MALDI imaging mass spectrometry and chemometric tools to discriminate highly similar colorectal cancer tissues.

Intratumour heterogeneity due to cancer cell clonal evolution and microenvironment composition and tumor differences due to genetic variations between patients suffering of the same cancer pathology play a crucial role in patient response to therapies. This study is oriented to show that matrix-assisted laser-desorption ionization-Mass spectrometry imaging (MALDI-MSI), combined with an advanced multivariate data processing pipeline can be used to discriminate subtle variations between highly similar colorectal tumors. To this aim, experimental tumors reproducing the emergence of drug-resistant clones were generated in athymic mice using subcutaneous injection of different mixes of two isogenic cell lines, the irinotecan-resistant HCT116-SN50 (R) and its sibling human colon adenocarcinoma sensitive cell line HCT116 (S). Because irinotecan-resistant and irinotecan-sensitive are derived from the same original parental HCT116 cell line, their genetic characteristics and molecular compositions are closely related. The multivariate data processing pipeline proposed relies on three steps: (a) multiset multivariate curve resolution (MCR) to separate biological contributions from background; (b) multiset K-means segmentation using MCR scores of the biological contributions to separate between tumor and necrotic parts of the tissues; and (c) partial-least squares discriminant analysis (PLS-DA) applied to tumor pixel spectra to discriminate between R and S tumor populations. High levels of correct classification rates (0.85), sensitivity (0.92) and specificity (0.77) for the PLS-DA classification model were obtained. If previously labelled tissue is available, the multistep modeling strategy proposed constitutes a good approach for the identification and characterization of highly similar phenotypic tumor subpopulations that could be potentially applicable to any kind of cancer tissue that exhibits substantial heterogeneity.

Use of physiological information based on grayscale images to improve mass spectrometry imaging data analysis from biological tissues.

The characterization of cancer tissues by matrix-assisted laser desorption ionization-mass spectrometry images (MALDI-MSI) is of great interest because of the power of MALDI-MS to understand the composition of biological samples and the imaging side that allows for setting spatial boundaries among tissues of different nature based on their compositional differences. In tissue-based cancer research, information on the spatial location of necrotic/tumoral cell populations can be approximately known from grayscale images of the scanned tissue slices. This study proposes as a major novelty the introduction of this physiologically-based information to help in the performance of unmixing methods, oriented to extract the MS signatures and distribution maps of the different tissues present in biological samples. Specifically, the information gathered from grayscale images will be used as a local rank constraint in Multivariate Curve Resolution-Alternating Least Squares (MCR-ALS) for the analysis of MALDI-MSI of cancer tissues. The use of this constraint, setting absence of certain kind of tissues only in clear zones of the image, will help to improve the performance of MCR-ALS and to provide a more reliable definition of the chemical MS fingerprint and location of the tissues of interest. The general strategy to address the analysis of MALDI-MSI of cancer tissues will involve the study of the MCR-ALS results and the posterior use of MCR-ALS scores as dimensionality reduction for image segmentation based on K-means clustering. The resolution method will provide the MS signatures and their distribution maps for each tissue in the sample. Then, the resolved distribution maps for each biological component (MCR scores) will be submitted as initial information to K-means clustering for image segmentation to obtain information on the boundaries of the different tissular regions in the samples studied. MCR-ALS prior to K-means not only provides the desired dimensionality reduction, but additionally resolved non-biological signal contributions are not used and the weight given to the different biological components in the segmentation process can be modulated by suitable preprocessing methods.

Kinetics of butyrylcholinesterase in reversed micelles under high pressure.

The combined effects of high pressure and reversed micelles have been studied to modulate the catalytic behaviour of butyrylcholinesterase. The purpose of this study was to determine whether the conformational plasticity of the enzyme is altered by entrapment in reversed micelles. The presence of soman, an irreversible inhibitor of cholinesterase was used to bring to the fore a possible modification of the enzyme behaviour in this system under pressure. Results show differences between enzyme in conventional medium and in reversed micelles regarding the mechanism of BuChE catalyzed hydrolysis of acetylthiocholine. In both systems, the enzyme displays a non-Michaelian behaviour with this substrate. In conventional medium the kinetics is multiphasic with an activation phase followed by an inhibition phase at high concentration. In reversed micelles there is inhibition by excess substrate but the activation phase is missing. This behaviour may be the result of a change of the enzyme conformational plasticity when is entrapped in reversed micelles.

High-pressure stabilization of alpha-chymotrypsin entrapped in reversed micelles of aerosol OT in octane against thermal inactivation.

alpha-Chymotrypsin (CT) solubilized in reversed micelles of sodium bis-(2-ethylhexyl)-sulfosuccinate (AOT) undergoes thermal inactivation and the enzyme stability decreases significantly when temperature increases (25-40 degrees C). The half-life of CT in micelles shows a bell-shaped dependence on the degree of hydration of AOT (wo) analogous to the previously obtained dependence on wo for the enzyme activity. The optima of catalytic activity and thermal stability have been observed under conditions where the diameter of the inner aqueous cavity of the micelle is close to the size of the enzyme molecule (wo = 10). Application of high hydrostatic pressure in the range of 1-1500 atm (bar) stabilizes CT against thermal inactivation at all hydration degrees (wo) from 7 to 20; the stabilization effect is most pronounced under the experimental conditions being far from the optimum for catalytic activity.

The role of tetrahydrobiopterin in the activation of oxygen by nitric-oxide synthase.

We have studied the reaction of reduced nitric-oxide synthase (NOS) with molecular oxygen at -30 degrees C. In the first reaction cycle (from L-Arg to hydroxy-L-Arg), an oxygen adduct complex formed rapidly. Experiments in the absence of the reductase domain demonstrated that this complex was then further reduced by one electron stemming from the cofactor tetrahydrobiopterin (BH4). Spectral evidence suggested an iron(IV) porphyrin pi-cation radical as an intermediate. The nature of the oxidized BH4 was identified by EPR as a BH3* radical. Within the second cycle (from hydroxy-L-Arg to citrulline and NO), an iron(III)-NO complex could be identified clearly by its spectral characteristics. The strict requirement of BH4 for its formation suggests that BH4 plays a redox role, although transient, also in the second reaction cycle.

Use of high pressure to study elementary steps in P450 and nitric oxide synthase.

Chemical reactions are often highly pressure-dependent. A perturbation of the elementary steps by pressure therefore offers the possibility of a detailed characterization of enzyme mechanisms. We used this method to study distinct steps in the reaction of nitric-oxide synthase (NOS), and compared them to analogous steps in the reaction of cytochrome P450 BM3 (BM3). Our results indicate that, in BM3, electron transfer depends on electrostatic interactions. In NOS, pressure, similarly to chemical denaturants, can mimic the structural effects of Ca/calmodulin. This helps to better understand the structural basis of the regulatory effect of Ca/calmodulin. Furthermore, stopped-flow kinetics under high pressure show that CO binding to the heme iron is hindered by substrate in NOS, but not in BM3. This indicates a relatively large or flexible substrate binding site in BM3, and a more narrow and rigid binding site in NOS.

A central role for water in the control of the spin state of cytochrome P-450scc.

A previous thermodynamic study [Lange, R., Larroque, C. & Anzenbacher, P. (1992) Eur. J. Biochem. 207, 69-73] demonstrated two conformations (A and B) of cytochrome P-450scc (SCC), the enzyme which initiates steroid biosynthesis by cleaving the side chain of cholesterol. The conformation found at the lowest temperatures (form A) displays a six-ligand high-spin heme iron [Hildebrandt, P., Heibel, G., Anzenbacher, P., Lange, R., Krüger, V. & Stier, A. (1994) Biochemistry 33, 12920-12929]. Analytical centrifugation shows that the oligomeric composition of SCC is the same for the A and the B conformers. However, as revealed by fourth-derivative ultraviolet spectroscopy, the two conformers differ in the mean environment of the tryptophan residues, which was more polar in the A form. The structural role of water in these two conformations was investigated using the pressure-jump technique under various pH, temperature and osmotic-stress conditions. Applying hydrostatic pressure to SCC induced very slow (tau >30 min) biexponential relaxation kinetics corresponding to the high-spin to low-spin transition. Analysis of the activation volumes suggested a dissociative mechanism for the A conformer (+45 ml/mol), and an associative mechanism for the B conformer (-39 ml/mol). Applying osmotic stress to the A form changed its kinetic characteristics to those of the B form. These results are consistent with a model comprising a solvent intake (ten water molecules) between the B and the A conformers and protonation of their respective high-spin states. The sixth ligand of the high-spin form in the A conformer involves a water molecule and an unknown constraining structure.

Pressure effects on enzyme reactions in mainly organic media: alpha-chymotrypsin in reversed micelles of Aerosol OT in octane.

Biocatalytic transformations in reversed micelles formed by anionic surfactant Aerosol OT in octane have been studied at high pressures by an example of alpha-chymotrypsin-catalyzed hydrolysis of N-carbobenzoxy-L-tyrosine p-nitrophenyl ester and N-succinyl-L-phenylalanine p-nitroanilide. For the first time it has been found that the enzyme retains high activity in these water-in-oil microemulsions up to a pressure of 2 kbar. The value of the activation volume (delta V*) for the enzyme reactions shows a dependence on the water content in the system. When the size of the micellar aqueous inner cavity (as evaluated at 1 atm) approaches the molecular size of alpha-chymotrypsin, delta V* becomes significantly different from the value in aqueous solution and in the micelles with a larger size. Possibilities of regulating the enzyme activity by pressure in systems with a low content of water are discussed.

A stable partly denatured state of trypsin induced by high hydrostatic pressure.

The effect of hydrostatic pressure on the unfolding of trypsin was studied by fluorescence spectroscopy under pressure from 1 to 7000 bar. It was found that, at pH 3.0 or pH 7.3, a stable partly denatured state of trypsin was obtained when the applied pressure was about 6.5 kbar. This transient denatured state did not show any enzymatic activity and was different from that denatured by 8 M urea or high temperature in both intrinsic fluorescence spectrum and 8-anilino-1-naphtalene sulfonate (ANS) binding, having some obvious characteristics of 2 molten globule state of protein. It was also found that the formation of this partly denatured state of trypsin was temperature dependent. Energenic values of the process were also given.

Low-temperature optical absorption spectra suggest a redox role for tetrahydrobiopterin in both steps of nitric oxide synthase catalysis.

To investigate the role of tetrahydrobiopterin (BH4) in the catalytic mechanism of nitric oxide synthase (NOS), we analyzed the spectral changes following addition of oxygen to the reduced oxygenase domain of endothelial nitric oxide synthase (NOS) in the presence of different pteridines at -30 degrees C. In the presence of N(G)-hydroxy-L-arginine (NOHLA) and BH4 or 5-methyl-BH4, both of which support NO synthesis, the first observable species were mixtures of high-spin ferric NOS (395 nm), ferric NO-heme (439 nm), and the oxyferrous complex (417 nm). With Arg, no clear intermediates could be observed under the same conditions. In the presence of the BH4-competitive inhibitor 7,8-dihydrobiopterin (BH2), intermediates with maxima at 417 and 425 nm were formed in the presence of Arg and NOHLA, respectively. In the presence of 4-amino-BH4, the maxima of the intermediates with Arg and NOHLA were at 431 and 423 nm, respectively. We ascribe all four spectra to oxyferrous heme complexes. The intermediates observed in this study slowly decayed to the high-spin ferric state at -30 degrees C, except for those formed in the presence of 4-amino-BH4, which required warming to room temperature for regeneration of high-spin ferric NOS; with Arg, regeneration remained incomplete. From these observations, we draw several conclusions. (1) BH4 is required for reductive oxygen activation, probably as a transient one-electron donor, not only in the reaction with Arg but also with NOHLA; (2) in the absence of redox-active pterins, reductive oxygen activation does not occur, which results in accumulation of the oxyferrous complex; (3) the spectral properties of the oxyferrous complex are affected by the presence and identity of the substrate; (4) the slow and incomplete formation of high-spin ferric heme with 4-amino-BH4 suggests a structural cause for inhibition of NOS activity by this pteridine.

Thermobarostability of alpha-chymotrypsin in reversed micelles of aerosol OT in octane solvated by water-glycerol mixtures.

Thermostability of alpha-chymotrypsin at normal pressure in reversed micelles depends on both an effective surfactant solvation degree and glycerol content in the system. The difference in alpha-chymotrypsin stability in reversed micelles at various glycerol concentrations [up to 60% (v/v)] was more pronounced at high surfactant degrees of solvation, R >/= 16. After a 1-h incubation at 40 degrees C in "aqueous" reversed micelles (in the absence of glycerol), alpha-chymotrypsin retained only 1% of initial catalytic activity and 10, 22, 59, and 48% residual activity in glycerol-solvated micelles with 20, 30, 50, and 60% (v/v) glycerol, respectively. The explanation of the observed effects is given in the frames of micellar matrix structural order increasing in the presence of glycerol as a water-miscible cosolvent that leads to the decreasing mobility of the alpha-chymotrypsin molecule and, thus the increase of its stability. It was found that glycerol or hydrostatic pressure could be used to stabilize alpha-chymotrypsin in reversed micelles; a lower pressure is necessary to reach a given level of enzyme stability in the presence of glycerol.

Reaction of neuronal nitric-oxide synthase with oxygen at low temperature. Evidence for reductive activation of the oxy-ferrous complex by tetrahydrobiopterin.

The reaction of reduced NO synthase (NOS) with molecular oxygen was studied at -30 degreesC. In the absence of substrate, the complex formed between ferrous NOS and O2 was sufficiently long lived for a precise spectroscopic characterization. This complex displayed similar spectral characteristics as the oxyferrous complex of cytochrome P450 (lambda max = 416.5 nm). It then decomposed to the ferric state. The oxidation of the flavin components was much slower and could be observed only at temperatures higher than -20 degreesC. In the presence of substrate (L-arginine), another, 12-nm blue-shifted, intermediate spectrum was formed. The breakdown of the latter species resulted in the production of Nomega-hydroxy-L-arginine in a stoichiometry of maximally 52% per NOS heme. This product formation took place also in the absence of the reductase domain of NOS. Both formation of the blue-shifted intermediate and of Nomega-hydroxy-L-arginine required the presence of tetrahydrobiopterin (BH4). We propose that the blue-shifted intermediate is the result of reductive activation of the oxygenated complex, and the electron is provided by BH4. These observations suggest that the reduction of the oxyferroheme complex may be the main function of BH4 in NOS catalysis.

Pressure-exploration of the 33-kDa protein from the spinach photosystem II particle.

The 33-kDa protein isolated from the spinach photosystem II particle is an ideal model to explore high-pressure protein-unfolding. The protein has a very low free energy as previously reported by chemical unfolding studies, suggesting that it must be easy to modulate its unfolding transition by rather mild pressure. Moreover, the protein molecule consists of only one tryptophan residue (Trp241) and eight tyrosine residues, which can be conveniently used to probe the protein conformation and structural changes under pressure using either fluorescence spectroscopy or fourth derivative UV absorbance spectroscopy. The different experimental methods used in the present study indicate that at 20 degrees C and pH 6, the 33-kDa protein shows a reversible two-state unfolding transition from atmospheric pressure to about 180 MPa. This value is much lower than those found for the unfolding of most proteins studied so far. The unfolding transition induces a large red shift of the maximum fluorescence emission of 34 nm (from 316 nm to 350 nm). The change in standard free energy (DeltaGo) and in volume (DeltaV) for the transition at pH 6.0 and 20 degrees C are -14.6 kJ.mol-1 and -120 mL.mol-1, respectively, in which the DeltaGo value is consistent with that obtained by chemical denaturation. We found that pressure-induced protein unfolding is promoted by elevated temperatures, which seem largely attributed to the decrease in the absolute value of DeltaGo (only a minor variation was observed for the DeltaV value). However, the promotion of the unfolding by alkaline pH seems mainly related to the increase in DeltaV without any significant changes in DeltaGo. It was also found that NaCl significantly protects the protein from pressure-induced unfolding. In the presence of 1 M NaCl, the pressure needed to induce the half-unfold of the protein is shifted to a higher value (shift of 75 MPa) in comparison with that observed without NaCl. Interestingly, in the presence of NaCl, the value of DeltaV is significantly reduced whilst that of DeltaGo remains as before. The unfolding-refolding kinetics of the protein has also been studied by pressure-jump, in which it was revealed that both reactions are a two-state transition process with a relatively slow relaxation time of about 102 s.

Pressure denaturation of phosphorylating glyceraldehyde-3-phosphate dehydrogenase from Bacillus stearothermophilus.

The effects of hydrostatic pressure on apo wild-type glyceraldehyde-3-phosphate dehydrogenase (wtGAPDH) from Bacillus stearothermophilus (B. stearothermophilus) have been studied by fluorescence spectroscopy under pressure from 0.1 to 650 MPa. Unlike yeast GAPDH [Ruan, K. C., and Weber, G. (1989) Biochemistry 28, 2144-2153], denaturation of the tetrameric apo wtGAPDH from B. stearothermophilus is likely to precede dissociation into subunits. As expected, denaturation is accompanied by the loss of enzymatic activity. B. stearothermophilus apo wtGAPDH interfaces are less pressure sensitive than apo yeast GAPDH ones, while NAD does not protect B. stearothermophilus wtGAPDH against denaturation by pressure. The pressure effects on B. stearothermophilus GAPDH whose R and Q-axis interfaces were destabilized by disruption of interfacial hydrogen bonds are similar to that of apo wtGAPDH.

Spectral properties of the oxyferrous complex of the heme domain of cytochrome P450 BM-3 (CYP102).

Here we describe for the first time the formation of a complex of reduced CYP102 (cytochrome P450 BM-3) heme domain with molecular oxygen. To stabilize the oxycomplex, the experiments had to be done under argon atmosphere at cryogenic temperatures (-25 degrees C) in the presence of 50% glycerol. The spectral properties of this species were different from those of another P450-type autosuffisant enzyme, i.e., the neuronal nitric oxide synthase. On the contrary, the oxyferrous complex of CYP102 possesses spectral properties similar to those of complexes of microsomal cytochromes P450, e.g., CYP2B4.

Flexibility and stability of the structure of cytochromes P450 3A4 and BM-3.

The flexibility of the structure and compressibility of the respective active site of cytochromes P450 3A4 (CYP3A4) and BM-3 (CYP102) were studied using absorption spectroscopy in the ultraviolet and visual regions. Conformational changes in the overall protein structures of both CYP3A4 and CYP102 due to the effects of temperature and pressure are reversible. However, the enzymes differ in the properties of their active sites. The CYP3A4 enzyme denatures to the inactive P420 form relatively easy, at 3000 bar over half is converted to P420. The compressibility of its active site is lower than that of CYP102 and is greater with the substrate bound, which is in line with the observed lack of a stabilizing effect of the substrate on its conformation under pressure. In contrast, CYP102, although having the most compressible active site among the P450s, possesses a structure that does not denature easily to the inactive (P420) form under pressure. In this respect, it resembles the P450 isolated from acidothermophilic archaebacteria [McLean, M.A., Maves, S.A., Weiss, K.E., Krepich, S. & Sligar, S.G. (1998) Biochem. Biophys. Res. Commun. 252, 166-172].

Expression of recombinant human apolipoprotein A-I in Chinese hamster ovary cells and Escherichia coli.

Human recombinant apolipoprotein (apo) A-I was produced by Chinese hamster ovary (CHO) cells and Escherichia coli with expression vectors containing cDNAs encoding preproapoA-I or apoA-I, respectively. The apoA-I from CHO cells was purified from the culture medium by ammonium sulfate precipitation, phenyl-Sepharose chromatography, and affinity purification on anti-apoA-I immunoabsorber. Human apoA-I was produced in E. coli as a fusion protein with glutathione S-transferase. A four amino acid linker, which separated the two proteins, was specifically recognized and cut by Factor Xa. The purification was accomplished by chromatography of E. coli extracts on glutathione-Sepharose and digestion with Factor Xa. The highest production level was found to be 0.5 micrograms/ml of culture medium per 48 h for a clone of stable transformant of CHO cells, whereas E. coli could produce as much as 20 micrograms/ml of bacterial culture. These apoA-I forms were compared in terms of molecular weight, isoelectric point, and expression of several epitopes. Recombinant apoA-I obtained from CHO cells appears intact and its isoelectric point is compatible with that of the mature form and the proform of apoA-I, whereas a part of the apoA-I produced by E. coli does not contain the COOH-terminus. Also, two of six epitopes are expressed to a greater extent in apoA-I obtained from E. coli than in apoA-I obtained from human plasma.