Druggability of the guanosine/adenosine/cytidine nucleoside hydrolase from Trichomonas vaginalis

Insights into nucleoside hydrolase from Leishmania donovani inhibition: A new bioaffinity chromatography-based screening assay and docking studies

Leishmaniasis, a group of neglected infectious diseases, encompasses a serious health concern, particularly with visceral leishmaniasis exhibiting potentially fatal outcomes. Nucleoside hydrolase (NH) has a fundamental role in the purine salvage pathway, crucial for Leishmania donovani survival, and presents a promising target for developing new drugs for visceral leishmaniasis treatment. In this study, LdNH was immobilized into fused silica capillaries, resulting in immobilized enzyme reactors (IMERs). The LdNH-IMER activity was monitored on-flow in a multidimensional liquid chromatography system, with the IMER in the first dimension. A C18 analytical column in the second dimension furnished the rapid separation of the substrate (inosine) and product (hypoxanthine), enabling direct enzyme activity monitoring through product quantification. LdNH-IMER exhibited high stability and was characterized by determining the Michaelis-Menten constant. A known inhibitor (1-(β-d-Ribofuranosyl)-4-quinolone derivative) was used as a model to validate the established method in inhibitor recognition. Screening of three additional derivatives of 1-(β-d-Ribofuranosyl)-4-quinolone led to the discovery of novel inhibitors, with compound 2a exhibiting superior inhibitory activity (Ki = 23.37 ± 3.64 µmol/L) compared to the employed model inhibitor. Docking and Molecular Dynamics studies provided crucial insights into inhibitor interactions at the enzyme active site, offering valuable information for developing new LdNH inhibitors. Therefore, this study presents a novel screening assay and contributes to the development of potent LdNH inhibitors.

A riboside hydrolase that salvages both nucleobases and nicotinamide in the auxotrophic parasite Trichomonas vaginalis

Pathogenic parasites of the Trichomonas genus are causative agents of sexually transmitted diseases affecting millions of individuals worldwide and whose outcome may include stillbirths and enhanced cancer risks and susceptibility to HIV infection. Trichomonas vaginalis relies on imported purine and pyrimidine nucleosides and nucleobases for survival, since it lacks the enzymatic activities necessary for de novo biosynthesis. Here we show that T. vaginalis additionally lacks homologues of the bacterial or mammalian enzymes required for the synthesis of the nicotinamide ring, a crucial component in the redox cofactors NAD+ and NADP. Moreover, we show that a yet fully uncharacterized T. vaginalis protein homologous to bacterial and protozoan nucleoside hydrolases is active as a pyrimidine nucleosidase but shows the highest specificity toward the NAD+ metabolite nicotinamide riboside. Crystal structures of the trichomonal riboside hydrolase in different states reveals novel intermediates along the nucleoside hydrolase-catalyzed hydrolytic reaction, including an unexpected asymmetry in the homotetrameric assembly. The active site structure explains the broad specificity toward different ribosides and offers precise insights for the engineering of specific inhibitors that may simultaneously target different essential pathways in the parasite.

The correlation between Trichomonas vaginalis infection and reproductive system cancer: a systematic review and meta-analysis

BACKGROUND

Trichomonas vaginalis (T. vaginalis) is a microaerophilic protozoan parasite which is responsible for trichomoniasis, the most common non-viral sexually transmitted infection in the world. The infection greatly damages the reproductive system. However, whether T. vaginalis infection can cause reproductive system cancer remains controversial.

METHODS

This study systematically searched PubMed, EMBASE, Ovid and Google scholar, and 144 relevant articles were retrieved and classified into three categories: epidemiological investigations (68), reviews (30) and research articles (46). These three types of articles were verified according to their respective inclusion and exclusion criteria. Stata 16 was used to conduct a meta-analysis on the articles of epidemiological investigations for analysing the correlation between T. vaginalis infection and reproductive system cancer.

RESULTS

The result of meta-analysis indicated that the rate of T. vaginalis infection in the cancer group was significantly higher than that in the non-cancer group (OR = 1.87, 95% CI 1.29-2.71, I² = 52%). Moreover, the cancer rate of the population infected with T. vaginalis was significantly higher than that of the population without T. vaginalis infection (OR = 2.77, 95% CI 2.37-3.25, I² = 31%). The review articles and most research articles stated that the infection of T. vaginalis could lead to cancer and the pathogenic mechanisms were as follows: T. vaginalis promoting inflammatory response, T. vaginalis infection changing the internal environment around parasitic sites and signal transduction pathway, the metabolites secreted by T. vaginalis inducing carcinogenesis and T. vaginalis increasing other pathogenic microbial infection to promote the occurrence of cancer.

CONCLUSIONS

Our study confirmed that there was a correlation between the infection of T. vaginalis and reproductive system cancer, and provided some possible research directions for clarifying the carcinogenic mechanisms caused by T. vaginalis infection.

Structure-Guided Insight into the Specificity and Mechanism of a Parasitic Nucleoside Hydrolase

Trichomonas vaginalis is the causative parasitic protozoan of the disease trichomoniasis, the most prevalent, nonviral sexually transmitted disease in the world. T. vaginalis is a parasite that scavenges nucleosides from the host organism via catalysis by nucleoside hydrolase (NH) enzymes to yield purine and pyrimidine bases. One of the four NH enzymes identified within the genome of T. vaginalis displays unique specificity toward purine nucleosides, adenosine and guanosine, but not inosine, and atypically shares greater sequence similarity to the pyrimidine hydrolases. Bioinformatic analysis of this enzyme, adenosine/guanosine-preferring nucleoside ribohydrolase (AGNH), was incapable of identifying the residues responsible for this uncommon specificity, highlighting the need for structural information. Here, we report the X-ray crystal structures of holo, unliganded AGNH and three additional structures of the enzyme bound to fragment and small-molecule inhibitors. Taken together, these structures facilitated the identification of residue Asp231, which engages in substrate interactions in the absence of those residues that typically support the canonical purine-specific tryptophan-stacking specificity motif. An altered substrate-binding pose is mirrored by repositioning within the protein scaffold of the His80 general acid/base catalyst. The newly defined structure-determined sequence markers allowed the assignment of additional NH orthologs, which are proposed to exhibit the same specificity for adenosine and guanosine alone and further delineate specificity classes for these enzymes.

Structure, Oligomerization and Activity Modulation in N-Ribohydrolases

Enzymes catalyzing the hydrolysis of the N-glycosidic bond in nucleosides and other ribosides (N-ribohydrolases, NHs) with diverse substrate specificities are found in all kingdoms of life. While the overall NH fold is highly conserved, limited substitutions and insertions can account for differences in substrate selection, catalytic efficiency, and distinct structural features. The NH structural module is also employed in monomeric proteins devoid of enzymatic activity with different physiological roles. The homo-oligomeric quaternary structure of active NHs parallels the different catalytic strategies used by each isozyme, while providing a buttressing effect to maintain the active site geometry and allow the conformational changes required for catalysis. The unique features of the NH catalytic strategy and structure make these proteins attractive targets for diverse therapeutic goals in different diseases.

The Role of Purinergic Signaling in Trichomonas vaginalis Infection

Trichomoniasis, one of the most common non-viral sexually transmitted infections worldwide, is caused by the parasite Trichomonas vaginalis. The pathogen colonizes the human urogenital tract, and the infection is associated with complications such as adverse pregnancy outcomes, cervical cancer, and an increase in HIV transmission. The mechanisms of pathogenicity are multifactorial, and controlling immune responses is essential for infection maintenance. Extracellular purine nucleotides are released by cells in physiological and pathological conditions, and they are hydrolyzed by enzymes called ecto-nucleotidases. The cellular effects of nucleotides and nucleosides occur via binding to purinoceptors, or through the uptake by nucleoside transporters. Altogether, enzymes, receptors and transporters constitute the purinergic signaling, a cellular network that regulates several effects in practically all systems including mammals, helminths, protozoa, bacteria, and fungi. In this context, this review updates the data on purinergic signaling involved in T. vaginalis biology and interaction with host cells, focusing on the characterization of ecto-nucleotidases and on purine salvage pathways. The implications of the final products, the nucleosides adenosine and guanosine, for human neutrophil response and vaginal epithelial cell damage reveal the purinergic signaling as a potential new mechanism for alternative drug targets.

Vaginitis: Review on Drug Resistance

Female genital tract infections have a high incidence among different age groups and represent an important impact on public health. Among them, vaginitis refers to inflammation of the vulva and/or vagina due to the presence of pathogens that cause trichomoniasis, bacterial vaginosis, and vulvovaginal candidiasis. Several discomforts are associated with these infections, as well as pregnancy complications and the facilitation of HIV transmission and acquisition. The increasing resistance of microorganisms to drugs used in therapy is remarkable, since women report the recurrence of these infections and associated comorbidities. Different resistant mechanisms already described for the drugs used in the therapy against Trichomonas vaginalis, Candida spp., and Gardnerella vaginalis, as well as aspects related to pathogenesis and treatment, are discussed in this review. This study aims to contribute to drug design, avoiding therapy ineffectiveness due to drug resistance. Effective alternative therapies to treat vaginitis will reduce the recurrence of infections and, consequently, the high costs generated in the health system, improving women's well-being.

An NMR-Based Chemometric Strategy to Identify Leishmania donovani Nucleoside Hydrolase Inhibitors from the Brazilian Tree Ormosia arborea

Nucleoside hydrolases are a strategic target for the development of drugs to treat leishmaniasis, a neglected disease that affects 700 thousand to one million people annually. The present study aimed to identify Leishmania donovani nucleoside hydrolase (LdNH) inhibitors from the leaves of Ormosia arborea, a tree endemic to Brazilian ecosystems, through a strategy based on ¹H NMR analyses and chemometrics. The aqueous EtOH extract of O. arborea leaves inhibited LdNH activity by 95%. The extract was fractionated in triplicate (13 in each step, making a total of 39 fractions). Partial least squares discriminant analysis (PLS-DA) was used to correlate the ¹H NMR spectra of the fractions with their LdNH inhibitory activity and thus to identify the spectral regions associated with the bioactivity. The strategy aimed at isolating the probable bioactive substances and led to two new A-type proanthocyanidins, linked to a p-coumaroyl unit (1 and 2), which appeared as noncompetitive inhibitors of LdNH (IC₅₀: 28.2 ± 3.0 μM and 25.6 ± 4.1 μM, respectively). This study confirms the usefulness of the NMR-based chemometric methods to accelerate the discovery of drugs from natural products.

Discovery of Ligand-Efficient Scaffolds for the Design of Novel Trichomonas vaginalis Uridine Nucleoside Ribohydrolase Inhibitors Using Fragment Screening

Trichomoniasis is caused by the parasitic protozoan Trichomonas vaginalis. The increasing prevalence of strains resistant to the current 5-nitroimidazole treatments creates the need for novel therapies. T. vaginalis cannot synthesize purine and pyrimidine rings and requires salvage pathway enzymes to obtain them from host nucleosides. The uridine nucleoside ribohydrolase was screened using an 19F NMR-based activity assay against a 2000-compound fragment diversity library. Several series of inhibitors were identified including scaffolds based on acetamides, cyclic ureas or ureas, pyridines, and pyrrolidines. A number of potent singleton compounds were identified, as well. Eighteen compounds with IC50 values of 20 μM or lower were identified, including some with ligand efficiency values of 0.5 or greater. Detergent and jump-dilution counter screens validated all scaffold classes as target-specific, reversible inhibitors. Identified scaffolds differ substantially from 5-nitroimidazoles. Medicinal chemistry using the structure-activity relationship emerging from the fragment hits is being pursued to discover nanomolar inhibitors.

Ligand-Efficient Inhibitors of Trichomonas vaginalis Adenosine/Guanosine Preferring Nucleoside Ribohydrolase

Trichomoniasis is caused by the parasitic protozoan Trichomonas vaginalis and is the most prevalent, nonviral sexually transmitted disease. The parasite has shown increasing resistance to the current 5-nitroimidazole therapies indicating the need for new therapies with different mechanisms. T. vaginalis is an obligate parasite that scavenges nucleosides from host cells and then uses salvage pathway enzymes to obtain the nucleobases. The adenosine/guanosine preferring nucleoside ribohydrolase was screened against a 2000-compound diversity fragment library using a ¹H NMR-based activity assay. Three classes of inhibitors with more than five representatives were identified: bis-aryl phenols, amino bicyclic pyrimidines, and aryl acetamides. Among the active fragments were 10 compounds with ligand efficiency values greater than 0.5, including five with IC₅₀ values <10 μM. Jump-dilution and detergent counter screens validated reversible, target-specific activity. The data reveals an emerging SAR that is guiding our medicinal chemistry efforts aimed at discovering compounds with nanomolar potency.

New Leishmania donovani nucleoside hydrolase inhibitors from Brazilian flora

This study presents new inhibitors of the nucleoside hydrolase from Leishmania donovani (LdNH) with in vitro leishmanicidal activity. Biological screening of 214 Brazilian plant extracts was performed to select plants with enzyme inhibitory activity. Two plants were selected for their results, and for their lack of prior phytochemical description: Leandra amplexicaulis DC. (Melastomataceae) and Urvillea rufescens Cambess (Sapindaceae). Three flavonoids were isolated by bioguided fractionation of the hydroethanolic extracts: kaempferol 3-O-α-l-rhamnopyranoside (1) and kaempferol 3-O-β-d-xylopyranosyl-(1→2)-α-l-rhamnopyranoside (2) from L. amplexicaulis, as well as tricetin-4′-O-methyl flavone (3) from U. rufescens. These flavonoids showed inhibitory activities (IC₅₀) of 197.4 μM (1), 74.7 μM (2) and 1.1 μM (3) on the LdNH. Their binding mode was proposed based on molecular docking with LdNH and by NMR Saturation Transfer Difference studies. Kinetic studies demonstrate that the most potent inhibitor (3) acts by uncompetitive inhibition. This study reports for the first time the inhibition of LdNH by naturally sourced flavonoids.

This study presents flavonoids as new inhibitors of the nucleoside hydrolase from Leishmania donovani (LdNH) and the first uncompetitive inhibitor described for LdNH.

Enzymatic Transition States and Drug Design

Transition state theory teaches that chemically stable mimics of enzymatic transition states will bind tightly to their cognate enzymes. Kinetic isotope effects combined with computational quantum chemistry provides enzymatic transition state information with sufficient fidelity to design transition state analogues. Examples are selected from various stages of drug development to demonstrate the application of transition state theory, inhibitor design, physicochemical characterization of transition state analogues, and their progress in drug development.