Interaction of Citrinin with Human Serum Albumin

Microplate fluorescence quenching for high throughput screening of affinity constants - Serum albumins and zearalenones case study

Measurements of changes in fluorescence signal is one of the most commonly applied methods for studying protein-ligand affinities. These measurements are generally carried out using cuvettes in spectrofluorometers, which can only measure one sample at a time. This makes screening procedures for multiple ligands and proteins extremely laborious, as each protein must be measured with multiple ligand concentrations, and usually in triplicate. Moreover, multiple equations exist to extract the affinity constants and other information from the data, and their underlying assumptions are often disregarded. In this study, the affinities of human, bovine and rat serum albumins for the mycotoxin zearalenone and five of its common derivatives were measured in 96-well microplates, allowing quick measurements of multiple samples using less reagent amounts. In comparison to measurements using a cuvette in a spectrofluorometer, the microplate method was shown to reproduce the affinity constants accurately. The results were discussed in terms of common pitfalls regarding experimental setup and available equations to analyze protein-ligand binding in fluorescence quenching assays. The commonly used Stern-Volmer equation was discussed in detail and the results used to show how inaccurate it is when a fluorescent protein-ligand complex is formed, and when other underlying approximations are ignored.

Reactions of citrinin with amino compounds modelling thermal food processing

Citrinin (CIT) is a nephrotoxic mycotoxin, produced by several species of Penicillium, Aspergillus, and Monascus. The foodstuffs most frequently contaminated with CIT include cereals, cereal products, and red yeast rice. Studies on the occurrence of CIT in food have shown that the CIT concentrations in processed cereal-based products are generally lower than in unprocessed industry cereal samples. One possible explanation is the reaction of CIT with major food components such as carbohydrates or proteins to form modified CIT. Such modified forms of CIT are then hidden from conventional analyses, but it is possible that they are converted back into the parent mycotoxin during digestion. The aim of this study is therefore to investigate reactions of CIT with food matrix during thermal processes and to gain a deeper understanding of the degradation of CIT during food processing. In this study, we could demonstrate that CIT reacts with amino compounds such as proteins, under typical food processing conditions, leading to modified forms of CIT.

Citrinin Provoke DNA Damage and Cell-Cycle Arrest Related to Chk2 and FANCD2 Checkpoint Proteins in Hepatocellular and Adenocarcinoma Cell Lines

Citrinin (CIT), a polyketide mycotoxin produced by Penicillium, Aspergillus, and Monascus species, is a contaminant that has been found in various food commodities and was also detected in house dust. Several studies showed that CIT can impair the kidney, liver, heart, immune, and reproductive systems in animals by mechanisms so far not completely elucidated. In this study, we investigated the CIT mode of action on two human tumor cell lines, HepG2 (hepatocellular carcinoma) and A549 (lung adenocarcinoma). Cytotoxic concentrations were determined using an MTT proliferation assay. The genotoxic effect of sub-IC50 concentrations was investigated using the alkaline comet assay and the impact on the cell cycle using flow cytometry. Additionally, the CIT effect on the total amount and phosphorylation of two cell-cycle-checkpoint proteins, the serine/threonine kinase Chk2 and Fanconi anemia (FA) group D2 (FANCD2), was determined by the cell-based ELISA. The data were analyzed using GraphPad Prism statistical software. The CIT IC50 for HepG2 was 107.3 µM, and for A549, it was >250 µM. The results showed that sensitivity to CIT is cell-type dependent and that CIT in sub-IC50 and near IC50 induces significant DNA damage and cell-cycle arrest in the G2/M phase, which is related to the increase in total and phosphorylated Chk2 and FANCD2 checkpoint proteins in HepG2 and A549 cells.

pH-induced interaction mechanism of zearalenone with zein: Binding characteristics, conformational structure and intermolecular forces

Zein-bound zearalenone (ZEN) complexes are naturally existed in maize by their spontaneous interaction, which significantly impacts the risk assessment of ZEN. Additionally, the pH levels in processing could affect the binding or release of zein-bound ZEN. In this study, pH-induced interaction mechanism of ZEN with zein were studied. Results showed that the acid conditions increased the binding constant (Ka) from 3.46 to 10.0 × 104 L/mol, binding energy from -17.38 to -43.49 kJ mol-1. By increasing hydrophobic interaction and hydrogen bond of ZEN with zein, the binding of ZEN with zein was promoted, forming zein-bound ZEN. Whereas, alkaline conditions decreased the Ka to 1.45 × 104 L/mol and binding energy to 148.48 kJ mol-1, weakened ZEN-zein interaction and stretched zein molecules, resulting the release of ZEN from zein. This study could provide important theoretical basis for perfecting risk assessment and controlling zein-bound ZEN during processing.

Multi-omics reveals the protective effects of curcumin against AFB1-induced oxidative stress and inflammatory damage in duckling intestines

Aflatoxin B1 (AFB1) is the most prevalent and toxic class of aflatoxins, which is considered a significant risk factor for food safety. Curcumin, a phytoconstituent with anti-inflammatory and antioxidant properties, has potential therapeutic value for intestinal inflammatory diseases. In this study, the duckling model susceptible to AFB1 was selected for toxicity testing, aiming to explore the effect of curcumin on AFB1 enterotoxicity and its possible mechanism of action. The results showed that curcumin promoted the growth and development of ducklings and mitigated the changes in morphology and permeability serological index (DAO and D-LA) after AFB1 exposure. Curcumin also mitigated AFB1-induced oxidative stress by activating the Nrf2 pathway, and ameliorated intestinal inflammation by inhibiting the NF-κB/IκB signaling pathway and boosting intestinal autophagy. In terms of gut flora and their metabolites, we found that curcumin supplementation significantly increased the intestinal flora's abundance index and diversity index compared to the AFB1 group, mitigating the decline in the abundance of Actinobacteria and the rise in that of harmful bacteria Clostridia. Furthermore, untargeted metabolomic analysis revealed that the protective effect of curcumin on the intestine was mainly through the regulation of AFB1-induced disorders of lipid metabolism, involving linoleic acid metabolism, α-linolenic acid metabolism, and glycerolipid metabolism. Overall, the enteroprotective effects of curcumin may be of significant value in the future for treating chronic AFB1 poisoning and also provide new therapeutic ideas for other mycotoxicosis.

Human serum albumin stabilizes streptolysin S activity secreted in the extracellular milieu by streptolysin S-producing streptococci

Anginosus group streptococci (AGS) are opportunistic pathogens of the human oral cavity; however, their pathogenicity has not been discussed in detail. Oral streptococci live in the gingival sulcus, from where they can easily translocate into the bloodstream due to periodontal diseases and dental treatment and cause hazardous effects on the host through their virulence factors. Streptolysin S (SLS), a pathogenic factor produced by β-hemolytic species/strains belonging to AGS, plays an important role in damaging host cells. Therefore, we investigated the SLS-dependent cytotoxicity of β-hemolytic Streptococcus anginosus subsp. anginosus (SAA), focusing on different growth conditions such as in the bloodstream. Consequently, SLS-dependent hemolytic activity/cytotoxicity in the culture supernatant of β-hemolytic SAA was stabilized by blood components, particularly human serum albumin (HSA). The present study suggests that the secreted SLS, not only from β-hemolytic SAA, but also from other SLS-producing streptococci, is stabilized by HSA. As HSA is the most abundant protein in human plasma, the results of this study provide new insights into the risk of SLS-producing streptococci which can translocate into the bloodstream.

Inhibition of Aflatoxin Production by Citrinin and Non-Enzymatic Formation of a Novel Citrinin-Kojic Acid Adduct

Screening for microorganisms that inhibit aflatoxin production from environments showed that Penicillium citrinum inhibited aflatoxin production by Aspergillus parasiticus. The inhibitory substance in the culture medium of P. citrinum was confirmed to be citrinin (CTN). RT-PCR analyses showed that CTN did not inhibit expressions of aflatoxin biosynthetic genes (aflR, pksL1, and fas-1) of A. parasiticus, whereas feeding experiments using A. parasiticus showed that CTN inhibited the in vivo conversion of dihydrosterigmatocystin to AFB₂·AFG₂. These results suggest that CTN inhibits a certain post-transcriptional step in aflatoxin biosynthesis. CTN in the culture medium of A. parasiticus was found to be decreased or lost with time, suggesting that a certain metabolite produced by A. parasiticus is the cause of the CTN decrease; we then purified, characterized, and then analyzed the substance. Physico-chemical analyses confirmed that the metabolite causing a decrease in CTN fluorescence was kojic acid (KA) and the resulting product was identified as a novel substance: (1R,3S,4R)-3,4-dihydro-6,8-dihydroxy-1-(3-hydroxy-6-(hydroxymethyl)-4-oxo-4H-pyran-2-yl)-3,4,5-trimethyl-1H-isochromene-7-carboxylic acid, which was named "CTN-KA adduct". Our examination of the metabolites' toxicities revealed that unlike CTN, the CTN-KA adduct did not inhibit aflatoxin production by A. parasiticus. These results indicate that CTN's toxicity was alleviated with KA by converting CTN to the CTN-KA adduct.

Interaction of the Emerging Mycotoxins Beauvericin, Cyclopiazonic Acid, and Sterigmatocystin with Human Serum Albumin

Beauvericin (BEA), cyclopiazonic acid (CPA), and sterigmatocystin (STC) are emerging mycotoxins. They appear as contaminants in food and animal feed, leading to economic losses and health risks. Human serum albumin (HSA) forms stable complexes with certain mycotoxins, including ochratoxins, alternariol, citrinin, and zearalenone. HSA binding can influence the toxicokinetics of xenobiotics, and albumin can also be considered and applied as a relatively cheap affinity protein. Therefore, we examined the potential interactions of BEA, CPA, and STC with HSA employing fluorescence spectroscopy, ultracentrifugation, ultrafiltration, and molecular modeling. Spectroscopic and ultracentrifugation studies demonstrated the formation of low-affinity BEA–HSA (K_a ≈ 10³ L/mol) and moderately strong CPA–HSA and STC–HSA complexes (K_a ≈ 10⁴ L/mol). In ultrafiltration experiments, CPA slightly displaced each site marker (warfarin, naproxen, and camptothecin) tested, while BEA and STC did not affect significantly the albumin binding of these drugs. Modeling studies suggest that CPA occupies Sudlow’s site I, while STC binds to the Heme site (FA1) on HSA. Considering the interactions of CPA with the site markers, the CPA–HSA interaction may have toxicological importance.

Biomonitoring of ochratoxin A, 2'R-ochratoxin A and citrinin in human blood serum from Switzerland

Biomonitoring of mycotoxins and their metabolites in biological fluids is increasingly used to assess human exposure. In this study, biomarkers of ochratoxin A (OTA) and citrinin (CIT) exposure were determined in a large number of serum samples from healthy blood donors in Switzerland. In 2019, 700 samples from different regions were obtained. From 240 donors, a second sample (taken 2-9 months later) was available for analysis. Moreover, 355 blood donor samples from 2005 from all regions in Switzerland and 151 additional samples from the southern Swiss region of Ticino from 2005 could be analysed.OTA, 2'R-ochratoxin A (2'R-OTA), ochratoxin alpha (OTα), CIT and dihydrocitrinone (DH-CIT) were analysed using validated targeted methods including precipitation and online SPE clean-up.OTA and 2'R-OTA were frequently detected (OTA in 99%; 2'R-OTA in 51% of the tested samples). The mean concentration in all positive samples was 0.4 ng/mL for OTA and 0.2 ng/mL for 2'R-OTA. OTα was not detected in any sample above the limit of quantification (LOQ). In contrast to OTA, CIT and DH-CIT were only quantifiable in 2% and 0.1% of the samples, respectively. No significant trend was observed between the samples from 2005 and the more recent samples, but OTA concentrations were usually higher in serum samples from the southern Swiss region of Ticino and in males compared to females.Our extensive data fit well within the framework of previously published values for the healthy adult European population.

The individual and combined effects of ochratoxin A with citrinin and their metabolites (ochratoxin B, ochratoxin C, and dihydrocitrinone) on 2D/3D cell cultures, and zebrafish embryo models

Ochratoxin A and citrinin are nephrotoxic mycotoxins produced by Aspergillus, Penicillium, and/or Monascus species. The combined effects of ochratoxin A and citrinin have been examined in more studies; however, only limited data are available regarding the co-exposure to their metabolites. In this investigation, the individual toxic effects of ochratoxin A, ochratoxin B, ochratoxin C, citrinin, and dihydrocitrinone were tested as well as the combinations of ochratoxin A with the latter mycotoxins were examined on 2D and 3D cell cultures, and on zebrafish embryos. Our results demonstrate that even subtoxic concentrations of certain mycotoxins can increase the toxic impact of ochratoxin A. In addition, typically additive effects or synergism were observed as the combined effects of mycotoxins tested. These observations highlight that different cell lines (e.g. MDBK vs. MDCK), cell cultures (e.g. 2D vs. 3D), and models (e.g. in vitro vs. in vivo) can show different (sometimes opposite) impacts. Mycotoxin combinations considerably increased miR-731 levels in zebrafish embryos, which is an early marker of the toxicity on kidney development. These results underline that the co-exposure to mycotoxins (and/or mycotoxin metabolites) should be seriously considered, since even the barely toxic mycotoxins (or metabolites) in combinations can cause significant toxicity.

Efflux at the Blood-Brain Barrier Reduces the Cerebral Exposure to Ochratoxin A, Ochratoxin α, Citrinin and Dihydrocitrinone

Recent studies have implied that environmental toxins, such as mycotoxins, are risk factors for neurodegenerative diseases. To act directly as neurotoxins, mycotoxins need to penetrate or affect the integrity of the blood-brain barrier, which protects the mammalian brain from potentially harmful substances. As common food and feed contaminants of fungal origin, the interest in the potential neurotoxicity of ochratoxin A, citrinin and their metabolites has recently increased. Primary porcine brain capillary endothelial cells were used to investigate cytotoxic or barrier-weakening effects of ochratoxin A, ochratoxin α, citrinin and dihydrocitrinone. The transfer and transport properties of the mycotoxins across the barrier formed by porcine brain capillary endothelial cell monolayers were analysed using HPLC-MS/MS. High levels of Ochratoxin A caused cytotoxic and barrier-weakening effects, whereas ochratoxin α, citrinin and dihydrocitrinone showed no adverse effects up to 10 µM. Likely due to efflux transporter proteins, the transfer to the brain compartment was much slower than expected from their high lipophilicity. Due to their slow transfer across the blood-brain barrier, cerebral exposure of ochratoxin A, ochratoxin α, citrinin and dihydrocitrinone is low and neurotoxicity is likely to play a subordinate role in their toxicity at common physiological concentrations.

Efficacy of a Combined Antiplatelet Therapy Is Not Affected by a Simultaneous Binding of Cangrelor and PSB 0777 to Albumin

Concurrent administration of two drugs may complicate the management of acute coronary syndromes: competitive drug displacement diminishes drug binding and alters drug pharmacodynamics. We investigated the interaction of two antiplatelet compounds (PSB 0777 and cangrelor) with human serum albumin (HSA) to determine whether they compete with one another for the binding to albumin. Both examined compounds have been earlier claimed to bind to HSA (PSB 0777) or plasma proteins (cangrelor). Fluorescence spectroscopy, surface plasmon resonance spectroscopy and molecular modeling indicated that PSB 0777 and cangrelor interacted with HSA with moderate affinity (K_D∼10⁻⁵ M). The binding of cangrelor to HSA involved primarily hydrophobic interactions, while the interaction of PSB 0777 with HSA was driven by hydrophobic and electrostatic forces. It was found that PSB 0777 and cangrelor do not share the same binding site on the protein. Our findings highlight the importance of albumin in the transport of PSB 0777 and cangrelor and suggest that the antiplatelet activity of the examined compounds used in combination is not affected by competition-induced changes in drug binding to HSA.

Probing the Interactions of Ochratoxin B, Ochratoxin C, Patulin, Deoxynivalenol, and T-2 Toxin with Human Serum Albumin

Ochratoxins, patulin, deoxynivalenol, and T-2 toxin are mycotoxins, and common contaminants in food and drinks. Human serum albumin (HSA) forms complexes with certain mycotoxins. Since HSA can affect the toxicokinetics of bound ligand molecules, the potential interactions of ochratoxin B (OTB), ochratoxin C (OTC), patulin, deoxynivalenol, and T-2 toxin with HSA were examined, employing spectroscopic (fluorescence, UV, and circular dichroism) and ultrafiltration techniques. Furthermore, the influence of albumin on the cytotoxicity of these xenobiotics was also evaluated in cell experiments. Fluorescence studies showed the formation of highly stable OTB–HSA and OTC–HSA complexes. Furthermore, fluorescence quenching and circular dichroism measurements suggest weak or no interaction of patulin, deoxynivalenol, and T-2 toxin with HSA. In ultrafiltration studies, OTB and OTC strongly displaced the Sudlow’s site I ligand warfarin, while other mycotoxins tested did not affect either the albumin binding of warfarin or naproxen. The presence of HSA significantly decreased or even abolished the OTB- and OTC-induced cytotoxicity in cell experiments; however, the toxic impacts of patulin, deoxynivalenol, and T-2 toxin were not affected by HSA. In summary, the complex formation of OTB and OTC with albumin is relevant, whereas the interactions of patulin, deoxynivalenol, and T-2 toxin with HSA may have low toxicological importance.

Comprehensive toxicokinetic analysis reveals major interspecies differences in absorption, distribution and elimination of citrinin in pigs and broiler chickens

A comprehensive toxicokinetic analysis of citrinin (CIT) revealed interspecies differences for all toxicokinetic parameters and in absolute oral bioavailability. Oral bioavailability for CIT was complete for broilers (113-131%), while ranging from 37 to 44% in pigs. CIT was more rapidly absorbed in pigs (T_max = 0.92 h) compared to broiler chickens (T_max = 7.33 h). The elimination of CIT was slower in pigs (T_1/2el = 26.81 h after intravenous (IV) administration) compared to chickens (T_1/2el = 1.97 h after IV administration), due to the striking difference in clearance (Cl_iv=9.87 mL/h/kg for pigs versus Cl_iv = 863.09 mL/h/kg for broilers). Also, the volume of distribution differed significantly between pigs (Vd = 0.30 L/kg after IV administration) and chickens (Vd = 2.46 L/kg after IV administration). However, plasma protein binding did not differ statistically significant (91-98%). It is imperative to further investigate biotransformation and elimination pathways in different species, including humans.

Citrinin biomarkers: a review of recent data and application to human exposure assessment

The mycotoxin citrinin (CIT) deserves attention due to its known toxic effects in mammalian species and a widespread occurrence in food commodities, often along with ochratoxin A, another nephrotoxic mycotoxin. Human exposure, a key element in assessing risks related to these food contaminants, depends upon mycotoxin levels in food and on food consumption. Yet, data available for CIT levels in food are insufficient for reliable intake estimates. Now biomonitoring, i.e., analysis of parent compound and/or metabolites in human specimen (blood, urine, breast milk), is increasingly used to investigate mycotoxin exposure. Biomonitoring requires sensitive methods for determining biomarkers of exposure, combined with kinetic data to conclude on the absorbed internal dose in an individual. Recent advances in LC-MS/MS-based analytical techniques have facilitated biomonitoring studies on the occurrence of CIT biomarkers in body fluids, mainly in urine samples. This review compiles evidence on human exposure to CIT in different countries, on CIT kinetics in humans, and on biomarker-based CIT intake estimates. Human CIT exposures are discussed in light of an intake value defined as 'level of no concern for nephrotoxicity' by the European Food Safety Agency, and some uncertainties in the toxicological data base. Further studies on CIT, including biomarker-based studies are warranted along with regular food surveys for this mycotoxin to protect consumers against undesirable health effects.

Interaction of Mycotoxin Alternariol with Serum Albumin

Alternariol (AOH) is a mycotoxin produced by Alternaria species. In vitro studies suggest the genotoxic, mutagenic, and endocrine disruptor effects of AOH, and an increased incidence of esophageal cancer has been reported related to higher AOH exposure. Human serum albumin (HSA) is the most abundant plasma protein in the circulation, it is able to affect toxicokinetic properties of numerous xenobiotics. HSA forms stable complexes with several mycotoxins, however, the interaction of AOH with albumin has not been examined. In this study, the complex formation of AOH with HSA was tested, employing fluorescence spectroscopy, ultrafiltration, and molecular modeling. Each spectroscopic measurement shows the formation of stable AOH-HSA complexes (K = 4 × 10⁵ L/mol). Investigations with site markers (in spectroscopic and ultrafiltration models) as well as modeling studies suggest that AOH occupies Sudlow’s site I as a high-affinity binding site in HSA. The binding affinity of AOH towards bovine, porcine, and rat albumins was also tested, suggesting that AOH binds to rat albumin with considerably higher affinity than other albumins tested. Our results demonstrate the strong interaction of AOH with serum albumins, suggesting the potential in vivo importance of these interactions.

Fluorescence Spectroscopic Investigation of Competitive Interactions between Quercetin and Aflatoxin B₁ for Binding to Human Serum Albumin

Aflatoxin B₁ (AFB₁) is a highly toxic mycotoxin found worldwide in cereals, food, and animal feeds. AFB₁ binds to human serum albumin (HSA) with high affinity. In previous experiments, it has been revealed that reducing the binding rate of AFB₁ with HSA could speed up the elimination rate of AFB₁. Therefore, we examined the ability of quercetin to compete with AFB₁ for binding HSA by fluorescence spectroscopy, synchronous spectroscopy, ultrafiltration studies, etc. It was shown that AFB₁ and quercetin bind to HSA in the same Sudlow site Ӏ (subdomain IIA), and the binding constant (K_a) of the quercetin-HSA complex is significantly stronger than the complex of AFB₁-HSA. Our data in this experiment showed that quercetin is able to remove the AFB₁ from HSA and reduce its bound fraction. This exploratory work may be of significance for studies in the future regarding decreasing its bound fraction and then increasing its elimination rate for detoxification. This exploratory study may initiate future epidemiological research designs to obtain further in vivo evidence of the long-term (potential protective) effects of competing substances on human patients.

Interaction of Chrysin and Its Main Conjugated Metabolites Chrysin-7-Sulfate and Chrysin-7-Glucuronide with Serum Albumin

Chrysin (5,7-dihydroxyflavone) is a flavonoid aglycone, which is found in nature and in several dietary supplements. During the biotransformation of chrysin, its conjugated metabolites chrysin-7-sulfate (C7S) and chrysin-7-glucuronide (C7G) are formed. Despite the fact that these conjugates appear in the circulation at much higher concentrations than chrysin, their interactions with serum albumin have not been reported. In this study, the complex formation of chrysin, C7S, and C7G with human (HSA) and bovine (BSA) serum albumins was investigated employing fluorescence spectroscopic, ultrafiltration, and modeling studies. Our major observations/conclusions are as follows: (1) Compared to chrysin, C7S binds with a threefold higher affinity to HSA, while C7G binds with a threefold lower affinity; (2) the albumin-binding of chrysin, C7S, and C7G did not show any large species differences regarding HSA and BSA; (3) tested flavonoids likely occupy Sudlow’s Site I in HSA; (4) C7S causes significant displacement of Sudlow’s Site I ligands, exerting an even stronger displacing ability than the parent compound chrysin. Considering the above-listed observations, the high intake of chrysin (e.g., through the consumption of dietary supplements with high chrysin contents) may interfere with the albumin-binding of several drugs, mainly due to the strong interaction of C7S with HSA.

Interaction of the mycotoxin metabolite dihydrocitrinone with serum albumin

Citrinin (CIT) is a nephrotoxic mycotoxin produced by Penicillium, Monascus, and Aspergillus species. CIT appears as a contaminant in cereals, cereal-based products, fruits, nuts, and spices. During the biotransformation of CIT, its major urinary metabolite dihydrocitrinone (DHC) is formed. Albumin interacts with several compounds (including mycotoxins) affecting their tissue distribution and elimination. CIT-albumin interaction is known; however, the complex formation of DHC with albumin has not been reported previously. In this study, we aimed to investigate the interaction of DHC with albumin, employing fluorescence spectroscopy, circular dichroism, and molecular modeling studies. Furthermore, species differences and thermodynamics of the interaction as well as the effects of albumin on the acute in vitro toxicity of DHC and CIT were also tested. Our main observations/conclusions are as follows: (1) Fluorescence signal of DHC is strongly enhanced by albumin. (2) Formation of DHC-albumin complexes is supported by both fluorescence spectroscopic and circular dichroism studies. (3) DHC forms similarly stable complexes with human albumin (K~10⁵ L/mol) as CIT. (4) DHC-albumin interaction did not show significant species differences (tested with human, bovine, porcine, and rat albumins). (5) Based on modeling studies and investigations with site markers, DHC occupies the Heme binding site (subdomain IB) on human albumin. (6) The presence of albumin significantly decreased the acute in vitro cytotoxic effects of both DHC and CIT on MDCK cell line.

Anti-cancer study and whey protein complexation of new lanthanum(III) complex with the aim of achieving bioactive anticancer metal-based drugs

In this study, a new lanthanum (III)-amino acid complex utilizing cysteine has been synthesized and characterized. The anticancer activities of the prepared La(III) complex against MCF-7 cell lines were studied. Results of MTT assay showed that at all three incubation times, the cytotoxic effect of prepared La(III) complex on MCF-7 breast cancer cell lines displays a time- and dose-dependent inhibitory effects. The interactions of the La(III) complex with two whey proteins (bovine serum albumin, BSA, and Bovine β-lactoglobulin, βLG) have been explored by using spectroscopic and molecular dicking methods. The obtained results indicated that La(III) complex strongly quenched the fluorescence of two carrier proteins in static quenching mode and also, BSA hah stronger binding affinity toward studied complex than βLG whit binding constant values of K_{BSA-La Complex} ∼ 0.11 × 10⁴ M^-1 and K_{βLG-La Complex} ∼ 0.63 × 10³ M^-1 at 300 K. The thermodynamic parameters revealed the contribution of hydrogen bond and Vander Waals interactions in both systems. The distances of the La(III) complex whit whey proteins were calculated using Förster energy transfer theory and proved existence of the energy transfer between two proteins and prepared La(III) complex with a high probability. FT-IR and UV-Vis absorption measurements indicated that the binding of the La(III) to BSA and βLG may induce conformational and micro-environmental changes of the proteins. The docking results indicate that the La(III) complex bind to residues located in the site II of BSA and second site of βLG. Communicated by Ramaswamy H. Sarma.

Identification differential behavior of Gd@C82(OH)22 upon interaction with serum albumin using spectroscopic analysis

The interaction between Gd@C₈₂(OH)₂₂ and serum albumin (HSA and BSA) were investigated by spectroscopic analysis. From the characteristic feature of fluorescence quenching spectra at different temperatures, the inherent binding information including quenching mechanism, association constants, number of binding site, fraction of initial fluorescence and basic thermodynamic parameters were calculated. The binding of Gd@C₈₂(OH)₂₂ to serum albumin caused strong quenching of protein intrinsic fluorescence and the structural changes of serum albumin. At lower concentrations, Gd@C₈₂(OH)₂₂ was likely to rise fluorescence quenching of serum albumin through individual static quenching process by forming a ground-state complex, while dynamic and static coexisting quenching mechanism occurred in high concentration. Bimolecular quenching (K_q) value is twice the diffusion-controlled quenching constant (2.0 × 10¹⁰ L mol^-1 s^-1); binding sites of BSA were slightly more than those of HAS, and all of them reached to 1; the distance r between donor and acceptor was found to be 3.1494 nm and 3.6479 nm for HSA and BSA, respectively, both of which were fewer than 7 nm. It is confirmed that binding interaction for proteins in the presence of drugs was strong, the binding ratio was 1:1, and non-radiative energy transfer from protein to drug was extremely high probability in lower density. Binding process of Gd@C₈₂(OH)₂₂-HSA was driven mainly through van der Waals forces and hydrogen bonding formation, however more likely to be electrostatic interaction involved in the Gd@C₈₂(OH)₂₂-BSA binding process; Binding sites of Gd@C₈₂(OH)₂₂ to serum albumin were near tryprophan (HSA) and tyrosine residues (BSA), respectively. Moreover, a theoretical model of predicting the binding rate of drug to serum albumin was estimated, further analyzed that the binding rate was dynamically altered in various dose of protein and drug. Overall, these results provide potentially significant information for elucidating the distribution, transportation, the apparent relationship between pharmacologic activity and total plasma drug concentration as well as anti-carcinogenic activity and mechanisms in vivo.

Interaction of 2'R-ochratoxin A with Serum Albumins: Binding Site, Effects of Site Markers, Thermodynamics, Species Differences of Albumin-binding, and Influence of Albumin on Its Toxicity in MDCK Cells

Ochratoxin A (OTA) is a nephrotoxic mycotoxin. Roasting of OTA-contaminated coffee results in the formation of 2′R-ochratoxin A (2′R-OTA), which appears in the blood of coffee drinkers. Human serum albumin (HSA) binds 2′R-OTA (and OTA) with high affinity; therefore, albumin may influence the tissue uptake and elimination of ochratoxins. We aimed to investigate the binding site of 2′R-OTA (verses OTA) in HSA and the displacing effects of site markers to explore which molecules can interfere with its albumin-binding. Affinity of 2′R-OTA toward albumins from various species (human, bovine, porcine and rat) was tested to evaluate the interspecies differences regarding 2′R-OTA-albumin interaction. Thermodynamic studies were performed to give a deeper insight into the molecular background of the complex formation. Besides fluorescence spectroscopic and modeling studies, effects of HSA, and fetal bovine serum on the cytotoxicity of 2′R-OTA and OTA were tested in MDCK kidney cell line in order to demonstrate the influence of albumin-binding on the cellular uptake of ochratoxins. Site markers displaced more effectively 2′R-OTA than OTA from HSA. Fluorescence and binding constants of 2′R-OTA-albumin and OTA-albumin complexes showed different tendencies. Albumin significantly decreased the cytotoxicity of ochratoxins. 2′R-OTA, even at sub-toxic concentrations, increased the toxic action of OTA.

Noncovalent Interaction of Tilmicosin with Bovine Serum Albumin

Tilmicosin is a widely used antibiotic in veterinary applications. Its antimicrobial activity is ranged from Gram-positive and some Gram-negative bacteria towards activities against Mycoplasma and Chlamydia. Adsorption affinity of tilmicosin antibiotics towards bovine serum albumin was investigated by both spectroscopic (UV-vis, Photoluminescence) and calorimetric methods. The interaction was determined on the basis of quenching of albumin by tilmicosin. Results confirm noncovalent binding of tilmicosin on bovine serum albumin with 1:1 stoichiometry associated with pK = 4.5, highlighting possible removal of tilmicosin molecules from the albumin surface through exchange reactions by known competitor molecules. Calorimetric measurements have confirmed the weak interaction between tilmicosin and albumin and reflect enhanced denaturation of the albumin in the presence of tilmicosin antibiotic. This process is associated with the decreased activation energy of conformational transition of the albumin. It opens a new, very quick reaction pathway without any significant effect on the product by noncovalent binding the tilmicosin molecules to the protein molecules. Results highlight the medical importance of these investigations by considerable docking of the selected antibiotic molecules on serum albumins. Although the binding may cause toxic effects in living bodies, the strength of the binding is weak enough to find competitor molecules for effective removals from their surface.

Interactions of zearalenone and its reduced metabolites α-zearalenol and β-zearalenol with serum albumins: species differences, binding sites, and thermodynamics

Zearalenone (ZEN) is a mycotoxin produced by Fusarium species. ZEN mainly appears in cereals and related foodstuffs, causing reproductive disorders in animals, due to its xenoestrogenic effects. The main reduced metabolites of ZEN are α-zearalenol (α-ZEL) and β-zearalenol (β-ZEL). Similarly to ZEN, ZELs can also activate estrogen receptors; moreover, α-ZEL is the most potent endocrine disruptor among these three compounds. Serum albumin is the most abundant plasma protein in the circulation; it affects the tissue distribution and elimination of several drugs and xenobiotics. Although ZEN binds to albumin with high affinity, albumin-binding of α-ZEL and β-ZEL has not been investigated. In this study, the complex formation of ZEN, α-ZEL, and β-ZEL with human (HSA), bovine (BSA), porcine (PSA), and rat serum albumins (RSA) was investigated by fluorescence spectroscopy, affinity chromatography, thermodynamic studies, and molecular modeling. Our main observations are as follows: (1) ZEN binds with higher affinity to albumins than α-ZEL and β-ZEL. (2) The low binding affinity of β-ZEL toward albumin may result from its different binding position or binding site. (3) The binding constants of the mycotoxin-albumin complexes significantly vary with the species. (4) From the thermodynamic point of view, the formation of ZEN-HSA and ZEN-RSA complexes are similar, while the formation of ZEN-BSA and ZEN-PSA complexes are markedly different. These results suggest that the toxicological relevance of ZEN-albumin and ZEL-albumin interactions may also be species-dependent.

Investigation of Non-Covalent Interactions of Aflatoxins (B1, B2, G1, G2, and M1) with Serum Albumin

Aflatoxins are widely spread mycotoxins produced mainly by Aspergillus species. Consumption of aflatoxin-contaminated foods and drinks causes serious health risks for people worldwide. It is well-known that the reactive epoxide metabolite of aflatoxin B1 (AFB1) forms covalent adducts with serum albumin. However, non-covalent interactions of aflatoxins with human serum albumin (HSA) are poorly characterized. Thus, in this study the complex formation of aflatoxins was examined with HSA applying spectroscopic and molecular modelling studies. Our results demonstrate that aflatoxins form stable complexes with HSA as reflected by binding constants between 2.1 × 10⁴ and 4.5 × 10⁴ dm³/mol. A binding free energy value of −26.90 kJ mol⁻¹ suggests a spontaneous binding process between AFB1 and HSA at room-temperature, while the positive entropy change of 55.1 JK⁻¹ mol⁻¹ indicates a partial decomposition of the solvation shells of the interacting molecules. Modeling studies and investigations with site markers suggest that Sudlow’s Site I of subdomain IIA is the high affinity binding site of aflatoxins on HSA. Interaction of AFB1 with bovine, porcine, and rat serum albumins was also investigated. Similar stabilities of the examined AFB1-albumin complexes were observed suggesting the low species differences of the albumin-binding of aflatoxins.

Interaction of mycotoxin zearalenone with human serum albumin

Zearalenone (ZEN) is a mycotoxin produced mainly by Fusarium species. Fungal contamination of cereals and plants can result in the formation of ZEN, leading to its presence in different foods, animal feeds, and drinks. Because ZEN is an endocrine disruptor, it causes reproductive disorders in farm animals and hyperoestrogenic syndromes in humans. Despite toxicokinetic properties of ZEN were studied in more species, we have no information regarding the interaction of ZEN with serum albumin. Since albumin commonly plays an important role in the toxicokinetics of different toxins, interaction of ZEN with albumin has of high biological importance. Therefore the interaction of ZEN with human serum albumin (HSA) was investigated using spectroscopic methods, ultrafiltration, and molecular modeling studies. Fluorescence spectroscopic studies demonstrate that ZEN forms complex with HSA. Binding constant (K) of ZEN-HSA complex was quantified with fluorescence quenching technique. The determined binding constant (logK=5.1) reflects the strong interaction of ZEN with albumin suggesting the potential biological importance of ZEN-HSA complex formation. Based on the results of the investigations with site markers as well as docking studies, ZEN occupies a non-conventional binding site on HSA. Considering the above listed observations, we should keep in mind this interaction if we would like to precisely understand the toxicokinetic behavior of ZEN.

Interaction of quercetin and its metabolites with warfarin: Displacement of warfarin from serum albumin and inhibition of CYP2C9 enzyme

Flavonoids are ubiquitous molecules in nature with manifold pharmacological effects. Flavonoids interact with several proteins, and thus potentially interfere with the pharmacokinetics of various drugs. Though much is known about the protein binding characteristics of flavonoid aglycones, the behaviour of their metabolites, which are extensively formed in the human body has received little attention. In this study, the interactions of the flavonoid aglycone quercetin and its main metabolites with the albumin binding of the oral anticoagulant warfarin were investigated by fluorescence spectroscopy and ultrafiltration. Furthermore, the inhibitory effects of these flavonoids on CYP2C9 enzyme were tested because the metabolic elimination of warfarin is catalysed principally by this enzyme. Herein, we demonstrate that each tested flavonoid metabolite can bind to human serum albumin (HSA) with high affinity, some with similar or even higher affinity than quercetin itself. Quercetin metabolites are able to strongly displace warfarin from HSA suggesting that high quercetin doses can strongly interfere with warfarin therapy. On the other hand, tested flavonoids showed no or weaker inhibition of CYP2C9 compared to warfarin, making it very unlikely that quercetin or its metabolites can significantly inhibit the CYP2C9-mediated inactivation of warfarin.