prepared by in situ encapsulation of horseradish peroxidase (HRP) in the frame of zeolitic imidazolate framework-8 (ZIF-8) with a simple one-pot method. The HRP@ZIF-8 nanocomposite displays outstanding thermal stability and efficiently catalyzes the chemiluminescence (CL) reaction of luminol with hydrogen peroxide (H₂O₂) under near-neutral pH condition (pH 7-8). This CL system has a good response to H₂O₂ with a linear range of 0.1-100.0 μmol L^-1. The limit of detection (LOD) is 0.06 μmol L^-1 H₂O₂. By marriage with cholesterol oxidase, cholesterol is determined with a linear range from 0.1 to 100.0 μmol L^-1 and a LOD of 0.04 μmol L^-1. The relative standard deviations (RSD) are 1.7% and 2.5%, respectively, in 11 repeated measurements of 50.0 μmol L^-1 solutions of H₂O₂ and cholesterol, indicating excellent precision of the method. The method shows good selectivity and has been applied to the determination of total cholesterol in real serum samples. No significant difference has been observed between the results obtained by this method and the cholesterol oxidase-peroxidase coupling method. Graphical abstract Schematic presentation of in situ one-pot synthesis of horseradish peroxidase@zeolitic imidazolate framework-8 (HRP@ZIF-8) nanocomposite and chemiluminescence determination of cholesterol with HRP@ZIF-8 catalyzing luminol-H₂O₂ system.

Enzymatic Reactions in a Lab-on-Valve System: Cholesterol Evaluations

The micro sequential injection analysis / lab-on-valve (µSIA-LOV) system is a miniaturized SIA system resulting from the implementation of a lab-on-valve (LOV) atop of the selection valve. It integrates the detection cell and the sample processing channels into the same device, promoting the reduction of reagent consumption and waste generation, the improvement of the versatility, and the reduction of the time of analysis. All of these characteristics are really relevant to the implementation of enzymatic reactions. Additionally, the evaluation of cholesterol in serum samples is widely relevant in clinical diagnosis, since higher values of cholesterol in human blood are actually an important risk factor for cardiovascular problems. An automatic methodology was developed based on the µSIA-LOV system in order to evaluate its advantages in the implementation of enzymatic reactions performed by cholesterol esterase, cholesterol oxidase and peroxidase. Considering these reactions, the developed methodology was also used for the evaluation of cholesterol in human serum samples, showing reliable and accurate results. The developed methodology presented detection and quantification limits of 1.36 and 4.53 mg dL⁻¹ and a linear range up to 40 mg dL⁻¹. This work confirmed that this µSIA-LOV system is a simple, rapid, versatile, and robust analytical tool for the automatic implementation of enzymatic reactions performed by cholesterol esterase, cholesterol oxidase, and peroxidase. It is also a useful alternative methodology for the routine determinations of cholesterol in real samples, even when compared with other automatic methodologies.

Expression optimization, purification, and functional characterization of cholesterol oxidase from Chromobacterium sp. DS1

Cholesterol oxidase is a bifunctional bacterial flavoenzyme which catalyzes oxidation and isomerization of cholesterol. This valuable enzyme has attracted a great deal of attention because of its wide application in the clinical laboratory, synthesis of steroid derived drugs, food industries, and its potentially insecticidal activity. Therefore, development of an efficient protocol for overproduction of cholesterol oxidase could be valuable and beneficial in this regard. The present study examined the role of various parameters (host strain, culture media, induction time, isopropyl ß-D-1-thiogalactopyranoside concentration, as well as post-induction incubation time and temperature) on over-expression of cholesterol oxidase from Chromobacterium sp. DS1. Applying the optimized protocol, the yield of recombinant cholesterol oxidase significantly increased from 92 U/L to 2115 U/L. Under the optimized conditions, the enzyme was produced on a large-scale, and overexpressed cholesterol oxidase was purified from cell lysate by column nickel affinity chromatography. K_m and V_max values of the purified enzyme for cholesterol were estimated using Lineweaver-Burk plot. Further, the optimum pH and optimum temperature for the enzyme activity were determined. This study reports a straightforward protocol for cholesterol oxidase production which can be performed in any laboratory.

Colorimetric Quantification of Glucose and Cholesterol in Human Blood Using a Nanocomposite Entrapping Magnetic Nanoparticles and Oxidases

In this study, a microscale well-plate colorimetric assay for the multiplexed detection of glucose and cholesterol in clinical human blood samples has been developed. This system utilized one-pot nanocomposite entrapping Fe3O4 magnetic nanoparticles (MNPs) as peroxidase mimetics and glucose oxidase (GOx)/cholesterol oxidase (ChOx) in mesoporous silica to detect glucose and cholesterol in blood samples. The sensing mechanism involves the generation of H2O2 by the catalytic action of an immobilized oxidase on the target molecules in the sample. This subsequently activates the MNPs in the mesopores, thereby leading to the conversion of the substrate into a colored end product. This strategy is used to detect the target glucose or cholesterol molecules in the concentration range of 15-250 mg/dL. The response is highly linear and the lower detection limit is 7.5 mg/dL. The aforementioned colorimetric assay is extremely convenient, and it exhibits a high degree of linearity, precision, and reproducibility when employing real human blood samples. Therefore, this assay can be used in clinical practice for the multiplexed and reliable quantification of glucose and cholesterol.

Purification and characterisation of the extracellular cholesterol oxidase enzyme from Enterococcus hirae

BACKGROUND

Recently many efforts are being carried out to reduce cholesterol in foods. Out of the 50 selected isolates that were tested using the agar well diffusion method to assess their ability to decompose cholesterol, 24 bacterial isolates were screened based on their cholesterol-decomposition ability in liquid media.

RESULTS

The bacterial isolate that displayed the highest cholesterol oxidase activity was identified as Enterococcus hirae. The maximal growth and cholesterol decomposition were achieved with a 1-day incubation under static conditions at 37 °C in cholesterol basal medium adjusted to pH 7 supplemented with 1 g/l cholesterol as the substrate, no additional carbon or nitrogen sources and 0.5 % CaSO4. The cholesterol oxidase enzyme (ChoX) produced by E. hirae was extracted at an (NH4)2SO4 saturation level of 80 % and purified with 79 % yield, resulting in 2.3-fold purification. The molecular weight of (ChoX) was 60 kDa. The optimal conditions required for the maximal activity of the purified COD enzyme produced by E. hirae were 30 min, 40 °C, pH 7.8, substrate concentration of 1 g/l and 200 ppm of MgCl2. The enzyme maintained approximately 36 % and 58.5 % of its activity after 18 days of storage at 4-8 °C. Also, the enzyme loss its activity by gradual thermal treatment, but it maintained 58.5 % of its activity at 95 °C for 2 hr.

CONCLUSIONS

E. hirae Mil-31 isolated from milk had a great capacity to decompose cholesterol in basal medium supplemented with cholesterol under its optimal growth conditions. Decomposition process of cholesterol by this strain results from its production of cholesterol oxidase enzyme (ChoX). The highest specific enzyme activity and highest purification fold of purified enzyme were achieved after using Sephadex G-100.

Cholesterol Oxidase Functionalised Polyaniline/Carbon Nanotube Hybrids for an Amperometric Biosensor

Functional carbon nanotubes (CNT) have attracted much attention for analytical and biomedical applications. This paper describes the fabrication of a cholesterol oxidase (ChOx) immobilised polyaniline (PANI)/CNT composite electrode for the amperometric detection of cholesterol. The prepared ChOx/PANI/CNT/Au bioelectrode bound ChOx via the available functionalties of PANI (-NH2) and CNT (-COOH). Moreover, the CNT creates a network inside the matrix that strengthens the mechanical property of the bioelectrode. The multifunctional matrix is presumed to provide a 3D-mesoporous surface, which substantially enhances enzyme activity. The linear range of the biosensor for cholesterol oleate was 30-280 μM with a response time of 10 sec.

Reusable and mediator-free cholesterol biosensor based on cholesterol oxidase immobilized onto TGA-SAM modified smart bio-chips

A reusable and mediator-free cholesterol biosensor based on cholesterol oxidase (ChOx) was fabricated based on self-assembled monolayer (SAM) of thioglycolic acid (TGA) (covalent enzyme immobilization by dropping method) using bio-chips. Cholesterol was detected with modified bio-chip (Gold/Thioglycolic-acid/Cholesterol-oxidase i.e., Au/TGA/ChOx) by reliable cyclic voltammetric (CV) technique at room conditions. The Au/TGA/ChOx modified bio-chip sensor demonstrates good linearity (1.0 nM to 1.0 mM; R = 0.9935), low-detection limit (∼0.42 nM, SNR∼3), and higher sensitivity (∼74.3 µAµM⁻¹cm⁻²), lowest-small sample volume (50.0 μL), good stability, and reproducibility. To the best of our knowledge, this is the first statement with a very high sensitivity, low-detection limit, and low-sample volumes are required for cholesterol biosensor using Au/TGA/ChOx-chips assembly. The result of this facile approach was investigated for the biomedical applications for real samples at room conditions with significant assembly (Au/TGA/ChOx) towards the development of selected cholesterol biosensors, which can offer analytical access to a large group of enzymes for wide range of biomedical applications in health-care fields.

Dehydroepiandrosterone sulfate (DHEAS) stimulates the first step in the biosynthesis of steroid hormones

Dehydroepiandrosterone sulfate (DHEAS) is the most abundant circulating steroid in human, with the highest concentrations between age 20 and 30, but displaying a significant decrease with age. Many beneficial functions are ascribed to DHEAS. Nevertheless, long-term studies are very scarce concerning the intake of DHEAS over several years, and molecular investigations on DHEAS action are missing so far. In this study, the role of DHEAS on the first and rate-limiting step of steroid hormone biosynthesis was analyzed in a reconstituted in vitro system, consisting of purified CYP11A1, adrenodoxin and adrenodoxin reductase. DHEAS enhances the conversion of cholesterol by 26%. Detailed analyses of the mechanism of DHEAS action revealed increased binding affinity of cholesterol to CYP11A1 and enforced interaction with the electron transfer partner, adrenodoxin. Difference spectroscopy showed K_d-values of 40±2.7 µM and 24.8±0.5 µM for CYP11A1 and cholesterol without and with addition of DHEAS, respectively. To determine the K_d-value for CYP11A1 and adrenodoxin, surface plasmon resonance measurements were performed, demonstrating a K_d-value of 3.0±0.35 nM (with cholesterol) and of 2.4±0.05 nM when cholesterol and DHEAS were added. Kinetic experiments showed a lower K_m and a higher k_cat value for CYP11A1 in the presence of DHEAS leading to an increase of the catalytic efficiency by 75%. These findings indicate that DHEAS affects steroid hormone biosynthesis on a molecular level resulting in an increased formation of pregnenolone.

On/Off-switchable zipper-like bioelectronics on a graphene interface

An on/off-switchable graphene-based zipper-like interface is architectured for efficient bioelectrocatalysis. The graphene interface transduces a temperature input signal into structural changes of the membrane, resulting in the amplification of electrochemical signals and their transformation into the gated transport of molecules through the membrane.

A dual enzyme functionalized nanostructured thulium oxide based interface for biomedical application

In this paper, we present results of the studies related to fabrication of a rare earth metal oxide based efficient biosensor using an interface based on hydrothermally prepared nanostructured thulium oxide (n-Tm2O3). A colloidal solution of prepared nanorods has been electrophoretically deposited (EPD) onto an indium-tin-oxide (ITO) glass substrate. The n-Tm2O3 nanorods are found to provide improved sensing characteristics to the electrode interface in terms of electroactive surface area, diffusion coefficient, charge transfer rate constant and electron transfer kinetics. The structural and morphological studies of n-Tm2O3 nanorods have been carried out by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopic techniques. This interfacial platform has been used for fabrication of a total cholesterol biosensor by immobilizing cholesterol esterase (ChEt) and cholesterol oxidase (ChOx) onto a Tm2O3 nanostructured surface. The results of response studies of the fabricated ChEt-ChOx/n-Tm2O3/ITO bioelectrode show a broad linear range of 8-400 mg dL(-1), detection limit of 19.78 mg (dL cm(-2))(-1), and high sensitivity of 0.9245 μA (mg per dL cm(-2))(-1) with a response time of 40 s. Further, this bioelectrode has been utilized for estimation of total cholesterol with negligible interference (3%) from analytes present in human serum samples. The utilization of this n-Tm2O3 modified electrode for enzyme-based biosensor analysis offers an efficient strategy and a novel interface for application of the rare earth metal oxide materials in the field of electrochemical sensors and bioelectronic devices.

Highly efficient bienzyme functionalized nanocomposite-based microfluidics biosensor platform for biomedical application

This report describes the fabrication of a novel microfluidics nanobiochip based on a composite comprising of nickel oxide nanoparticles (nNiO) and multiwalled carbon nanotubes (MWCNTs), as well as the chip's use in a biomedical application. This nanocomposite was integrated with polydimethylsiloxane (PDMS) microchannels, which were constructed using the photolithographic technique. A structural and morphological characterization of the fabricated microfluidics chip, which was functionalized with a bienzyme containing cholesterol oxidase (ChOx) and cholesterol esterase (ChEt), was accomplished using X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy. The XPS studies revealed that 9.3% of the carboxyl (COOH) groups present in the nNiO-MWCNT composite are used to form amide bonds with the NH2 groups of the bienzyme. The response studies on this nanobiochip reveal good reproducibility and selectivity, and a high sensitivity of 2.2 mA/mM/cm2. This integrated microfluidics biochip provides a promising low-cost platform for the rapid detection of biomolecules using minute samples.

A simple method for the determination of the cholesterol esterase activity

The proposed method determines the activity of cholesterol esterase (CEH) and takes advantage of its ability to catalyze the hydrolysis of cholesterol esters naturally present in human serum. The assay is based on Allain's method of spectrophotometric determination of cholesterol by means of cholesterol oxidase, peroxidase, but using 3,5-dichloro-dihydroxybenzenesulfonic acid (DHBS) as phenolic chromogen and human serum as a source of substrate for the CEH as a novelty. Furthermore, it is characterized by low costs and high precision. It can be employed to control the activity of CE preparations used for the preparation of enzymatic kits for the determination of cholesterol or for screening of potential bacterial enzyme producers.

Enzymatic activity of cholesterol oxidase immobilized onto polymer nanoparticles mediated by Congo red

Poly(ethylene glycol), PEG, decorated polystyrene (PS) nanoparticles were synthesized and characterized by means of dynamic light scattering (DLS), zeta (ζ) potential measurements, Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The adsorption of Congo red (CR) onto PS/PEG particles was evidenced by the decrease of ζ potential values and increase in the particles mean diameter in comparison to bare particles. Cholesterol oxidase (ChOx), the main enzyme in the oxidation of cholesterol, adsorbed onto PS/PEG and PS/PEG/CR particles, as revealed by the increase in the particles mean size and spectrophotometry. The enzymatic activity of free and immobilized ChOx was determined as a function of time by means of a coupled reaction with horseradish peroxidase. The activity of free ChOx decreased with time, while the activity of immobilized ChOx increased with time; after 1h reaction the latter was half of the former. Freeze-drying the ChOx covered PS/PEG/CR particles allowed their storage for at least one month under room conditions without loss of enzymatic activity. Conjugation effects between CR and ChOx or cholesterol evidenced by circular dichroism and spectrophotometry rendered a conformational state of ChOx, such that the enzymatic action was favored. ChOx adsorbed onto PS/PEG presents no enzymatic activity, probably due to ChOx denaturation or unfavorable orientation. Freeze-dried and freshly prepared dispersions of ChOx immobilized onto PS/PEG/CR particles yielded linear response in the cholesterol concentration range of 100mgdL(-1) (lowest limit of normal blood concentration) to 300mgdL(-1) (high risk level).

A highly efficient microfluidic nano biochip based on nanostructured nickel oxide

We present results of the studies relating to fabrication of a microfluidic biosensor chip based on nickel oxide nanorods (NRs-NiO) that is capable of directly measuring the concentration of total cholesterol in human blood through electrochemical detection. Using this chip we demonstrate, with high reliability and in a time efficient manner, the detection of cholesterol present in buffer solutions at clinically relevant concentrations. The microfluidic channel has been fabricated onto a nickel oxide nanorod-based electrode co-immobilized with cholesterol esterase (ChEt) and cholesterol oxidase (ChOx) that serves as the working electrode. Bare indium tin oxide served as the counter electrode. A Ag/AgCl wire introduced to the outlet of the microchannel acts as a reference electrode. The fabricated NiO nanorod-based electrode has been characterized using X-ray diffraction, Raman spectroscopy, HR-TEM, FT-IR, UV-visible spectroscopy and electrochemical techniques. The presented NRs-NiO based microfluidic sensor exhibits linearity in the range of 1.5-10.3 mM, a high sensitivity of 0.12 mA mM(-1) cm(-2) and a low value of 0.16 mM of the Michaelis-Menten constant (Km).

[Biocatalytic synthesis of pharmacology perspective stigmast-4-en-3-one using Rhodococci cells]

The conditions for a directed biocatalytic oxidation of beta-sitosterol to a pharmacologically promising stigmast-4-en-3-one using Rhodococcus actinobacteria were selected. It was shown that palmitic acid induced the cholesterol oxidase reaction and allowed for the decrease in the bioconversion process duration from 7 to 5 days. The maximum level ofstigmast-4-ene-3-one formation was achieved using n-hexadecane as an additional growth substrate. With increased concentrations of beta-sitosterol (up to 2 g/L) an effective target product formation (80%) was achieved in the presence of Tween-80 and beta-cyclodextrine. R. erythropolis strains were 1.5-2 times more active than R. ruber strains in catalyzing the beta-sitosterol biotransformation process.

Synthesis of halogenated pregnanes, mechanistic probes of steroid hydroxylases CYP17A1 and CYP21A2

The human steroidogenic cytochromes P450 CYP17A1 (P450c17, 17α-hydroxylase/17,20-lyase) and CYP21A2 (P450c21, 21-hydroxylase) are required for the biosynthesis of androgens, glucocorticoids, and mineralocorticoids. Both enzymes hydroxylate progesterone at adjacent, distal carbon atoms and show limited tolerance for substrate modification. Halogenated substrate analogs have been employed for many years to probe cytochrome P450 catalysis and to block sites of reactivity, particularly for potential drugs. Consequently, we developed efficient synthetic approaches to introducing one or more halogen atom to the 17- and 21-positions of progesterone and pregnenolone. In particular, novel 21,21,21-tribromoprogesterone and 21,21,21-trichloroprogesterone were synthesized using the nucleophilic addition of either bromoform or chloroform anion onto an aldehyde precursor as the key step to introduce the trihalomethyl moieties. When incubated with microsomes from yeast expressing human CYP21A2 or CYP17A1 with P450-oxidoreductase, CYP21A2 metabolized 17-fluoroprogesterone to a single product, whereas incubations with CYP17A1 gave no products. Halogenated steroids provide a robust system for exploring the substrate tolerance and catalytic plasticity of human steroid hydroxylases.

Phospholipid-sepiolite biomimetic interfaces for the immobilization of enzymes

Biomimetic interfaces based on phosphatidylcholine (PC) assembled to the natural silicate sepiolite were prepared for the stable immobilization of the urease and cholesterol oxidase enzymes. This is an important issue in practical advanced applications such as biocatalysis or biosensing. The supported lipid bilayer (BL-PC), prepared from PC adsorption, was used for immobilization of enzymes and the resulting biomimetic systems were compared to several other supported layers including a lipid monolayer (ML-PC), a mixed phosphatidylcholine/octyl-galactoside layer (PC-OGal), a cetyltrimethylammonium monolayer (CTA), and also to the bare sepiolite surface. Interfacial characteristics of these layers were investigated with a focus on layer packing density, hydrophilicity/hydrophobicity, and surface charge, which are being considered as key points for enzyme immobilization and stabilization of their biological activity. Cytoplasmic urease and membrane-bound cholesterol oxidase, which served as model enzymes, were immobilized on the different PC-based hybrid materials to probe their biomimetic character. Enzymatic activity was assessed by cyclic voltammetry and UV-vis spectrophotometry. The resulting enzyme/bio-organoclay hybrids were applied as active phase of a voltammetric urea biosensor and cholesterol bioreactor, respectively. Urease supported on sepiolite/BL-PC proved to maintain its enzymatic activity over several months while immobilized cholesterol oxidase demonstrated high reusability as biocatalyst. The results emphasize the good preservation of bioactivity due to the accommodation of the enzymatic system within the biomimetic lipid interface on sepiolite.

Polyaniline-carboxymethyl cellulose nanocomposite for cholesterol detection

Cholesterol oxidase (ChOx) has been covalently immobilized onto polyaniline-carboxymethyl cellulose (PANI-CMC) nanocomposite film deposited onto indium-tin-oxide (ITO) coated glass plate using glutaraldehyde as a cross-linker. Fourier transform infrared (FTIR) spectroscopic and electrochemical studies have been used to characterize the PANI-CMC/ITO nanocomposite electrode and ChOx/PANI-CMC/ITO bioelectrode. Scanning electron microscopy (SEM) studies reveal the formation of PANI-CMC nanocomposite fibers of size approximately 150 nm in diameter. The ChOx/PANI-CMC/ITO bioelectrode exhibits linearity as 0.5-22 mM, detection limit as 1.31 mM, sensitivity as 0.14 mA/mM cm2, response time as 10 s and shelf-life of about 10 weeks when bioelectrode is stored at 4 degrees C. The low value of Michaelis-Menten constant (K(m)) obtained as 2.71 mM reveals high affinity of immobilized ChOx for PANI-CMC/ITO nanocomposite electrode.

Preparation, characterization and application of polyaniline nanospheres to biosensing

Polyaniline nanospheres (PANI-NS) prepared by morphological transformation of micelle polymerized camphorsulfonic acid (CSA) doped polyaniline nanotubes (PANI-NT) in the presence of ethylene glycol (EG) have been characterized by X-ray diffraction, atomic force microscopy, transmission electron microscopy, scanning electron microscopy, Fourier transform infra-red and UV-Visible spectroscopy. A PANI-NS (60-80 nm) film deposited onto an indium-tin-oxide (ITO) coated glass plate by the solution casting method has been utilized for covalent immobilization of biomolecules (cholesterol oxidase (ChOx)) viaN-ethyl-N'-(3-dimethylaminopropyl) carbodiimide (EDC) and N-hydroxysuccinimide (NHS) chemistry for fabrication of a cholesterol biosensor. The ChOx/PANI-NS/ITO bioelectrode detects cholesterol in the concentration range of 25 to 500 mg dL(-1) with sensitivity of 1.3 x 10(-3) mA mg(-1) dL and regression coefficient of 0.98. Further, this PANI-NS based bioelectrode shows fast response time (10 s), low Michaelis-Menten constant (2.5 mM) and shelf-life of 12 weeks. The spherical nanostructure observed in the final morphology of the PANI-NS film is attributed to hydrogen bonding interactions between PANI-NT and EG.

A hydrogen-bonding network is important for oxidation and isomerization in the reaction catalyzed by cholesterol oxidase

Cholesterol oxidase is a flavoenzyme that catalyzes the oxidation and isomerization of 3beta-hydroxysteroids. Structural and mutagenesis studies have shown that Asn485 plays a key role in substrate oxidation. The side chain makes an NH...pi interaction with the reduced form of the flavin cofactor. A N485D mutant was constructed to further test the role of the amide group in catalysis. The mutation resulted in a 1800-fold drop in the overall k(cat). Atomic resolution structures were determined for both the N485L and N485D mutants. The structure of the N485D mutant enzyme (at 1.0 A resolution) reveals significant perturbations in the active site. As predicted, Asp485 is oriented away from the flavin moiety, such that any stabilizing interaction with the reduced flavin is abolished. Met122 and Glu361 form unusual hydrogen bonds to the functional group of Asp485 and are displaced from the positions they occupy in the wild-type active site. The overall effect is to disrupt the stabilization of the reduced FAD cofactor during catalysis. Furthermore, a narrow transient channel that is shown to form when the wild-type Asn485 forms the NH...pi interaction with FAD and that has been proposed to function as an access route of molecular oxygen, is not observed in either of the mutant structures, suggesting that the dynamics of the active site are altered.

Immobilization of Cholesterol Oxidase on Cellulose Acetate Membrane for Free Cholesterol Biosensor Development

This article describes the immobilization of cholesterol oxidase on a cellulose acetate (CA) membrane activated by Sodium periodate, ethylenediamine, and glutaraldehyde etc. The properties of the immobilized enzyme membrane were investigated. The factors affecting the activity of immobilized enzyme such as the concentration of glutaraldehyde, the concentration of enzyme used during immobilization, temperature, pH, and immobilizing time etc. were also studied. The immobilized COD membrane has been used to construct fibre-optic fluorescent biosensor.

Purification and characterization of Chromobacterium sp. DS-1 cholesterol oxidase with thermal, organic solvent, and detergent tolerance

A new screening method for 6beta-hydroperoxycholest-4-en-3-one (HCEO)-forming cholesterol oxidase was devised in this study. As the result of the screening, a novel cholesterol oxidase producer (strain DS-1) was isolated and identified as Chromobacterium sp. Extracellular cholesterol oxidase of strain DS-1 was purified from the culture supernatant. The molecular mass of the purified enzyme was 58 kDa. This enzyme showed a visible adsorption spectrum having peaks at 355 and 450 nm, like a typical flavoprotein. The enzyme oxidized cholesterol to HCEO, with the consumption of 2 mol of O2 and the formation of 1 mol of H2O2 for every 1 mol of cholesterol oxidized. The enzyme oxidized 3beta-hydroxysteroids such as cholesterol, beta-cholestanol, and pregnenolone at high rates. The Km value for cholesterol was 26 microM. The enzyme was stable at pH 3 to 11 and most active at pH 7.0-7.5, showing optimal activity at pH 7.0 and 65 degrees C. The enzyme retained about 80% of its activity after incubation for 30 min at 85 degrees C. The thermal stability of the enzyme was the highest among the cholesterol oxidases tested. Moreover, the enzyme was more stable in the presence of various organic solvents and detergents than commercially available cholesterol oxidases.

Cholesterol reporter molecules

Cholesterol is a major constituent of the membranes in most eukaryotic cells where it fulfills multiple functions. Cholesterol regulates the physical state of the phospholipid bilayer, affects the activity of several membrane proteins, and is the precursor for steroid hormones and bile acids. Cholesterol plays a crucial role in the formation of membrane microdomains such as "lipid rafts" and caveolae. However, our current understanding on the membrane organization, intracellular distribution and trafficking of cholesterol is rather poor. This is mainly due to inherent difficulties to label and track this small lipid. In this review, we describe different approaches to detect cholesterol in vitro and in vivo. Cholesterol reporter molecules can be classified in two groups: cholesterol binding molecules and cholesterol analogues. The enzyme cholesterol oxidase is used for the determination of cholesterol in serum and food. Susceptibility to cholesterol oxidase can provide information about localization, transfer kinetics, or transbilayer distribution of cholesterol in membranes and cells. The polyene filipin forms a fluorescent complex with cholesterol and is commonly used to visualize the cellular distribution of free cholesterol. Perfringolysin O, a cholesterol binding cytolysin, selectively recognizes cholesterol-rich structures. Photoreactive cholesterol probes are appropriate tools to analyze or to identify cholesterol binding proteins. Among the fluorescent cholesterol analogues one can distinguish probes with intrinsic fluorescence (e.g., dehydroergosterol) from those possessing an attached fluorophore group. We summarize and critically discuss the features of the different cholesterol reporter molecules with a special focus on recent imaging approaches.

Polyaniline Langmuir-Blodgett film based cholesterol biosensor

Cholesterol oxidase (ChOx) has been covalently linked to Langmuir-Blodgett (LB) monolayers of polyaniline (PANI)-stearic acid (SA) prepared onto indium-tin-oxide (ITO) coated glass plates via glutaraldehyde (Glu) chemistry. These ChOx/Glu/PANI-SA LB film/ITO electrodes have been characterized by FT-IR, cyclic voltammetry, and scanning electron microscopy, respectively. The results of response measurements carried out on these bioelectrodes using linear sweep voltammetry (LSV) reveal linearity from 25 to 400 mg/dL of cholesterol concentration with sensitivity of 88.9 nA mg(-1) dL. The linear regression analysis of bioelectrode reveals standard deviation and correlation coefficient of 0.737 microA and 0.9988, respectively. The low value of the Michaelis-Menten constant of these bioelectrodes obtained as 1.21 mM for the immobilized enzyme indicates increased interaction between ChOx and cholesterol in the PANI-SA LB film.

Heterologous expression of cholesterol oxidase in Bifidobacterium longum under the control of 16S rRNA gene promoter of bifidobacteria

We have constructed a constitutive high-level-expression vector for the genus Bifidobacterium and used it to express cholesterol oxidase from Streptomyces coelicola. The promoter region of the 16S rRNA gene was amplified by inverse PCR and used for the construction of pBES16PR. The optimal ribosome-binding site (RBS) for Bifidobacterium was incorporated in pBES16PR. In order to test the efficacy of this expression vector, we constructed pBES16PR-CHOL with the structural gene for cholesterol oxidase under the control of the 16S rRNA promoter, and used it to transform Bifidobacterium longum. The gene was successfully expressed and high level of cholesterol oxidase activity was obtained in B. longum. This is the first report of an expression vector for the genus Bifidobacterium using a 16S rRNA gene promoter and successful expression of cholesterol oxidase.

Covalent attachment of cholesterol oxidase and horseradish peroxidase on perlite through silanization: Activity, stability and co-immobilization

In the present work, co-immobilization of cholesterol oxidase (COD) and horseradish peroxidase (POD) on perlite surface was attempted. The surface of perlite were activated by 3-aminopropyltriethoxysilane and covalently bonded with COD and POD via glutaraldehyde. Enzymes activities have been assayed by spectrophotometric technique. The stabilities of immobilized COD and POD to pH were higher than those of soluble enzymes and immobilization shifted optimum pH of enzymes to the lower pH. Heat inactivation studies showed improved thermostability of the immobilized COD for more than two times, but immobilized POD was less thermostable than soluble POD. Also activity recovery of immobilized COD was about 50% since for immobilized POD was 11%. The K(m) of immobilized enzymes was found slightly lower than that of soluble enzymes. Immobilized COD showed inhibition in its activity at high cholesterol concentration which was not reported for soluble COD before. Co-immobilized enzymes retained 65% of its initial activity after 20 consecutive reactor batch cycles.

Cholesterol biosensor based on amino-undecanethiol self-assembled monolayer using surface plasmon resonance technique

Cholesterol oxidase has been covalently immobilized onto 11-amino-1-undecanethiol hydrochloride (AUT) self-assembled monolayer (SAM) fabricated on gold (Au) substrates using glutaraldehyde as a cross-linker. These ChOx/AUT/Au bioelectrodes characterized using contact angle (CA) measurements; electrochemical technique and atomic force microscopy (AFM) have been utilized for the estimation of cholesterol in solution using the surface plasmon resonance (SPR) technique. These biosensing electrodes exhibiting linearity from 50 to 500 mg/dL of cholesterol in solution and sensitivity of 1.23 m0/(mg dL), can be used more than 20 times and have a shelf life of about 10 weeks when stored at 4 degrees C.

Combined quantum mechanical and molecular mechanical simulations of one- and two-electron reduction potentials of flavin cofactor in water, medium-chain acyl-CoA dehydrogenase, and cholesterol oxidase

Flavin adenine dinucleotide (FAD) is a common cofactor in redox proteins, and its reduction potentials are controlled by the protein environment. This regulation is mainly responsible for the versatile catalytic functions of flavoenzymes. In this article, we report computations of the reduction potentials of FAD in medium-chain acyl-CoA dehydrogenase (MCAD) and cholesterol oxidase (CHOX). In addition, the reduction potentials of lumiflavin in aqueous solution have also been computed. Using molecular dynamics and free-energy perturbation techniques, we obtained the free-energy changes for two-electron/two-proton as well as one-electron/one-proton addition steps. We employed a combined quantum mechanical and molecular mechanical (QM/MM) potential, in which the flavin ring was represented by the self-consistent-charge density functional tight-binding (SCC-DFTB) method, while the rest of the enzyme-solvent system was treated by classical force fields. The computed two-electron/two-proton reduction potentials for lumiflavin and the two enzyme-bound FADs are in reasonable agreement with experimental data. The calculations also yielded the pKa values for the one-electron reduced semiquinone (FH*) and the fully reduced hydroquinone (FH2) forms. The pKa of the FAD semiquinone in CHOX was found to be around 4, which is 4 units lower than that in the enzyme-free state and 2 units lower than that in MCAD; this supports the notion that oxidases have a greater ability than dehydrogenases to stabilize anionic semiquinones. In MCAD, the flavin ring interacts with four hydrophobic residues and has a significantly bent structure, even in the oxidized state. The present study shows that this bending of the flavin imparts a significant destabilization (approximately 5 kcal/mol) to the oxidized state. The reduction potential of lumiflavin was also computed using DFT (M06-L and B3LYP functionals with 6-31+G(d,p) basis set) with the SM6 continuum solvation model, and the results are in good agreement with results from explicit free-energy simulations, which supports the conclusion that the SCC-DFTB/MM computation is reasonably accurate for both 1e(-)/1H+ and 2e(-)/2H+ reduction processes. These results suggest that the first coupled electron-proton addition is stepwise for both the free and the two enzyme-bound flavins. In contrast, the second coupled electron-proton addition is also stepwise for the free flavin but is likely to be concerted when the flavin is bound to either the dehydrogenase or the oxidase enzyme.

Development of cholesterol biosensor based on immobilized cholesterol esterase and cholesterol oxidase on oxygen electrode for the determination of total cholesterol in food samples

The development of a cholesterol biosensor by co-immobilization of cholesterol esterase (ChEt) and cholesterol oxidase (ChOX) on oxygen electrode is described. The electrode consists of gold cathode and Ag/AgCl anode. The enzymes were immobilized by cross-linking with glutaraldehyde and Bovine Serum Albumin (BSA). The immobilized enzymatic membrane was attached to the tip of the electrode by a push cap system. The optimum pH and temperature of the sensor was determined, these are 6 and 25 degrees C respectively. The developed sensor was calibrated from 1-75 mg/dl of cholesterol palmiate and found linear in the range of 2-50 mg/dL. The calibration curve was drawn with V(i) (ppm/min)(initial velocity) vs different concentrations of cholesterol palmiate (mg/dL). The application of the sensor to determine the total cholesterol in different real food samples such as egg, meat was investigated. The immobilized enzymatic layer can be reused over 30 times and the stability of the enzymatic layer was studied up to 9 weeks.

Cholesterol biosensor based on N-(2-aminoethyl)-3-aminopropyl-trimethoxysilane self-assembled monolayer

Cholesterol oxidase (ChOx) has been covalently immobilized onto two-dimensional self-assembled monolayer (SAM) of N-(2-aminoethyl)-3-aminopropyl-trimethoxysilane (AEAPTS) deposited on the indium-tin oxide (ITO) coated glass plates using N-ethyl-N'-(3-dimethylaminopropyl) carbodiimide and N-hydroxysuccinimide (EDC/NHS) chemistry. These ChO x/AEAPTS/ITO bioelectrodes are characterized using contact angle (CA) measurements, UV-visible spectroscopy, atomic force microscopy (AFM), electrochemical impedance technique, and Fourier transform infrared (FT-IR) technique. The covalently immobilized ChOx-modified AEAPTS bioelectrodes are used for the estimation of cholesterol in solution using UV-visible technique. These cholesterol sensing bioelectrodes show linearity as 50 to 500 mg/dl for cholesterol solution, detection limit as 25mg/dl, sensitivity as 4.499 x 10(-5) Abs (mg/dl)(-1), K(m) value as 58.137 mg/dl (1.5mM), apparent enzyme activity as 1.81 x 10(-3) U cm(-2), shelf life of approximately 10 weeks, and electrode reusability as 10 times.

Coenzyme precursor-assisted expression of a cholesterol oxidase from Brevibacterium sp. in Escherichia coli

The gene (choB(b)), encoding cholesterol oxidase from Brevibacterium sp. CCTCC M201008, was cloned and sequenced by PCR (GenBank accession number: DQ345780). The gene consists of 1653 base pairs and encodes a protein of 551 amino acids. ChoB(b) exhibited a homology of 98% with cholesterol oxidase gene from Brevibacterium sterolicum ATCC 21387. The cholesterol oxidase gene, cloned in the vector pET-28a, was over-expressed in Escherichia coli BL21-CodonPlus (DE3)-RP grown at 23 degrees C in Luria-Bertani medium containing 50 microM riboflavin, the precursor of the FAD coenzyme of the enzyme. A maximum activity of 3.7 U/mg was obtained from cell free extract of E. coli BL21-CodonPlus (DE3)-RP harboring the pET-28a-choB(b).

Immobilization of cholesterol esterase and cholesterol oxidase onto sol–gel films for application to cholesterol biosensor

Cholesterol oxidase (ChOx) and cholesterol esterase (ChEt) have been covalently immobilized onto tetraethylorthosilicate (TEOS) sol-gel films. The tetraethylorthosilicate sol-gel/ChEt/ChOx enzyme films thus prepared have been characterized using scanning electron microscopic (SEM), UV-vis spectroscopic, Fourier-transform-infrared (FTIR) spectroscopic and amperometric techniques, respectively. The results of photometric measurements carried out on tetraethylorthosilicate sol-gel/ChEt/ChOx reveal thermal stability up to 55 degrees C, response time as 180 s, linearity up to 780 mg dL(-1) (12 mM), shelf life of 1 month, detection limit of 12 mg dL(-1) and sensitivity as 5.4 x 10(-5) Abs. mg(-1) dL(-1).

Fluorometric assay for detection of sterol oxidation in liposomal membranes

The authors have developed a fluorescence assay to measure the rate and extent of sterol oxidation in lipid bilayers. Dehydroergosterol (DHE), a fluorescent cholesterol analog, is used as a probe and at the same time as a membrane component. The assay can also be performed on bilayers containing a mixture of sterols including DHE and nonfluorescent sterols, such as cholesterol and ergosterol. The fluorescence intensity of DHE decreases on oxidation, so the rate and extent of free radical- or enzyme-induced sterol oxidation can be measured as a function of temperature and membrane composition. For the studies, two-component (e.g., phosphatidylcholine (PC)/DHE) and multicomponent (e.g., DHE/PC/bovine-brain sphingomyelin) large unilamellar vesicles were used, and sterol oxidation was initiated either by the peroxy radical generator 2,2'-azobis (2-amidinopropane) dihydrochloride or by the enzyme cholesterol oxidase. The data gathered from this assay may be used to examine the effects of water- and lipid-soluble antioxidants on membrane sterol oxidation produced by free radicals. This assay can be used to test the potency of antioxidants and pro-oxidants, and can be used to determine whether unknown substances demonstrate antioxidant activity against sterol oxidation. The assay can also be used as a tool to examine the effect of sterol lateral organization on sterol oxidation (in the presence or absence of antioxidants). In agreement with the sterol regular distribution model, it is found that both free radical- and enzyme-induced sterol oxidation vary with membrane sterol content in a well defined alternating manner.

Structural and kinetic analyses of the H121A mutant of cholesterol oxidase

Cholesterol oxidase is a monomeric flavoenzyme that catalyses the oxidation of cholesterol to cholest-5-en-3-one followed by isomerization to cholest-4-en-3-one. The enzyme from Brevibacterium sterolicum contains the FAD cofactor covalently bound to His121. It was previously demonstrated that the H121A substitution results in a approximately 100 mV decrease in the midpoint redox potential and a approximately 40-fold decrease in turnover number compared to wild-type enzyme [Motteran, Pilone, Molla, Ghisla and Pollegioni (2001) Journal of Biological Chemistry 276, 18024-18030]. A detailed kinetic analysis of the H121A mutant enzyme shows that the decrease in turnover number is largely due to a corresponding decrease in the rate constant of flavin reduction, whilst the re-oxidation reaction is only marginally altered and the isomerization reaction is not affected by the substitution and precedes product dissociation. The X-ray structure of the mutant protein, determined to 1.7 A resolution (1 A identical with 0.1 nm), reveals only minor changes in the overall fold of the protein, namely: two loops have slight movements and a tryptophan residue changes conformation by a rotation of 180 degrees about chi1 compared to the native enzyme. Comparison of the isoalloxazine ring moiety of the FAD cofactor between the structures of the native and mutant proteins shows a change from a non-planar to a planar geometry (resulting in a more tetrahedral-like geometry for N5). This change is proposed to be a major factor contributing to the observed alteration in redox potential. Since a similar distortion of the flavin has not been observed in other covalent flavoproteins, it is proposed to represent a specific mode to facilitate flavin reduction in covalent cholesterol oxidase.

Lipid headgroup superlattice modulates the activity of surface-acting cholesterol oxidase in ternary phospholipid/cholesterol bilayers

The relationship between the molecular organization of lipid headgroups and the activity of surface-acting enzyme was examined using a bacterial cholesterol oxidase (COD) as a model. The initial rate of cholesterol oxidation by COD in fluid state 1-palmitoyl-2-oleoyl-phosphatidylethanolamine/1-palmitoyl-2-oleoyl-phosphatidylcholine/cholesterol (POPE/POPC/CHOL) bilayers was measured as a function of POPE-to-phospholipid mole ratio (X(PE)) and cholesterol-to-lipid mole ratio (X(CHOL)) at 37 degrees C. At X(PE) = 0, the COD activity changed abruptly at X(CHOL) approximately 0.40, whereas major activity peaks were detected at X(PE) approximately 0.18, 0.32, 0.50, 0.64, and 0.73 when X(CHOL) was fixed to 0.33 or 0.40. At a fixed X(CHOL) of 0.50, the COD activity increased progressively with PE content and exhibited small peaks or kinks at X(PE) approximately 0.40, 0.50, 0.58, 0.69, and 0.81. When X(PE) and X(CHOL) were systematically varied within a narrow 2-D lipid composition window, an onset of COD activity at X(CHOL) approximately 0.40 and the elimination of the activity peak at X(PE) approximately 0.64 for X(CHOL) >0.40 were clearly observed. Except for X(PE) approximately 0.40 and 0.58, the observed critical PE mole ratios agree closely (+/-0.03) with those predicted by a headgroup superlattice model (Virtanen, J.A., et al. (1998) Proc. Natl. Acad. Sci. U.S.A. 95, 4964-4969; Cannon, B., et al. (2006) J. Phys. Chem. B 110, 6339-6350), which proposes that lipids with headgroups of different sizes tend to adopt regular, superlattice-like distributions at discrete and predictable compositions in fluid lipid bilayers. Our results indicate that headgroup superlattice domains exist in lipid bilayers and that they may play a crucial role in modulating the activity of enzymes acting on the cell membrane surface.

Protection of membrane cholesterol by sphingomyelin against free radical-mediated oxidation

Although the free radical-mediated oxidation of free cholesterol (FC) is critical in the generation of regulatory sterols and in atherogenesis, the physiological regulation of this process is poorly understood. We tested the hypothesis that sphingomyelin (SM), a major phospholipid of cell membranes, which is closely associated with FC, protects FC against oxidation, because of its unique structure, and affinity to the sterol. We employed phosphatidylcholine (PC) liposomes containing varying amounts of SM, and either radioactive FC or a fluorescent analog, dehydroergosterol (DHE), and determined the oxidative decay of the sterol in presence of 2,2'-azo-bis(2-amidinopropane hydrochloride) (AAPH). Incorporation of 25 mol% of SM in the liposomes inhibited the oxidation of FC or DHE by up to 50%. This inhibition was specific for SM among phospholipids, and was abolished by sphingomyelinase treatment. SM was not degraded during the oxidation reaction, and its effect was not dependent on the nature of the oxidizing agent, because it also inhibited sterol oxidation by FeSO(4)/ascorbate, and by cholesterol oxidase. These studies show that SM plays a physiological role in the regulation of cholesterol oxidation by free radicals.

Atomic resolution crystallography reveals how changes in pH shape the protein microenvironment

Hydrogen atoms are a vital component of enzyme structure and function. In recent years, atomic resolution crystallography (>or=1.2 A) has been successfully used to investigate the role of the hydrogen atom in enzymatic catalysis. Here, atomic resolution crystallography was used to study the effect of pH on cholesterol oxidase from Streptomyces sp., a flavoenzyme oxidoreductase. Crystallographic observations of the anionic oxidized flavin cofactor at basic pH are consistent with the UV-visible absorption profile of the enzyme and readily explain the reversible pH-dependent loss of oxidation activity. Furthermore, a hydrogen atom, positioned at an unusually short distance from the main chain carbonyl oxygen of Met122 at high pH, was observed, suggesting a previously unknown mechanism of cofactor stabilization. This study shows how a redox active site responds to changes in the enzyme's environment and how these changes are able to influence the mechanism of enzymatic catalysis.

Performance characteristics of cholesterol oxidase for kinetic determination of total cholesterol

The enzymatic method for cholesterol determination can use either an endpoint or a kinetic method. Not much is known concerning the properties (K(m) and V(max)) of the commercial enzyme for the kinetic method. We measured the K(m) and V(max) of Brevibacterium, Streptomyces, Pseudomonas fluorescens, and Cellulomonas cholesterol oxidase. Brevibacterium gave the highest K(m) value (230.3 x 10(-4) M), followed by Streptomyces (2.17 x 10(-4) M), Cellulomonas (0.84 x 10(-4) M), and Pseudomonas (0.61 x 10(-4) M). The K(m) values and the linearity obtained from Streptomyces (2.6 mmol/L), Pseudomonas (2.1 mmol/L), or Cellulomonas (2.1 mmol/L) were too low. Dichlorophenol isomers, acting as inhibitors, increased the enzyme's K(m). The addition of 3,4-dichlorophenol raised the K(m) of Streptomyces from 2.17 x 10(-4) to 24.89 x 10(-4) M. The linearity was increased from 2.6 to 13.0 mmol/L. The high K(m) of Brevibacterium resulted in an insensitive reaction and low cholesterol linearity (7.8 mmol/L). An increase in the sample-to-reagent ratio from 1:100 to 1:10 enhanced the reaction rate and the linearity from 7.8 to 20.7 mmol/L. We suggest that Brevibacterium and Streptomyces cholesterol oxidase (with the addition of 3,4 dichlorophenol) are good sources for serum cholesterol determination by the kinetic method.

Analysis of oxysterols by electrospray tandem mass spectrometry

Oxysterols are oxygenated derivatives of cholesterol. They are intermediates in cholesterol excretion pathways and may also be regarded as transport forms of cholesterol. The introduction of additional hydroxyl groups to the cholesterol skeleton facilitates the flux of oxysterols across the blood brain barrier, and oxysterols have been implicated in mediating a number of cholesterol-induced metabolic effects. Oxysterols are difficult to analyze by atmospheric pressure ionization mass spectrometry on account of the absence of basic or acidic functional groups in their structures. In this communication, we report a method for the derivatization and analysis of oxysterols by electrospray mass spectrometry. Oxysterols with a 3beta-hydroxy-Delta5 structure were converted by cholesterol oxidase to 3-oxo-Delta4 steroids and then derivatized with the Girard P reagent to give Girard P hydrazones, which were subsequently analyzed by tandem mass spectrometry. The improvement in sensitivity for the analysis of 25-hydroxycholesterol upon oxidation and derivatization was over 1000.

Enzymic conversion of 3β-hydroxy-5-ene-steroids and their sulfates to 3-oxo-4-ene-steroids for increasing sensitivity in LC–APCI-MS

A method of increasing the sensitivity of 3beta-hydroxy-5-ene (Delta5)-steroids in liquid chromatography-atmospheric pressure chemical ionization-mass spectrometry (LC-APCI-MS) based on the structural conversion by cholesterol oxidase (ChO) was demonstrated. The Delta5-steroids were rapidly converted to their 3-oxo-4-ene (Delta4)-forms by the treatment with ChO and the obtained Delta4-forms provided 3-14-fold higher sensitivity compared to intact steroids in the positive-APCI-MS. This enzymic conversion method was also applied to the sulfated conjugates of Delta5-steroids after solvolysis. The method enabled the detection of trace levels of dehydroepiandrosterone and androstenediol 3-sulfate in human serum, which could not be detected by the usual LC-MS.

Functional conformational motions in the turnover cycle of cholesterol oxidase

Reexamining experimental data of single-molecule fluorescence correlation spectroscopy for cholesterol oxidase, we find that the existing Michaelis-Menten models with dynamical disorder cannot explain strong correlations between subsequent turnover cycles revealed in the diagonal feature in the joint statistical distribution of adjacent "on" times of this enzyme. We suggest that functional conformational motions representing ordered sequences of transitions between a set of conformational substates are involved, along with equilibrium conformational fluctuations in the turnover cycle of cholesterol oxidase. A two-channel model of single-enzyme dynamics, including a slow functional conformational motion in one of the channels, is proposed that allows us to reproduce such strong correlations.