Asymmetric Total Synthesis of Cyclocitrinol

The first and asymmetric total synthesis of cyclocitrinol, an unusual C25 steroid, has been accomplished in a linear sequence of 18 steps from commercially available compound 11. The synthetically challenging bicyclo[4.4.1] A/B ring system with a strained bridgehead (anti-Bredt) double bond of cyclocitrinol was constructed efficiently and diastereoselectively via a type II intramolecular [5 + 2] cycloaddition.

Two-stage operation to treat destructive midfoot tuberculosis: 14 cases experience

INTRODUCTION

Tuberculosis (TB) in midfoot is often secondary to tuberculosis elsewhere in the body. The experience and literatures to treat midfoot tuberculosis are rare. Up until now, no successful method is reported to treat midfoot tuberculosis.

HYPOTHESIS

Stage surgery is an effective method to treat midfoot bone TB.

MATERIAL AND METHODS

Between January 2008 to January 2011, 14 patients who were diagnosed midfoot tuberculosis and suffered stage operation were enrolled. All the patients had been diagnosed definitely relying on imaging examination and laboratory tests preoperatively. Two-stage operation was performed to all patients. At the first stage, TB tissue and infective tissue were completely removed and replaced by antibiotic bone cement. Normal foot length and arch would be restored and maintained by K-wires and external fixators. At the second stage, autologous iliac and allogeneic bone graft were used to replace bone cement and by fixed by locking plates. American Orthopaedic Foot and Ankle Society (AOFAS), SF-36 and visual analogue scale (VAS) pain score were recorded at the last follow-up.

RESULTS

The average bone union time was 3.8 (range 3-6) months. There is no case of local recurrence or skin sinus. Neither implant broken nor screw loosen was present in this study. The AOFAS score was increased from 51.7±6.8 (range 43-61) preoperatively to 82.9±3.9 (range 76-90) postoperatively (P<0.001). The SF-36 score increased from 46.1±6.1 preoperatively to 83.1±5.4 postoperatively (P<0.001). The VAS score decreased from 6.1±1.1 preoperatively to 1.4±0.9 points postoperatively (P<0.001).

DISCUSSION

Stage operation is an effective treatment to stage III, IV midfoot tuberculosis.

LEVELS OF EVIDENCE

Level IV, retrospective.

Misfolding of apoprotein B-100, LDL aggregation and 17-β -estradiol in atherogenesis

The long quest for a missing mechanistic rationale accounting for the correlation between plasma cholesterol levels and cardiovascular disease (CVD) has been focused on various possible modifications of low density lipoprotein (LDL), turning this physiological cholesterol carrier into a damaging agent able to trigger atherogenesis and later the onset of the disease. In addition to the debated oxidized LDL (oxLDL), a modified LDL with a misfolded apoprotein B-100, called electronegative LDL(-) for its negative charge due to an increased amount of free fatty acids, is commonly present in plasma. LDL(-) is generated by the action of secretory calcium dependent phospholipase A2. LDL(-) primes LDL aggregation and amyloid formation according to mechanisms very similar to those observed in other misfolding diseases. The LDL particle aggregates recall the structure and size of the subendothelial lipid droplets described in early atherogenesis and elicit a powerful inflammatory response. The use of 17-β-estradiol (E2) confirmed that the suggested atherogenicity of LDL (-) is mostly dependent on the misfolded character of its apoprotein. E2 binding to the apoprotein of native LDL, through a specific and saturable receptor, inhibits misfolding phenomenon despite an unaffected production of LDL (-) by phospholipase A2, ultimately preventing LDL aggregation. The apoprotein misfolding in LDL(-) emerges as a possible significant trigger mechanism of atherogenesis. Potential implications for the development of novel therapeutic approaches might be hypothesized in perspective. The existing evidence is discussed and reported in this review.

Total synthesis of aplykurodinone-1

The concise total synthesis of aplykurodinone-1 with an unusual cis-fused hydrindane moiety has been accomplished without the need for any protecting group chemistry using a unique SmI2 mediated reductive cascade cyclization reaction and a direct cuprate mediated 1,4-addition. This work represents the first example of the use of a SmI2-mediated intramolecular cascade cyclization reaction between "halide, alkene and aldehyde" groups.

The Role of the Atrial Neural Network In Atrial Fibrillation: The Metastatic Progression Hypothesis

With the advent of catheter ablation of atrial fibrillation (AF) there has been acceleration in our understanding of the mechanisms underlying the etiology of this common clinical arrhythmia. In this regard, the role of the intrinsic cardiac autonomic nervous system in the initiation and maintenance of AF began to receive attention in numerous experimental and clinical investigations. Up to now, the focus has been on the large ganglionated plexi (GP) which are located in the posterior left atrium mainly at the pulmonary vein-atrial junctions. As long term outcomes have been reported and single procedures have indicated diminished success rates particularly for persistent/long standing persistent AF, emphasis has begun to shift away from the pulmonary vein isolation (PVI) alone as well as GP ablation with or without PVI. An understanding of the atrial substrate represented by the extensions of the intrinsic cardiac autonomic system constituting the atrial neural network is beginning to evolve. In this review, the contribution of the intrinsic cardiac autonomic nervous system to the etiology of AF is addressed, particularly in regard to the greater prevalence of AF in the elderly. In addition, we emphasize the involvement of the atrial neural network in the "metastatic" progression of paroxysmal to persistent and long standing persistent forms of AF.

Detection of Quantitative Trait Loci Affecting Milk Production Traits on Bovine Chromosome 6 in a Chinese Holstein Population by the Daughter Design

Fourteen microsatellite markers with a coverage of 63.5 cM on bovine chromosome 6 were selected, and 26 sire families with 2,260 daughters were analyzed for mapping quantitative trait loci (QTL) affecting 5 milk production traits in a Chinese Holstein population. In the analyses across 26 families and within the largest significant families with a one-QTL model fitted, a QTL near BMS470 was detected that affected fat yield at the 5% experiment-wide significance level. When a 2-QTL model was fitted in the across-family analysis, it was found that there might exist 2 QTL affecting the 3 yield traits, although the exact or empirical thresholds for the significance testing were unknown. In all analyses, the results for milk yield and protein yield were generally consistent, which might have resulted from the same genetic background for milk and protein yield.

Cytochrome c reconstituted from two peptide fragments displays native-like redox properties

Recombination of two fragments of horse cytochrome c (the heme-containing N-fragment, residues 1-56, and the C-fragment, residues 57-104), which are substantially unstructured at neutral pH, gives rise to a 1:1 fragment complex with a compact conformation, in which the alpha helical structure and the native Met80-Fe(III) axial bond are recovered. With respect to the native protein, the ferric complex shows a less rigid atomic packing and a decreased stability [Delta(DeltaG(o))D = 14.7 kJ.mol(-1)], ascribed to perturbations involving the Trp59 microenvironment and, to a lower extent, the heme pocket region. The redox potential, E1/2 = 234 +/- 5 mV vs. normal hydrogen electrode at 25 degrees C, is close to that of the intact protein, consistent with recovery of the native Met80-heme Fe(III) axial bond. Furthermore, the fragment complex shows reactivity similar to intact cytochrome c, in the reaction with cytochrome c oxidase. We conclude that the absence in the complex of some native cross-links and interlocked packing important for protein rigidity and stability is not as relevant for maintaining the native redox properties of the protein, provided that some structural requirements (i.e. recovering of the native-like alpha helical structure) are fulfilled and coordination of Met80 to the heme-iron is restored.

Loss of apoB-100 secondary structure and conformation in hydroperoxide rich, electronegative LDL(-)

A subpopulation of low-density lipoproteins (LDL) is present in human plasma that contains lipid hydroperoxides and is more negatively charged (LDL(-)) than normal native LDL. By circular dichroism and tryptophan lifetime measurements we found that apoB-100 secondary structure is markedly decreased and its conformation is severely altered in LDL(-). The low tryptophan fluorescence intensity confirms the oxidative degradation of the lipoprotein, and the very long lifetime value of one of its decay components indicates a low polarity environment for the remaining unbleached residues. Either a peculiar folding or, most likely, a sinking of the apoB-100 into the lipid core can account for the observed long lifetime component. Oxidation in vitro produces a similar unfolding of the apolipoprotein but the lifetime of tryptophan fluorescence is shifted to lower values, indicating that the denatured apoprotein remains at the hydrophilic surface of the lipoprotein particle. A disordering and an increased polarity of the LDL(-) surface lipids was demonstrated by measuring the generalized polarization of 2-dimethylamino-6-lauroylnaphthalene (Laurdan). The looser monolayer packing apparently favors the new conformation of apoB-100 and its sinking into a more hydrophobic environment, possibly accounting for it reduced receptor binding properties.

Estradiol enhances the resistance of LDL to oxidation by stabilizing apoB-100 conformation

Among different proposed mechanisms to account for the protection exerted by estrogens against cardiovascular diseases, the antioxidant effect has attracted considerable attention. We confirmed that 17-beta-estradiol (E2), when added to human LDL at a 6:1 ratio to apoB-100, markedly delays the phase of massive LDL lipid peroxidation induced by Cu(2+). We also observed an increased oxidative resistance of E2-treated LDL by monitoring the early phase of oxidative degradation on the basis of increased LDL surface polarity by the generalized polarization of the lipophilic fluorescent probe 2-(dimethylamino)-6-lauroylnaphthalene (Laurdan). A scavenging of free radicals by E2 is ruled out since, consistent with its structure, its rate constant for the reduction of peroxy radicals is extremely low, i.e., 0.02% of that of vitamin E. Tryptophan fluorescence lifetime and circular dichroism measurements revealed that (i) apoB-100 undergoes a conformational modification and a progressive loss of secondary structure during lipid peroxidation; (ii) E2 increases apoB-100 secondary structure and modifies its conformation; and (iii) the apoB-100 conformational change induced by E2 makes this protein resistant to modifications brought about by lipid peroxidation. We propose that E2, by affecting apoB-100 secondary structure and conformation, modifies the interaction of this protein with the outer layer of the LDL particle thus increasing its overall oxidative resistance.

Anion size modulates the structure of the A state of cytochrome c

Several studies have shown that anions induce collapse of acid-denatured cytochrome c into the compact A state having the properties of the molten globule and that the anion charge is the main determinant for the A state stabilization. The results here reported show that the anion size plays a role in determining the overall structure of the A state. In particular, small anions induce formation of an A state in which the native Met80-Fe(III) axial bond is recovered and the nativelike redox properties restored. On the other hand, the A state stabilized by large anions shows a histidine (His26 or His33) as the sixth ligand of the heme-iron, a very weak interaction between Trp59 and the heme propionate, and lacks nativelike redox properties. The two anion-stabilized states show similar stability, indicating that (i) the hydrophobic core (which is equally stabilized by all the anions investigated, independently of their size) is the region that mainly contributes to the macromolecule stabilization, and (ii) the flexible loops are responsible for the spectroscopic (and, thus, structural) and redox differences observed.

Role of the tertiary and quaternary structures in the stability of dimeric copper, zinc superoxide dismutases

The equilibrium unfolding process of human Cu,Zn superoxide dismutase has been quantitatively monitored through circular dichroism and fluorescence spectroscopy as a function of increasing guanidinium hydrochloride concentration. The process occurs through the formation of a monomeric intermediate species following a three-state transition equilibrium. Comparison with the stability of the prokaryotic Cu,Zn SOD from P. leiognathi shows that the eukaryotic enzyme is more stable than the prokaryotic enzyme by approximately 3 kcal/mol. This difference is due to the monomer-to-unfolded equilibrium, while the dimer-to-monomer equilibrium is comparable for the two enzymes despite their different intersubunit interactions. These results are confirmed by the unfolding of the copper-depleted derivatives. The Cu,Zn superoxide dismutase represents a good example of how evolution has found two independent quaternary assemblies maintaining the same dimer stability.

Octamer-primed sequencing technology: development of primer identification software

Octamer sequencing technology (OST) is a primer-directed sequencing strategy in which an individual octamer primer is selected from a pre-synthesized octamer primer library and used to sequence a DNA fragment. However, selecting candidate primers from such a library is time consuming and can be a bottleneck in the sequencing process. To accelerate the sequencing process and to obtain high quality sequencing data, a computer program, electronic OST or eOST, was developed to automatically identify candidate primers from an octamer primer library. eOST integrates the base calling software PHRED to provide a quality assessment for target sequences and identifies potential primer binding sites located within a high quality target region. To increase the sequencing success rate, eOST includes a simple dynamic folding algorithm to automatically calculate the free energy and predict the secondary structure within the template in the vicinity of the octamer-binding site. Several parameters were found to be important, including base quality threshold, the window size of the template sequence segment, and the threshold [Delta] G value. OST, coupled with the eOST software, can be used to sequence short DNA fragments or in the finishing assembly stage of large-scale sequencing of genomic DNA.

Opposite effects of Ca(2+) and GTP binding on tissue transglutaminase tertiary structure

Tissue transglutaminase (tTG) belongs to a class of enzymes that catalyze a cross-linking reaction between proteins or peptides. The protein activity is known to be finely tuned by Ca(2+) and GTP binding. In this study we report the effects of these ligands on the enzyme structure, as revealed by circular dichroism, and steady-state and dynamic fluorescence measurements. We have found that calcium and GTP induced opposite conformational changes at the level of the protein tertiary structure. In particular the metal ions were responsible for a small widening of the protein molecule, as indicated by anisotropy decay measurements and by the binding of a hydrophobic probe such as 1-anilino-8-naphthalenesulfonic acid (ANS). Unlike Ca(2+), the nucleotide binding increased the protein dynamics, reducing its rotational correlation lifetime from 32 to 25 ns, preventing also the binding of ANS into the protein matrix. Unfolding of tTG by guanidinium hydrochloride yielded a three-state denaturation mechanism, involving an intermediate species with the characteristics of the so-called "molten globule" state. The effect of GTP binding (but not that of Ca(2+)) had an important consequence on the stability of tissue transglutaminase, increasing the free energy change from the native to the intermediate species by at least approximately 0.7 kcal/mol. Also a greater stability of tTG to high hydrostatic pressure was obtained in presence of GTP. These findings suggest that the molecular mechanism by which tTG activity is inhibited by GTP is essentially due to a protein conformational change which, decreasing the accessibility of the protein matrix to the solvent, renders more difficult the exposure of the active site.

Evidence of stable monomeric species in the unfolding of Cu,Zn superoxide dismutase from Photobacterium leiognathi

The equilibrium unfolding process of Photobacterium leiognathi Cu,Zn superoxide dismutase has been quantitatively monitored through circular dichroism (CD) and fluorescence spectroscopy, upon increasing the guanidinium hydrochloride concentration. The study has been undertaken for both the holo- and the copper-free derivative to work out the role of copper in protein stability. In both cases the unfolding was reversible. The denaturation curve derived from CD and fluorescence spectroscopy was not coincident, suggesting that the denaturation process occurs through a three-state model with formation of an intermediate monomeric species. The occurrence of an intermediate species has been unambiguously demonstrated following CD and steady-state fluorescence spectra of the enzyme at various concentrations in presence of a fixed amounts of guanidinium hydrochloride.

The effect of pressure and guanidine hydrochloride on azurins mutated in the hydrophobic core

The unfolding of the blue-copper protein azurin from Pseudomonas aeruginosa by guanidine hydrochloride, under nonreducing conditions, has been studied by fluorescence techniques and circular dichroism. The denaturation transition may be fitted by a simple two-state model. The total free energy change from the native to the unfolded state was 9.4 +/- 0.4 kcal.mol-1, while a lower value (6.4 +/- 0.4 kcal.mol-1) was obtained for the metal depleted enzyme (apo-azurin) suggesting that the copper atom plays an important stabilization role. Azurin and apo-azurin were practically unaffected by hydrostatic pressure up to 3000 bar. Site-directed mutagenesis has been used to destabilize the hydrophobic core of azurin. In particular either hydrophobic residue Ile7 or Phe110 has been substituted with a serine. The free energy change of unfolding by guanidinium hydrochloride, resulted to be 5.8 +/- 0.3 kcal.mol-1 and 4.8 +/- 0.3 kcal.mol-1 for Ile7Ser and Phe110Ser, respectively, showing that both mutants are much less stable than the wild-type protein. The mutated apoproteins could be reversible denatured even by high pressure, as demonstrated by steady-state fluorescence measurements. The change in volume associated to the pressure-induced unfolding was estimated to be -24 mL.mol-1 for Ile7Ser and -55 mL.mol-1 for Phe110Ser. These results show that the tight packing of the hydrophobic residues that characterize the inner structure of azurin is fundamental for the protein stability. This suggests that the proper assembly of the hydrophobic core is one of the earliest and most crucial event in the folding process, bearing important implication for de novo design of proteins.

An endothelin receptor antagonist ameliorates injuries of sinusoid lining cells in porcine liver transplantation

BACKGROUND

TAK-044 is an endothelin receptor antagonist. Whether the agent has protective effects on liver graft injuries from non-heart-beating donors is unknown.

METHODS

In donor pigs, cardiac arrest was induced by stopping the respirator. Forty-five minutes after cessation of the respirator, the liver was flushed with University of Wisconsin (UW) solution, preserved for 8 hours at 4 degrees C, and transplanted orthotopically. The pigs were divided into two groups: a control group and a drug-treated group in which TAK-044 was given in the UW solutions (10 mg/L) and was administered to recipients (10 mg/kg body weight).

RESULTS

TAK-044 treatment significantly increased recipient survival rate. After reperfusion of the graft, portal venous pressure and 15-minute retention rate of indocyanine green were significantly reduced in the drug-treated group. Electron microscopic findings indicated that TAK-044 attenuated endothelial cell injuries.

CONCLUSION

TAK-044 treatment improves the viability of livers harvested from non-heart-beating donors. The main effect of the agent is protection of endothelial cells from ischemia/reperfusion injuries.

Endothelin antagonist treatment for successful liver transplantation from non-heart-beating donors

BACKGROUND

With the shortage of cadaveric donors, non-heart-beating donors (NHBDs) are a potential source of liver allografts. However, warm ischemic injury in NHBDs seriously affects the viability of graft liver. Endothelin (ET)-1 has been reported to be involved in the hepatic microcirculatory disturbances after ischemia-reperfusion.

METHODS

In a porcine orthotopic liver transplantation model, changes in the serum and liver tissue ET-1 concentration were measured and the effects of an ET receptor antagonist, TAK-044, were evaluated. After cardiac arrest of the donors, liver allografts were subjected to 90 min of warm ischemia, flushed, and preserved for 4 hr at 4 degrees C. The pigs were divided into two groups: a control group (no drug treatment) and a drug-treated group, in which donors and recipients were treated with TAK-044 (10 mg/kg body, drip intravenous injection). Both groups had six donor/recipient pairs.

RESULTS

-The ET-1 concentration in the hepatic venous blood increased after reperfusion of the graft in the control group recipients. ET-1 in the graft liver significantly increased during the cold preservation period. TAK-044 treatment significantly increased recipient 7-day survival rate. After reperfusion of the graft, the concentrations of serum liver enzymes and arterial lactate in the drug-treated group were significantly lower than in the control group. The postoperative increase in portal venous pressure was significantly reduced in the drug-treated group. Measurements of liver enzymes in the washed-out preservation fluid at the time of graft rinsing indicated that TAK-044 treatment of the donors significantly suppressed liver enzyme release during ischemia.

CONCLUSIONS

These findings indicate TAK-044 treatment has protective effects on postoperative function of hepatic allografts procured from NHBDs.

Resolution of the heterogeneous fluorescence in multi-tryptophan proteins: ascorbate oxidase

Ascorbate oxidase is a copper-containing enzyme which catalyzes a redox reaction between vitamin C and molecular oxygen. The protein, which shows a complex tertiary structure, is an homodimer of monomers, each containing three domains and 14 tryptophan residues. Recently, we have demonstrated by spectroscopic and ultracentrifugation techniques the existence of a stable dimeric intermediate along the unfolding pathway of this enzyme [Mei, G., Di Venere, A., Buganza, M., Vecchini, P., Rosato, N. & Finazzi Agrò, A. (1997) Biochemistry 36, 10917-10922]. In this study, the steady-state and dynamic fluorescence features of ascorbate oxidase have been exploited in order to find a way of monitoring the individual subsystems of the protein. The fluorescence intensity and anisotropy upon excitation at 295 nm are extremely sensitive functions of the emission wavelength, indicating a great heterogeneity of the system. The emission decay collected through a cut-off filter can be analyzed in terms of two continuous distributions of lifetimes. Using a monochromator in emission or an optical multichannel analyzer, the two distributions may be attributed to distinct components of the fluorescence spectrum. Differential quenching by cesium chloride also confirmed that the several tryptophan residues present in the protein structure may be grouped into two main classes, each with a different environment. Once the complex fluorescence decay of ascorbate oxidase was analyzed and resolved, a comparison with the crystallographic data allowed a first, approximate attribution of the protein spectroscopic properties to some of the tryptophan residues. This might provide a powerful tool of investigation about the role of definite segments of the protein in its three-dimensional structure and catalytic activity. Furthermore, the methodology set up for ascorbate oxidase can be usefully extended to other multitryptophan proteins.

New stable folding of beta-lactoglobulin induced by 2-propanol

beta-lactoglobulin A has been studied in 2-propanol-water mixtures by means of circular dichroism, fluorescence and small angle X-ray scattering. At a low ionic strength, 2-propanol induces an increase in alpha-helix structure followed by a further transformation which gives rise to a new feature, rich of beta-sheet fragments. The second step of the secondary structure transformation is time-dependent and depressed at high ionic strength. As a consequence, the tertiary structure is completely modified and a new stable protein folding may be hypothesized. Small angle X-ray scattering measurements reveal that 2-propanol induces a diffuse protein aggregation, but the complex equilibria among intra- and inter-molecular hydrophobic and electrostatic interactions may be modulated by balancing the ionic strength and/or alcohol percentage.

Formation and movement of Fe(III) in horse spleen, H- and L-recombinant ferritins. A fluorescence study

Iron oxidation and incorporation into apoferritins of different subunit composition, namely the recombinant H and L homopolymers and the natural horse spleen heteropolymer (10-15% H), have been followed by steady-state and time-resolved fluorescence. After aerobic addition of 100 Fe(II) atoms/polymer, markedly different kinetic profiles are observed. In the rL-homopolymer a slow monotonic fluorescence quenching is observed which reflects binding, slow oxidation at the threefold apoferritin channels, and diffusion into the protein cavity. In the rH-homopolymer a fast fluorescence quenching is followed by a partial, slow recovery. The two processes have been attributed to Fe(II) binding and oxidation at the ferroxidase centers and to Fe(III) released into the cavity, respectively. The fluorescence kinetics of horse spleen apoferritin is dominated by the H chain contribution and resembles that of the H homopolymer. It brings out clearly that the rate of the overall process is limited by the rate at which Fe(III) leaves the ferroxidase centers of the H chains where binding of incoming Fe(II) and its oxidation take place. The data obtained upon stepwise addition of iron and the results of optical absorption measurements confirm this picture. The correspondence between steady-state and time-resolved data is remarkably good; this is manifest when the latter are used to calculate the change in fluorescence intensity as apparent in the steady-state measurements.

Role of quaternary structure in the stability of dimeric proteins: the case of ascorbate oxidase

Equilibrium denaturation experiments have been performed in order to study the dissociation into monomers and unfolding of the dimeric copper-containing enzyme ascorbate oxidase by urea and guanidine hydrochloride. The process has been followed by fluorescence intensity and anisotropy, by optical density, and by circular dichroism as a function of denaturant concentration. The noncoincidence of the unfolding curves obtained by different techniques suggests that a multiphasic process is occurring. The study of enzymatic activity and aromatic circular dichroism as a function of denaturant concentration shows that the first transition involves a change in the protein tertiary structure which is accompanied by the loss of biological function. Gel electrophoresis, ultracentrifugation, and protein dilution experiments demonstrate that a large fraction of protein molecules is still dimeric during this first transition with a stability which is strictly dependent on the denaturant used. The free energy change from the native form to this intermediate species was estimated to be approximately 3.5 kcal/mol. The binding of 1-anilino-8-naphthalenesulfonic acid to the partially unfolded, inactive ascorbate oxidase dimer also suggests a large conformational change accompanied by copper release, allowing the probe to penetrate deep inside the protein structure. Further denaturation to give a fully unfolded form is protein concentration dependent, suggesting that dissociation into monomers is occurring. The monomers appear to be very unstable. No evidence for structured intermediates was in fact detected in the last step of the denaturation process. A three-state model has been used to fit the fluorescence data, and the fractions of different species have been calculated as a function of denaturant concentration. The total free energy change of the unfolding transition using either urea or guanidine hydrochloride is rather small ( approximately 15-16 kcal/mol) and quite comparable to the value found for smaller proteins. The loss of secondary structure which occurs in the second part of the unfolding transition may be described by a simple two-state process which is characterized by a free energy change of 12-13 kcal/mol. These results suggest that the folding process of ascorbate oxidase follows a hierarchical model (Jaenicke, 1991). In this context, the assembly of monomers in a dimeric molecule plays a fundamental role by enhancing the protein stability and driving the final organization of the tertiary structure.

Spectroscopic properties of an engineered maltose binding protein

The maltose binding protein (MBP) has been site specifically labelled with a nitrobenzoxadiazole (NBD) group following mutation of a serine to a cysteine residue at position 337. The resulting protein shows a large ligand (maltose or beta-cyclodextrin) dependent increase in its steady-state fluorescence intensity. Analysis of the static (intensity and anisotropy) and dynamic (lifetime distributions) fluorescence of the NBD label as well as the tryptophan residues in both ligand-bound and ligand-free states of this molecule reveals complex multi-component decays that are interpreted in terms of a ligand-induced solvent shielding mechanism. In the context of the known crystal structures of the various forms of the maltose-binding protein (MBP), ligand-dependent changes in both the fluorescence parameters as well as the circular dichroism spectra of the NBD group are interpreted by a twisted intramolecular charge-transfer (TICT) mechanism, wherein ligand binding locks the NBD group into a conformation that prevents efficient relaxation of the excited state.

Protein unfolding by peptidylarginine deiminase. Substrate specificity and structural relationships of the natural substrates trichohyalin and filaggrin

Peptidylarginine deiminases, which are commonly found in mammalian cells, catalyze the deimination of protein-bound arginine residues to citrullines. However, very little is known about their substrate requirements and the significance or consequences of this postsynthetic modification. We have explored this reaction in vitro with two known substrates filaggrin and trichohyalin. First, the degree and rate of modification of arginines to citrullines directly correlates with the structural order of the substrate. In filaggrin, which has little structural order, the reaction proceeded rapidly to >95% completion. However, in the highly alpha-helical protein trichohyalin, the reaction proceeded slowly to about 25% and could be forced to a maximum of about 65%. Second, the rate and degree of modification depends on the sequence location of the target arginines. Third, we show by gel electrophoresis, circular dichroism, and fluorescence spectroscopy that the reaction interferes with organized protein structure: the net formation of >/=10% citrulline results in protein denaturation. Cyanate modification of the lysines in model alpha-helix-rich proteins to homocitrullines also results in loss of organized structure. These data suggest that the ureido group on the citrulline formed by the peptidylarginine deiminase enzyme modification functions to unfold proteins due to decrease in net charge, loss of potential ionic bonds, and interference with H bonds.

Probing the structure and mobility of Pseudomonas aeruginosa azurin by circular dichroism and dynamic fluorescence anisotropy

The UV dynamic fluorescence and CD of several Pseudomonas aeruginosa azurins bearing single amino acid mutation have been studied. Two classes of mutants were examined. In the first class, two hydrophobic residues in the core of the protein, Ile 7 and Phe 110, nearest to the azurin single tryptophan Trp 48, were substituted by a serine (mutants 17S and F110S). In the second class, two residues in the outer sphere of the copper ligand field were changed, obtaining the following mutants: M44K, H35F, H35L, and H35Q. All these proteins showed two fluorescence lifetimes in the copper-containing form, but only one in the copper-free form. The lifetime of the latter derivatives was different from either those of the metal-bound samples, definitely ruling out the presence of apo-like species in the holo protein. Copper-free 17S and F110S showed a more complex fluorescence decay profile requiring a distribution of lifetimes rather than a single lifetime. Holo F110S was also better fitted, in the limit of confidence, with two distributions rather than a pair of lifetimes. Time-resolved anisotropy of these two mutants as well as of wild-type (wt) protein showed two components (rotational times for wt < or = 200 ps and 7 ns, respectively). These components were not affected significantly by copper removal in the case of wt protein. Instead, the short rotational component of the mutants dropped dramatically to values near zero, indicating a much greater mobility of the tryptophanyl residue in the mutant apo azurins. These data were supported by CD measurements showing a small effect of the copper presence in the region below 250 nm, i.e., in the secondary structure, but almost a collapse of the aromatic asymmetry at 270-295 nm related to a relaxation of the structural constraint around the tryptophan. Altogether these data show that copper does not play a structural role in wt azurin, whereas it is crucial in the stabilization of 17S and F110S mutants. Furthermore, although the metal site geometry is rigidly kept in wt apo-azurin, it regains the native form only in the presence of the metal in the "core" mutants. This finding is important for the theory of entatic states in metalloproteins (Williams RJP, 1995, Eur J Biochem 234:363-381).

Structural flexibility modulates the activity of human glutathione transferase P1-1. Role of helix 2 flexibility in the catalytic mechanism

Presteady-state and steady-state kinetic studies performed on human glutathione transferase P1-1 (EC 2.5.1.18) with 1-chloro-2, 4-dinitrobenzene as co-substrate indicate that the rate-determining step is a physical event that occurs after binding of the two substrates and before the final sigma-complex formation. It may be a structural transition involving the ternary complex. This event can be related to diffusion-controlled motions of protein portions as kcat degrees /kcat linearly increases by raising the relative viscosity of the solution. Similar viscosity dependence has been observed for Km GSH, while Km CDNB is independent. No change of the enzyme structure by viscosogen has been found by circular dichroism analysis. Thus, kcat and Km GSH seem to be related to the frequency and extent of enzyme structural motions modulated by viscosity. Interestingly, the reactivity of Cys-47 which can act as a probe for the flexibility of helix 2 is also modulated by viscosity. Its viscosity dependence parallels that observed for kcat and Km GSH, thereby suggesting a possible correlation between kcat, Km GSH, and diffusion-controlled motion of helix 2. The viscosity effect on the kinetic parameters of C47S and C47S/C101S mutants confirms the involvement of helix 2 motions in the modulation of Km GSH, whereas a similar role on kcat cannot be ascertained unequivocally. The flexibility of helix 2 modulates also the homotropic behavior of GSH in these mutants. Furthermore, fluorescence experiments support a structural motion of about 4 A occurring between helix 2 and helix 4 when GSH binds to the G-site.

A stochastic model for the sigmoidal behaviour of cooperative biological systems

A stochastic model for cooperative transitions in biological systems based on a Markov chain is proposed. This model requires only two parameters, the mean probability, p, and the coupling capacity, Deltap, which measure the probability of forming a new weak bond depending on the number of similar bonds already formed and it is also responsible for the transition. In this paper we show how the model works for a large number of identical molecules and how it can be useful for studying the noise around the centre of the transition where, increasing the degree of cooperativity, i.e. the number n in the well-known Hill equation, the width of the noise increases along with its fractal dimension. A simple relationship between the degree of cooperativity and the parameter Deltap is proposed, suggesting that the cooperativity of real biological transitions is related to the coupling capacity Deltap of the present model.

Biochemical, structural, and transglutaminase substrate properties of human loricrin, the major epidermal cornified cell envelope protein

Loricrin is the major protein of the cornified cell envelope of terminally differentiated epidermal keratinocytes which functions as a physical barrier. In order to understand its properties and role in cornified cell envelope, we have expressed human loricrin from a full-length cDNA clone in bacteria and purified it to homogeneity. We have also isolated loricrin from newborn mouse epidermis. By circular dichroism and fluorescence spectroscopy, the in vivo mouse and bacterially expressed human loricrins possess no alpha or beta structure but have some organized structure in solution associated with their multiple tyrosines and can be reversibly denatured by either guanidine hydrochloride or temperature. The transglutaminase (TGase) 1, 2, and 3 enzymes expressed during epidermal differentiation utilized loricrin in vitro as a complete substrate, but the types of cross-linking were different. The TGase 3 reaction favored certain lysines and glutamines by forming mostly intrachain cross-links, whereas TGase 1 formed mostly large oligomeric complexes by interchain cross-links involving different lysines and glutamines. Together, the glutamines and lysines used in vitro are almost identical to those seen in vivo. The data support a hypothesis for the essential and complementary roles of both TGase 1 and TGase 3 in cross-linking of loricrin in vivo. Failure to cross-link loricrin by TGase 1 may explain the phenotype of lamellar ichthyosis, a disease caused by mutations in the TGase 1 gene.

New polarized light microscope with precision universal compensator

A new type of polarized light microscope ('new pol-scope') for fast and orientation-independent measurement of birefringent fine structure has been developed. The design of the new pol-scope incorporates a precision universal compensator made from two liquid crystal variable retarders. A video camera and digital image processing system provide fast measurements of specimen anisotropy (retardance magnitude and azimuth) at all points of the image forming the field of view. The images document fine structural and molecular organization within a thin optical section of the specimen. The sensitivity of the current instrument is 0.1 nm of specimen retardance, measured with data gathered in 0.43 s at all 640 x 480 image points. Examples of birefringence measurements in biological (microtubule arrays) and industrial (magneto-optical disc substrate) specimens are presented.

Biotin and biotin analogues specifically modify the fluorescence decay of avidin

Avidin, a basic tetrameric glycoprotein, isolated from hen egg-white, binds up to four molecules of biotin with exceptionally high affinity. The presence of tryptophanyl residues in the active site pointed out the opportunity of correlating the protein fluorescence with biotin binding. We have performed both steady state and dynamic fluorescence experiments using biotin or biotin-derived molecules (biotinamine, diaminobiotin and iminobiotin) as ligands. The fluorescence decay data can only be fitted by two continuous distributions of lifetimes which may reflect the presence of static or dynamic microheterogeneity in the environment of the tryptophan residues. We observed that the binding of biotin, biotinamine and iminobiotin reduces the widths of both distributions to discrete lifetimes thus indicating a more homogenous environment for the emitting tryptophan residues. Instead, the binding of diaminobiotin, which lacks the imidazolone ring, affects one lifetime distribution only. The binding of biotin also affects the rotational correlation time of avidin, which becomes shorter, suggesting a more compact structure of the ligated protein. The utility of analyzing the fluorescence in terms of distributions appears to be further warranted.

Unique environment of Trp48 in Pseudomonas aeruginosa azurin as probed by site-directed mutagenesis and dynamic fluorescence spectroscopy

Two mutants of the blue copper protein azurin from Pseudomonas aeruginosa, Ile7Ser and Phe110Ser, were prepared. The mutations were aimed at affecting the mobility and the fluorescence properties of Trp48, the only tryptophan residue present, which in the wild-type protein is located in a highly hydrophobic and rigid environment. EPR, UV-vis, and NMR spectroscopy show that the copper binding site and the overall structure of the wild-type protein are preserved and that structural effects occur only on a local scale. Steady-state fluorescence spectra of both mutants, particularly in the copper-free form, show that tryptophan fluorescence is dramatically affected by the introduction of a polar residue close to it. The emission maximum is red-shifted and dependent on the excitation wavelength. This indicates a loosening of the matrix around the indolyl side chain and an increase of the effective dielectric constant of the microenvironment. Time-resolved fluorescence spectroscopy also shows substantial changes in the fluorescence lifetimes and in the distribution of the lifetimes of the mutants; these variations are interpreted in terms of a change in solvation of the Trp48 side chain.

Molten globule monomers in human superoxide dismutase

The time-resolved fluorescence decay and anisotropy of Cu/Zn human superoxide dismutase (HSOD) were studied as a function of temperature and denaturant concentration. In addition, circular dichroism (CD) measurements were performed on HSOD as a function of denaturant concentration in the amide and aromatic regions. The time-resolved fluorescence decay results reveal the existence of structural microheterogeneity in HSOD. Furthermore, CD measurements and a global analysis decomposition of the time-resolved fluorescence decay over denaturant concentration shows the presence of an intermediate in the unfolding of HSOD by guanidinium hydrochloride. Considering our previous measurements of partially denatured HSOD as a function of protein concentration (Mei et al., Biochemistry 31 (1992) 7224-7230), our results strongly suggest that the unfolding intermediate is a monomer that displays a molten globule state.

Some pitfalls in the measurement of blood glutathione

1. We report some pitfalls in the measurement of whole blood and plasma glutathione in man. 2. Using a simple tourniquet to the forearm and a 21-gauge needle, blood samples were collected by brachial vein puncture from healthy subjects. Whole blood and plasma were analysed for total glutathione, including the reduced and oxidized forms, by a spectrophotometric recycling method involving the glutathione reductase/NADPH couple. 3. The concentration of oxidized glutathione was determined after treatment of aliquots with either 2-vinylpyridine or N-ethylmaleimide to trap reduced glutathione. Reduced glutathione in the native samples could then be obtained by subtraction. 4. When the reagents were added to separated plasma, 2-vinylpyridine yielded values for oxidized glutathione that were twice as high as with N-ethylmaleimide. In whole blood studies the discrepancy was even greater, and the problem was not resolved by deproteinization of samples with 5-sulphosalicylic acid. Using N-ethylmaleimide, levels of oxidized glutathione were less than 1% of total glutathione in whole blood. 5. Despite attempts to minimize haemolysis, lysed erythrocytes contributed on average 25% to the 'plasma' glutathione concentration.

Denaturation of human Cu/Zn superoxide dismutase by guanidine hydrochloride: a dynamic fluorescence study

The unfolding of holo and apo forms of human Cu/Zn superoxide dismutase by guanidine hydrochloride has been investigated by steady-state and dynamic fluorescence. In agreement with previous observations, a stabilizing effect of the metal ions on the protein tertiary structure was apparent from comparison of apo- and holoproteins, which both showed a sharp sigmoidal transition though at different denaturant concentrations. The transition was also followed by circular dichroism to check the extent of secondary structure present at each denaturant concentration. The results are incompatible with a simple two-state mechanism for denaturation. The occurrence of a more complicated process is supported by the emission decay properties of the single tryptophanyl residue at different denaturant concentrations. A complex decay function, namely, two discrete exponentials or a continuous distribution of lifetimes, was always required to fit the data. In particular, the width of the lifetime distribution, which is maximum at the transition midpoint, reflects heterogeneity of the tryptophan microenvironment and thus the presence of different species along the denaturation pathway. In the unfolded state, the width of the lifetime distribution is broader than in the folded state probably because the tryptophan residue is affected by a larger number of local conformations. The dissociation of the dimer was also studied by varying the protein concentration at different denaturant concentrations. This process affects primarly the surface of the protein rather than its secondary structure as shown by a comparison between the tryptophan emission decay and circular dichroism data under the same conditions. Another consequence of dissociation is a greater instability in the structure of the monomers, which are more easily unfolded.(ABSTRACT TRUNCATED AT 250 WORDS)

Affinity labeling of pig kidney 3,4-dihydroxyphenylalanine (Dopa) decarboxylase with N-(bromoacetyl)pyridoxamine 5'-phosphate. Modification of an active-site cysteine

Pig kidney 3,4-dihydroxyphenylalanine (Dopa) decarboxylase is inactivated by N-(bromoacetyl)pyridoxamine 5'-phosphate (BAPMP) in a reaction which follows first-order kinetics at pH 7.5 and 25 degrees C. The concentration dependence of inactivation reveals saturation kinetics with an apparent Ki of 0.16 mM and kinact of 0.086 min-1 at saturating inhibitor concentration. Enzyme can be protected from inactivation by pyridoxal 5'-phosphate. Inactivation of enzyme by [14C]BAPMP proceeds with the incorporation of a stoichiometric amount of labeled inhibitor. Proteolytic digestions of the radioactively labeled enzyme followed by high-performance liquid chromatography allow the isolation of the modified peptide corresponding to the sequence Ala-Ala-Ser-Pro-Ala-Cys-Thr-Glu-Leu in which cysteine (Cys111) is the modified residue. The conservation of this residue and also of an extended region around it in all Dopa decarboxylases so far sequenced is underlined. The overall conclusion of these findings is that Cys111 may be at, or near, the pyridoxal-5'-phosphate binding site of pig kidney Dopa decarboxylase and plays a critical role in the catalytic function of the enzyme. Furthermore, fluorescence studies of BAPMP-modified apoenzyme provide useful information on the microenvironment of the affinity label at its binding site.

Intrinsic fluorescence of the bacterial copper-containing protein amicyanin

The fluorescence properties of the single tryptophanyl residue present in amicyanin from Thiobacillus versutus are very similar to those of azurin from Pseudomonas aeruginosa and other mononuclear blue copper proteins. The emission maximum is well structured and centered at 318 nm. The quantum yield is strongly affected by the presence of copper, the removal of which is accompanied by a more than sixfold increase in fluorescence, without change in shape. The fluorescence decay of holo-amicyanin is heterogeneous with a longer component of 5.7 ns and a shorter one of 0.7 ns accounting for 90% of the total emitting molecules. Copper-free amicyanin shows instead a single exponential decay (3.3 ns) of intrinsic fluorescence. This lifetime decreases as the temperature increases as does the longer lifetime component of holoamicyanin.

Fluorescence lifetime distributions in human superoxide dismutase. Effect of temperature and denaturation

The internal dynamics of human superoxide dismutase has been studied using time-resolved fluorescence. The fluorescence decay has been analyzed using continuous distribution of lifetime values. The effect of temperature and conformational state on the lifetime distribution has been investigated. The emission of the single tryptophan residue depends on the nature and dynamics of the protein matrix. Conformational changes have been induced by increased concentration of guanidinium hydrochloride. We found that both temperature and conformation strongly effect the width of the lifetime distribution.

A time-resolved fluorescence study of human copper-zinc superoxide dismutase

The intrinsic fluorescence decay of human Cu,Zn superoxide dismutase was measured by frequency-domain techniques. The protein consists of two subunits, each containing one tryptophan and no tyrosine residues. Using a synchrotron radiation source, which allows facile selection of the excitation wavelength, the dependence of the emission decay upon excitation was studied. No significant excitation wavelength effects were found. The two tryptophans contained in the dimer, although fully equivalent and exposed to solvent, showed a fluorescence decay that cannot be described by a single lifetime. Either two lifetimes, or one Lorentzian-shaped continuous distribution of lifetimes, are needed to obtain a good fit. Under identical experimental conditions, control experiments showed that N-acetyltryptophanamide, an analogue of tryptophanyl residues in proteins, decays with a single lifetime. The heterogeneous decay of tryptophan fluorescence in superoxide dismutase is interpreted as due to the presence of static and/or dynamic conformers in the protein that decay with different lifetimes. The two models of discrete lifetimes and continuous distribution of lifetimes are discussed with reference to measurements on holo- and apo-human superoxide dismutase.