Antitrypanosomal Activity of Sterol 14α-Demethylase (CYP51) Inhibitors VNI and VFV in the Swiss Mouse Models of Chagas Disease Induced by the Trypanosoma cruzi Y Strain.

Chagas disease is a life-threatening infection caused by a variety of genetically diverse strains of the protozoan parasite Trypanosoma cruzi The current treatment (benznidazole and nifurtimox) is unsatisfactory, and potential alternatives include inhibitors of sterol 14α-demethylase (CYP51), the cytochrome P450 enzyme essential for the biosynthesis of sterols in eukaryotes and the major target of clinical and agricultural antifungals. Here we performed a comparative investigation of two protozoon-specific CYP51 inhibitors, VNI and its CYP51 structure-based derivative VFV, in the murine models of infection caused by the Y strain of T. cruzi The effects of different treatment regimens and drug delivery vehicles were evaluated in animals of both genders, with benznidazole serving as the reference drug. Regardless of the treatment scheme or delivery vehicle, VFV was more potent in both genders, causing a >99.7% peak parasitemia reduction, while the VNI values varied from 91 to 100%. Treatments with VNI and VFV resulted in 100% animal survival and 0% natural relapse after the end of therapy, though, except for the 120-day treatment schemes with VFV, relapses after three cycles of immunosuppression were observed in each animal group, and quantitative PCR analysis revealed a very light parasite load in the blood samples (sometimes below or near the detection limit, which was 1.5 parasite equivalents/ml). Our studies support further investigations of this class of compounds, including their testing against other T. cruzi strains and in combination with other drugs.

Different Therapeutic Outcomes of Benznidazole and VNI Treatments in Different Genders in Mouse Experimental Models of Trypanosoma cruzi Infection.

The lack of translation between preclinical assays and clinical trials for novel therapies for Chagas disease (CD) indicates a need for more feasible and standardized protocols and experimental models. Here, we investigated the effects of treatment with benznidazole (Bz) and with the potent experimental T. cruzi CYP51 inhibitor VNI in mouse models of Chagas disease by using different animal genders and parasite strains and employing distinct types of therapeutic schemes. Our findings confirm that female mice are less vulnerable to the infection than males, show that male models are less susceptible to treatment with both Bz and VNI, and thus suggest that male models are much more suitable for selection of the most promising antichagasic agents. Additionally, we have found that preventive protocols (compound given at 1 dpi) result in higher treatment success rates, which also should be avoided during advanced steps of in vivo trials of novel anti-T. cruzi drug candidates. Another consideration is the relevance of immunosuppression methods in order to verify the therapeutic profile of novel compounds, besides the usefulness of molecular diagnostic tools (quantitative PCR) to ascertain compound efficacy in experimental animals. Our study aims to contribute to the development of more reliable methods and decision gates for in vivo assays of novel antiparasitic compounds in order to move them from preclinical to clinical trials for CD.

Biodiversity of CYP51 in trypanosomes.

Sterol 14alpha-demethylases (CYP51) are metabolic cytochromes P450, found in each biological kingdom. They catalyse a single three-step reaction included in all sterol biosynthetic pathways. Plant CYP51s have strict preference towards their physiological substrate O (obtusifoliol), which is C-4-monomethylated. Natural substrates of animal/fungal CYP51 (lanosterol, 24,25-dihydrolanosterol or 24-methylenelanosterol) are C-4-dimethylated. CYP51 from the pathogenic protozoa TB (Trypanosoma brucei) is the first example of O-specific sterol 14alpha-demethylase in non-photosynthetic organisms. Surprisingly, at 83% amino acid identity to the TB orthologue, CYP51 from TC (Trypanosoma cruzi) clearly prefers C-4-dimethylated sterols. Replacement of animal/fungi-like Ile(105) in the B' helix of TC CYP51 with phenylalanine, the residue found in this position in all plant and other trypanosome CYP51s, dramatically increases the ability of the enzyme to metabolize O, converting it into a more plant-like sterol 14alpha-demethylase. A more than 100-fold increase in binding and turnover is observed for the 24-desmethyl analogue of O [N (norlanosterol)], which is found in vivo in procyclic forms of TB and is a good TB CYP51 substrate in vitro. We believe that (i) N is a non-conventional CYP51 substrate, preferred in TB and perhaps other Trypanosomatidae and (ii) functional similarity of TC CYP51 to animal/fungal orthologues is a result of evolutionary convergence (including F105I mutation), leading to different pathways for sterol production in TC versus TB.

Role of C-terminal sequence of cytochrome P450scc in folding and functional activity.

To elucidate the role of Arg472 and C-terminal sequence of the mature form of cytochrome P450scc, a mitochondrial cytochrome P450, in the present work we have performed sequential removal of the C-terminal amino acid residues of the hemeprotein and evaluated their functional role in folding and catalysis. The removal of 2, 4, 7, or 9 amino acid residues (cytochrome P450scc mutants Delta2, Delta4, Delta7, and Delta9) does not significantly affect the physicochemical properties of the truncated forms of cytochrome P450scc, but results in significant increase in the expression level of the hemeprotein in Escherichia coli (Delta4 cytochrome P450scc mutant). However, removal of 10 C-terminal amino acid residues (Delta10 cytochrome P450scc) of mature form of cytochrome P450scc (replacement of codon for Arg472 for stop-codon) is followed by loss of the ability for correct folding in E. coli. Based on these data, it is concluded that the C-terminal amino acid residues of cytochrome P450scc (DeltaArg472-Ala481) play an important role in correct recombinant protein folding and heme binding by cytochrome P450scc during its expression in E. coli, while folding of mitochondrial cytochrome P450scc during its heterologous expression in bacterial cells is more similar to the folding of prokaryotic soluble cytochrome P450's than to microsomal cytochrome P450's.

Role of positively charged residues lys267, lys270, and arg411 of cytochrome p450scc (CYP11A1) in interaction with adrenodoxin.

Cytochrome P450scc and adrenodoxin are redox proteins of the electron transfer chain of the inner mitochondrial membrane steroid hydroxylases. In the present work site-directed mutagenesis of the charged residues of cytochrome P450scc and adrenodoxin, which might be involved in interaction, was used to study the nature of electrostatic contacts between the hemeprotein and the ferredoxin. The target residues for mutagenesis were selected based on the theoretical model of cytochrome P450scc-adrenodoxin complex and previously reported chemical modification studies of cytochrome P450scc. In the present work, to clarify the molecular mechanism of hemeprotein interaction with ferredoxin, we constructed cytochrome P450scc Lys267, Lys270, and Arg411 mutants and Glu47 mutant of adrenodoxin and analyzed their possible role in electrostatic interaction and the role of these residues in the functional activity of the proteins. Charge neutralization at positions Lys267 or Lys270 of cytochrome P450scc causes no significant effect on the physicochemical and functional properties of cytochrome P450scc. However, cytochrome P450scc mutant Arg411Gln was found to exhibit decreased binding affinity to adrenodoxin and lower activity in the cholesterol side chain cleavage reaction. Studies of the functional properties of Glu47Gln and Glu47Arg adrenodoxin mutants indicate that a negatively charged residue in the loop covering the Fe2S2 cluster, being important for maintenance of the correct architecture of these structural elements of ferredoxin, is not directly involved in electrostatic interaction with cytochrome P450scc. Moreover, our results indicate the presence of at least two different binding (contact) sites on the proximal surface of cytochrome P450scc with different electrostatic input to interaction with adrenodoxin. In the binary complex, the positively charged sites of the proximal surface of cytochrome P450scc well correspond to the two negatively charged sites of adrenodoxin: the "interaction" domain site and the "core" domain site.

Site-directed mutagenesis of cytochrome P450scc. II. Effect of replacement of the Arg425 and Arg426 residues on the structural and functional properties of the cytochrome P450scc.

Cytochrome P450-dependent monooxygenases, in spite of their wide distribution, can be simply divided into a few groups differing in the location of the electron transfer chain and their composition. The two main groups of cytochrome P450-dependent monooxygenases are the mitochondrial and microsomal enzymes. While in two-component microsomal cytochrome P450-dependent monooxygenases electrons are supplied to cytochrome P450 by a flavoprotein (NADPH-cytochrome P450 reductase), in three-component mitochondrial monooxygenases the electrons are supplied to cytochrome P450 by a low molecular weight protein (ferredoxin). The interaction of cytochrome P450 with NADPH-cytochrome P450 reductase and ferredoxin is the subject of intensive studies. Using chemical modification, chemical cross-linking, and site-directed mutagenesis, we identified surface exposed positively charged residues of cytochrome P450scc which might be important for interaction with adrenodoxin. Theoretical analysis of the distribution of surface electrostatic potential in cytochrome P450 indicates that in contrast to microsomal monooxygenases, cytochromes P450 of mitochondrial type, and cholesterol side-chain cleavage cytochrome P450 (P450scc) in part, carry on the proximal surface an evidently positively charged site that is formed by residues Arg425 and Arg426. In the present work, to estimate the functional role of Arg425 and Arg426 of cytochrome P450scc, we used site-directed mutagenesis to replace these residues with glutamine. The results indicate that residues Arg425 and Arg426 are involved in the formation of a heme-binding center and electrostatic interaction of cytochrome P450scc with its physiological electron-transfer partner, adrenodoxin.

Folding requirements are different between sterol 14alpha-demethylase (CYP51) from Mycobacterium tuberculosis and human or fungal orthologs.

Upon sequence alignment of CYP51 sterol 14alpha-demethylase from animals, plants, fungi, and bacteria, arginine corresponding to Arg-448 of CYP51 in Mycobacterium tuberculosis (MT) is conserved near the C terminus of all family members. In MTCYP51 Arg-448 forms a salt bridge with Asp-287, connecting beta-strand 3-2 with helix J. Deletion of the three C-terminal residues of MTCYP51 has little effect on expression of P450 in Escherichia coli. However, truncation of the fourth amino acid (Arg-448) completely abolishes P450 expression. We have investigated whether Arg-448 has other structural or functional roles in addition to folding and whether its conservation reflects conservation of a common folding pathway in the CYP51 family. Characterization of wild type protein and three mutants, R448K, R448I, and R448A, including examination of catalytic activity, secondary and tertiary structure analysis by circular dichroism and tryptophan fluorescence, and studies of both equilibrium and temporal MTCYP51 unfolding behavior, shows that Arg-448 does not play any role in P450 function or maintenance of the native structure. C-terminal truncation of Candida albicans and human CYP51 orthologs reveals that, despite conservation in sequence, the requirement for arginine at the homologous C-terminal position in folding in E. coli is not conserved. Thus, despite similar spatial folds, functionally related but evolutionarily distinct P450s can follow different folding pathways.

Site-directed mutagenesis of cytochrome P450scc (CYP11A1). Effect of lysine residue substitution on its structural and functional properties.

Our previous chemical modification and cross-linking studies identified some positively charged amino acid residues of cytochrome P450scc that may be important for its interaction with adrenodoxin and for its functional activity. The present study was undertaken to further evaluate the role of these residues in the interaction of cytochrome P450scc with adrenodoxin using site-directed mutagenesis. Six cytochrome P450scc mutants containing replacements of the surface-exposed positively charged residues (Lys103Gln, Lys110Gln, Lys145Gln, Lys394Gln, Lys403Gln, and Lys405Gln) were expressed in E. coli cells, purified as a substrate-bound high-spin form, and characterized as compared to the wild-type protein. The replacement of the surface Lys residues does not dramatically change the protein folding or the heme pocket environment as judged from limited proteolysis and spectral studies of the cytochrome P450 mutants. The replacement of Lys in the N-terminal sequence of P450scc does not dramatically affect the activity of the heme protein. However, mutant Lys405Gln revealed rather dramatic loss of cholesterol side-chain cleavage activity, efficiency of enzymatic reduction in a reconstituted system, and apparent dissociation constant for adrenodoxin binding. The present results, together with previous findings, suggest that the changes in functional activity of mutant Lys405Gln may reflect the direct participation of this amino acid residue in the electrostatic interaction of cytochrome P450scc with its physiological partner, adrenodoxin.

Conformational dynamics and molecular interaction reactions of recombinant cytochrome p450scc (CYP11A1) detected by fluorescence energy transfer.

Bovine adrenocortical cytochrome P450scc (P450scc) was expressed in Escherichia coli and purified as the substrate bound, high-spin complex (16.7 nmol of heme per mg of protein, expression level in E. coli about 400-700 nmol/l). The recombinant protein was characterized by comparison with native P450scc purified from adrenal cortex mitochondria. To study the interaction of the electron transfer proteins during the functioning of the heme protein, recombinant P450scc was selectively modified with fluorescein isothiocyanate (FITC). The present paper shows that modified P450scc, purified by affinity chromatography using adrenodoxin-Sepharose to remove non-covalently bound FITC, retains the functional activity of the unmodified enzyme, including its ability to bind adrenodoxin. Based on the efficiency of resonance fluorescence energy transfer in the donor-acceptor pair, FITC-heme, we calculated the distance between Lys(338), selectively labeled with the dye, and the heme of P450scc. The intensity of fluorescence from the label dramatically changes during: (a) denaturation of P450scc; (b) changing the spin state or redox potential of the heme protein; (c) formation of the carbon monoxide complex of reduced P450scc; (d) as well as during reactions of intermolecular interactions, such as changes of the state of aggregation, complex formation with the substrate, binding to the electron transfer partner adrenodoxin, or insertion of the protein into an artificial phospholipid membrane. Selective chemical modification of P450scc with FITC proved to be a very useful method to study the dynamics of conformational changes of the recombinant heme protein. The data obtained indicate that functionally important conformational changes of P450scc are large-scale ones, i.e. they are not limited only to changes in the dynamics of the protein active center. The results of the present study also indicate that chemical modification of Lys(338) of bovine adrenocortical P450scc does not dramatically alter the activity of the heme protein, but does result in a decrease of protein stability.

Comparative structural-functional characterization of recombinant and natural adrenodoxin. Interaction with cytochrome P450scc.

The conditions for heterologous expression of recombinant bovine adrenodoxin in E. coli have been optimized, thus reaching expression levels up to 12-14 micromoles per liter of culture medium. A highly efficient method for purification of this recombinant ferredoxin from the E. coli cells has been developed. The structural-functional properties of the highly purified recombinant protein have been characterized and compared to those of natural adrenodoxin purified from bovine adrenocortical mitochondria. In contrast to natural adrenodoxin, which is characterized by microheterogeneity, the recombinant adrenodoxin is homogeneous as judged by N- and C-terminal amino acid sequencing, and its sequence corresponds to the full-length mature form of adrenodoxin containing 128 amino acid residues. The interactions of the natural and recombinant adrenodoxins with cytochrome P450scc have been studied and compared with respect to: the efficiency of their enzymatic reduction of cytochrome P450scc in a reconstituted system; the ability of the immobilized adrenodoxins to bind cytochrome P450scc; the ability of the adrenodoxins to induce a spectral shift of cytochrome P450scc and to effect the average polarity of exposed tyrosines in the low-spin cytochrome P450scc. The recombinant adrenodoxin is functionally active and in the reduced state as well as at low ionic strength it displays higher affinity to cytochrome P450scc as compared to the natural bovine adrenocortical adrenodoxin. The possible role of the C-terminal sequence of the adrenodoxin molecule in its interaction with cytochrome P450scc as well as the advantages of using the recombinant protein instead of the natural one are discussed.

Comparative structural and immunochemical characterization of recombinant and natural cytochrome p450scc (CYPXIAI).

Optimization of the conditions for heterologous expression of recombinant cytochrome P450scc in E. coli provided an expression level of about 420 nmoles of cytochrome P450scc per liter of bacterial culture. A new procedure for purification of recombinant protein in substrate-bound high-spin and substrate-free low-spin form is described. Highly purified electrophoretically homogeneous recombinant cytochrome P450scc contains 12.3 and 16.7 nmoles heme per mg protein for substrate-free and substrate-bound forms, respectively. The recombinant and natural cytochrome P450scc from bovine adrenocortical mitochondria were compared functionally and immunochemically. The dissociation constants for the complexes of cytochrome P450scc with cholesterol and adrenodoxin, the efficiency of enzymatic reduction in the reconstituted system (NADPH--adrenodoxin reductase--adrenodoxin), and cholesterol side-chain cleavage activity were determined. It was found that limited proteolysis of the recombinant cytochrome P450scc with trypsin forms two main fragments which are electrophoretically and immunochemically identical with the fragments F1 (29.8 kD) and F2 (26.6 kD) formed during proteolysis of bovine adrenocortical cytochrome P450scc. The quantitative values of the studied parameters are practically identical in natural and substrate-bound recombinant cytochrome P450scc, while there were great differences between substrate-bound and substrate-free forms of recombinant cytochrome P450scc both of functional (decrease of cholesterol side-chain cleavage activity, efficiency of enzymatic reduction in the reconstituted system, and affinity to adrenodoxin for substrate-free cytochrome P450scc) as well as structural (increase in accessibility to exogenous and endogenous proteolysis) character. The identity of the folding process for recombinant and natural proteins as well as the nature of a stabilizing and activating effect of cholesterol on cytochrome P450scc is discussed.

Dynamics and functional activity of cytochrome P450scc selectively labeled with fluorescein isothiocyanate.

Studies have been done to assess the effect of selective chemical modification of Lys338 on the functional activity and conformational mobility of cytochrome P450scc. It is found that fluorescently labelled cytochrome P450scc retains cholesterol side-chain cleavage activity and the ability for spectral response of type I during interaction with cholesterol and protein-protein interaction with the electron donor, adrenodoxin. Based on the quenching of the FITC fluorescence by iodide after incorporation of the labelled heme protein into an artificial phospholipid membrane, the orientation of Lys338 was determined. By measuring the efficiency of resonance fluorescence energy transfer in an FITC-heme pair during interaction of the labelled cytochrome P450scc with substrate and adrenodoxin as well as under insertion of the heme protein into membrane the changes of inter-molecular distance between Lys338 and heme were registered that indicate a functional importance of conformational mobility of cytochrome P450scc. It is shown that chemical modification of Lys338 does not directly affect the catalytic activity of enzyme but results in a significant decrease of its stability. Increasing th content of inactivated form of cytochrome P450scc is followed by an increase in the calculated distance between Lys338 and heme. However, under stabilizing conditions the decrease of the indicated distance is demonstrated. It is suggested that Lys338, being not directly involved in formation of the active sites of cytochrome P450scc, indirectly participates in the biological functions of this heme protein, providing the necessary conformational mobility to the protein molecule in the present fragment of the polypeptide chain. The disturbance of this mobility by inserting of the bulky fluorescent label into cytochrome P450scc polypeptide chain results in decrease of stability (lability of the structure) of the whole protein molecule.

[The use of selective chemical modification of cytochrome P450scc (CYPXIA1) with fluorescein isothiocyanate for evaluation of intramolecular distances and conformational changes by fluorescent resonance energy transfer]. / Ispol'zovanie selektivnoi khimicheskoi modifikatsii tsitokhroma P450scc (CYPXIA1) fluorestseinizotiotsianatom dlia otsenki vnutrimolekuliarnykh rasstoianii i konformatsionnykh izmenenii s pomoshch'iu rezonansnogo perenosa énergii fluorestsentsii.

Clinical modification of cytochrome P450scc with fluoresceine isothiocyanate (FITC) results in selective incorporation of the fluorescent label into Lys338. According to affinity chromatography on immobilized adrenodoxin, fluorescently labeled cytochrome P450scc can interact with ferredoxin. Fluorescent labeling did not significantly affect the enzymatic activity of the hemoprotein at the stage of the first electron transfer. Distance between the heme and labeled Lys338 (2.81 nm) was determined using the efficiency of the fluorescent resonance energy transfer within the donor-acceptor pair (FITC-heme); also changes in this distance were measured during the transitions of cytochrome P450scc from high to low spin state and from oligomeric to monomeric form as well as after its conversion to cytochrome P420 by temperature or alkaline treatment and after complete denaturation of the protein globule in 6 M guanidine chloride. Thus, chemical modification of cytochrome P450scc by selective labeling with FITC is informative method which can be used to monitor conformational changes in the cytochrome P450scc molecule during enzymatic catalysis.

[Direct detection of an antigen in immunoglobulin G Langmuir-Blodgett films based on a surface plasma resonance method and in a piezoelectric system]. / Priamaia detektsiia antigena v plenkakh Léngmiura-Blodzhett immunoglobulinov G na osnove metoda poverkhnostnogo plazmonnogo rezonansa i v p'ezoélektricheskoi sisteme.

The possibilities of direct antigen detection in unlabelled systems based on immunoglobulin G Langmuir-Blodgett films as a sensitive surface have been studied. It was shown that an increase in the monolayer number in an immunoglobulin G Langmuir-Blodgett film deposited onto a solid surface coated with a thin silver film (50 nm) resulted in the regeneration of the antigen-binding capacity of the upper antibody layer. This dependence can be used for the construction of a direct optical immunosensor based on surface plasmon resonance. Moreover, a model of a piesoelectric immunosensor on the basis of immunoglobulin G Langmuir-Blodgett films for direct ferritin detection has been proposed. The detection range of ferritin concentrations in solution is 10(-10)-10(-7) M.

[IgG, modified with coproporphyrin I and the possibility of direct measurement of the antigen-antibody complex]. / IgG, modifitsirovannyi koproporfirinom I, i vozmozhnost' priamoi registratsii kompleksa antigen-antitelo.

Modification of rabbit IgG by coproporphyrin I activated by Woodward's reagent K as well as antigen-binding and fluorescent properties of coproporphyrin-IgG have been studied. It was shown that coproporphyrin I modified IgG retains its capacity to bind the antigen. The formation of the immune antigen-antibody complex increases the intensity of coproporphyrin-IgG fluorescence.

[Modification of a disulfide in the hinge region of rabbit IgG and study of the interaction of polyvalent ferritin antigen, protein A, and anti-IgG]. / Modifikatsiia disul'fida sharnirnoi oblasti IgG krolika i issledovanie vzaimodeistviia s polivalentnym antigenom ferritinom, belkom A i anti-IgG.

Since immunoglobulins are used in a vast variety of immunoassays, the problem of obtaining antibodies with enhanced antigen-binding activity is of great importance. In order to discriminate between putative approaches to activating antibody modification, some functional characteristics of rabbit IgG modified at the hinge disulfide by three reagents: iodoacetamide, N-ethylmaleimide, and 2.2'-dipyridyl disulfide, have been studied. As can be judged from gel-permeation chromatography data, the molecular sizes of modified rabbit IgG were slightly increased in comparison with the native protein. Using enzyme immunoassay, it was shown that modification by each of the above reagents results in the same degree of activation of the antibody binding to the protein polyvalent antigen-human ferritin, due to the increase in segmental flexibility, i.e., Fab motion around the Fo fragment of IgG. The type of concentration dependencies of antigen binding suggest that another determinant stimulating the antigen binding in addition to the increase in segmental flexibility, can be attributed to intra- or interdomain flexibility of domains constituting the Fab fragments. Using protein A and anti-IgG as conformational probes for the antibody Fo fragment, the conformation and conformational dynamics of both CD2 domain epitopes and the switch region between the CD2 and CH3 domains have been shown to be essentially unaffected by modification.

[The use of biotin-streptavidin systems for enhancing the parameters of immunometric analysis]. / Primenenie biotin-streptavidinovykh sistem dlia usileniia parametrov immunometricheskogo analiza.

The possibility of improving analytical parameters of the immunometric assay with the use of biotinylated antibodies and biotin-streptavidin complexes in comparison with the commonly known approach of direct antibody modification with 125I has been studied. Experiments have been carried out with the use of low-affinity antibodies (Kass approximately 10(9) M-1) to ferritin. The signal-to-noise ratio in the immunometric increases 2.3 times when streptavidin labeled with horse-radish peroxidase is used and 4.3 times when the preformed streptavidin + biotin-peroxidase complex is used in comparison with assay systems based on 125I-labeled antibodies. The improvement of assay parameters of immunochemical systems by means of biotin-streptavidin complexes has been found to permit the use of low-affinity antibodies as assay reagents, thus ensuring analytical parameters attaining or close to those of immunoradiometric assay systems based on high-affinity 125I-labeled antibodies (Kass approximately 10(10) M-1). As shown in this study, the following factors ensure the signal enhancement in biotin-streptoavidin systems: (a) the biotin modification of several lysin residues per IgG molecule, the optimum extent of modification being 3-4 residues per molecule; (b) mild procedure for biotinylation. In contrast to oxidative iodination, the modification of NH2 groups with biotin esters does not significantly affect their antigen-binding properties.

[Effect of low-intensity laser radiation on the brain cholinesterase activity of rats]. / Vliianie nizkointensivnogo lazernogo izlucheniia na aktivnost' kholinésterazy mozga krys.

In studying the activity of cholinesterase in subcellular fractions of rat brain affected by laser radiation of low intensity (9 mW) symbasic changes were noted which were more pronounced in vitro than in vivo.