Oxygen quenching of sensitized terbium luminescence in complexes of terbium with small organic ligands and proteins

Water solubility differentiates the impact of tea polyphenols and rutin on the postprandial glycemic response to cooked maize starch

Postprandial hyperglycemia is an independent risk factor for cardiovascular diseases, and the impact of tea polyphenols (TP) and rutin, representative phenolic compounds with different water solubilities, on the postprandial glycemic response to cooked normal corn starch (CCS) was investigated. Comparatively, TP (DPPH50 = 0.12 mmol L-1) are more potent than rutin (DPPH50 = 0.50 mmol L-1) in scavenging the free radicals of DPPH, but both TP and rutin inhibited the activity of porcine pancreatic α-amylase (PPA), the major enzyme in starch digestion, with an IC50 of 4.09 mmol L-1 and 2.71 mmol L-1, respectively. However, an in vivo study showed that a significant reduction in postprandial blood glucose was only observed in the presence of rutin, and TP had no effect on the glycemic response to CCS. To find out the underlying mechanism, fluorescence spectroscopy and molecular docking were carried out and they showed that, compared to TP, rutin bound to the active site of PPA with higher affinity and a lower free energy (ΔG) driven by hydrogen bonds and π-stacking, and rutin also greatly increased the viscosity of starch. Collectively, water-soluble TP have a higher antioxidant property and a lower potency to inhibit PPA compared to water-insoluble rutin, and the weaker interaction between TP and PPA, and starch as well might synergistically contribute to TP's ineffectiveness in lowering the postprandial glycemic response, and water solubility linking the molecular structures and functions of phenolic compounds might be the fundamental basis for the observed difference in their biological functions, and water solubility can also be used to enrich specific phenolic compounds for desired functions.

Inhibition of starch digestion by phenolic acids with a cinnamic acid backbone: Structural requirements for the inhibition of α-amylase and α-glucosidase

This study investigated the inhibition mechanism of cinnamic acid-based phenolic acids (cinnamic acid: CIA, 3,4-dimethoxy cinnamic acid: 3,4-mCIA, caffeic acid: CA, ferulic acid: FA) on starch digestion. CA, FA, and 3,4-mCIA contributed to reducing the rapidly digested starch content and increasing the resistant starch content. The enzyme activity inhibition results responded that the four phenolic acids inhibited α-amylase activity better than α-glucosidase. The order of IC50 against α-amylase and α-glucosidase was CA > FA > 3,4-mCIA > CIA. Phenolic acid's benzene ring formed conjugated Pi-systems with the amino acid residues of α-amylase. Salt-bridge interactions were the main driving forces for the binding of phenolic acids to α-glucosidase. The binding was stabilized by the hydroxyl (OH) group and the methoxy on the benzene ring, where the OH exerted a better effect. These results illuminate the inhibition mechanism of starch digestion with cinnamic acid-based phenolic acids from an interaction perspective.

Physicochemical characterizations, α-amylase inhibitory activities and inhibitory mechanisms of five bacterial exopolysaccharides

Inhibiting pancreatic α-amylase activity can decrease the release rate of glucose, thereby delaying postprandial blood glucose. This study aimed to investigate the physicochemical properties and porcine pancreatic α-amylase (PPA) inhibitory activities of five bacterial exopolysaccharides (EPSs). We also aimed to analyze the differences of their inhibitory activities, exploring the inhibition mechanism between EPSs and PPA. Five EPSs had a low molecular weight (55-66 kDa), which were mainly composed of mannose and glucose with total content exceeding 86 %. The IC50 values of five EPSs (0.162-0.431 mg/mL) were significantly lower than that of acarbose (0.763 mg/mL), indicating that the inhibitory effects of five EPSs on PPA were stronger than acarbose, especially the EPS from Bacillus subtilis STB22 (BS-EPS). Moreover, BS-EPS was a mixed-type inhibitor, whereas other EPSs were noncompetitive inhibitors of PPA. Five EPSs quenched the fluorophore of PPA by the mixed quenching or apparent static quenching. Interestingly, BS-EPS showed stronger binding affinity to PPA than other EPSs. It can be speculated that EPSs with low molecular weight, high carboxylic acid content, and α-glycosidic bond exhibited high PPA inhibitory activity. These results suggest that BS-EPS can effectively inhibit PPA activity and has potential applications in reducing postprandial hyperglycemia.

Enzymic catalyzing affinity to substrate affects inhibitor-enzyme binding interactions: Inhibition behaviors of EGCG against starch digestion by individual and co-existing α-amylase and amyloglucosidase

The inhibition of (-)-epigallocatechin-gallate (EGCG) against starch digestion by α-amylase (AA), amyloglucosidase (AMG) and co-existing enzymes (AA/AMG) were comparatively studied. EGCG inhibited AA only at slowly-digestible-starch (SDS) phase. This resulted from high catalytic efficiency of AA for rapidly-digestible-starch (RDS), counteracting the inhibition at this phase. EGCG inhibited AMG and AA/AMG during whole process. At RDS phase, the catalytic velocity of AMG was always higher than AA/AMG because of an antagonistic effect of two enzymes. However, at SDS phase with EGCG, the catalytic velocity of AA/AMG was higher than AMG. This is because binding of EGCG with both enzymes caused more unbound AMG that generated more glucose in co-existing AA/AMG than AMG. Although EGCG-AA binding affinity was higher than EGCG-AMG, competitive inhibition of EGCG against AA was weaker than AMG, indicating relatively higher binding/catalyzing affinity of AA to starch significantly weakened EGCG-AA binding due to competitive relationship between starch and EGCG.

Number of galloyl moieties and molecular flexibility are both important in alpha-amylase inhibition by galloyl-based polyphenols

The inhibition of porcine pancreatic α-amylase (PPA) by 9 galloyl-based polyphenols was evaluatedviainitial digestion velocity, IC₅₀, inhibition kinetics, fluorescence quenching and molecular docking studies.

Research on the Influences of Five Food-Borne Polyphenols on In Vitro Slow Starch Digestion and the Mechanism of Action

Inhibiting starch digestion can effectively control postprandial blood sugar level. In this study, the in vitro digestion differences among the mixtures of five polyphenols (i.e., procyanidins [PAs], catechin [CA], tannic acid [TA], rutin [RU], and quercetin [QU]) and starch were analyzed through an in vitro simulation test of starch digestion. The interaction characteristics of these five polyphenols with α-amylase and α-glucosidase were investigated in terms of the inhibition effect, dynamics, fluorescence quenching, and circular dichroism (CD). The results revealed that the rapidly digestible starch (RDS) contents decreased, while the resistant starch (RS) contents increased. All five polyphenols inhibited the α-amylase activity through the noncompetitive approach but inhibited the α-glucosidase activity through the competitive approach. Five polyphenols combined with α-amylase spontaneously by using the hydrophobic effect. The interaction of PAs and QU with α-glucosidase were recognized as van der Waals forces and H bonding, whereas CA and TA interacted with α-glucosidase through the hydrophobic effect. All five polyphenols can cause conformational changes in enzymes.

Inhibition of α-amylase by flavonoids: Structure activity relationship (SAR)

Flavonoids are recognized to regulate animals' food digestion processes trough interaction with digestive enzymes. The binding capacity of hesperetin (HES), luteolin (LUT), quercetin (QUE), catechin (CAT) and rutin (RUT) with pancreatic α-amylase were evaluated, using UV-Vis spectroscopy, fluorescence and molecular docking. Using p-nitrophenyl-α-d-maltopentoside (pNPG5) as substrate analog, LUT showed the best inhibitory capacity, even better than that of the positive control, acarbose (ACA). A mixed-type inhibition was observed for HES, LUT and QUE, a competitive-type for ACA, while no inhibition was observed with CAT and RUT. In agreement with kinetic results, α-amylase presented a higher affinity for LUT, when analyzed by fluorescence quenching. The binding of flavonoids to amylase followed a static mechanism, where the binding of one flavonoid per enzyme molecule was observed. Docking analysis showed that flavonoids bound near to enzyme active site, while ACA bound in another site behind the catalytic triad. Extrinsic fluorescence analysis, together with docking analysis pointed out that hydrophobic interactions regulated the flavonoid-α-amylase interactions. The present study provides evidence to understand the relationship of flavonoids structure with their inhibition mechanism.

The mechanism of interactions between tea polyphenols and porcine pancreatic alpha-amylase: Analysis by inhibition kinetics, fluorescence quenching, differential scanning calorimetry and isothermal titration calorimetry

SCOPE

This study aims to use a combination of biochemical and biophysical methods to derive greater mechanistic understanding of the interactions between tea polyphenols and porcine pancreatic α-amylase (PPA).

METHODS AND RESULTS

The interaction mechanism was studied through fluorescence quenching (FQ), differential scanning calorimetry (DSC) and isothermal titration calorimetry (ITC) and compared with inhibition kinetics. The results showed that a higher quenching effect of polyphenols corresponded to a stronger inhibitory activity against PPA. The red-shift of maximum emission wavelength of PPA bound with some polyphenols indicated a potential structural unfolding of PPA. This was also suggested by the decreased thermostability of PPA with these polyphenols in DSC thermograms. Through thermodynamic binding analysis of ITC and inhibition kinetics, the equilibrium of competitive inhibition was shown to result from the binding of particularly galloylated polyphenols with specific sites on PPA. There were positive linear correlations between the reciprocal of competitive inhibition constant (1/Kic ), quenching constant (KFQ ) and binding constant (Kitc ).

CONCLUSION

The combination of inhibition kinetics, FQ, DSC and ITC can reasonably characterize the interactions between tea polyphenols and PPA. The galloyl moiety is an important group in catechins and theaflavins in terms of binding with and inhibiting the activity of PPA.

Pyridine-2,6-dicarboxylic acid for the sensitization of europium(iii) luminescence with very long lifetimes

Convenient and mild syntheses of two europium(iii) complexes, Na[Eu(dipic)₂·3H₂O]·4H₂O (dipic = dipicolinate anions) and Na[Eu(dipic)₂·phen]·H₂O (phen = 1,10-phenanthroline), were reported.

Structure Activity Relationships of Flavonoids as Potent α-Amylase Inhibitors

The effects of three flavonoids (quercetin, luteolin, diosmetin) on α-amylase were examined by enzymatic kinetics and fluorescence spectroscopy. The three test flavonoids were non-competitive inhibitors of the enzyme. Addition of flavonoids led to fluorescence quenching of α-amylase. The quenching was initiated from the formation of a complex between the flavonoids and the enzyme, corresponding to a static quenching process. An α-amylase molecule provides a binding site for the test flavonoid. The main binding force was hydrophobic. The decreasing order of inhibition of α-amylase by flavonoids and the binding force was luteolin, diosmetin, and quercetin. It is demonstrated that hydroxylation in ring C and methylation of the hydroxyl group in ring B of flavonoids may weaken the binding affinities to α-amylase.

Interaction of phenolic compounds with bovine serum albumin (BSA) and α-amylase and their relationship to astringency perception

The ability of grape seed extracts to bind to bovine serum albumin (BSA) and α-amylase was studied by fluorescence quenching of protein intrinsic fluorescence and nephelometry. The influence of grape seed ripeness on astringency was also evaluated. From the spectra obtained, the modified Sterm-Volmer (K(app)) and the bimolecular quenching constants were calculated. Results showed that grape seed extracts had good affinity for proteins. The association strength of tannin-protein interactions varied with changes in tannin structure associated with the degree of ripeness affecting the binding/quenching process. In all cases studied, higher values of K(app) were obtained in samples at harvest which have greater ability to bind to proteins than have samples at post-veraison time. Nephelometric assays show the same trend as do fluorescence quenching studies. A possible explanation for this is that, as seeds ripen, their tannins increase in molecular mass, which relates to an increase in hydrophobicity of the molecules, and this increases protein affinity. However, that is contrary to the reported decrease in astringency of grape seeds during maturity. This indicates that tannin-protein interactions are not the only explanation for the complex sensations of astringency of grape seeds.

Release of NO from a nitrosyl ruthenium complex through oxidation of mitochondrial NADH and effects on mitochondria

Nitrosyl ruthenium complexes are promising NO donor agents with numerous advantages for the biologic applications of NO. We have characterized the NO release from the nitrosyl ruthenium complex [Ru(NO(2))(bpy)(2)(4-pic)](+) (I) and the reactive oxygen/nitrogen species (ROS/RNS)-mediated NO actions on isolated rat liver mitochondria. The results indicated that oxidation of mitochondrial NADH promotes NO release from (I) in a manner mediated by NO(2) formation (at neutral pH) as in mammalian cells, followed by an oxygen atom transfer mechanism (OAT). The NO released from (I) uncoupled mitochondria at low concentrations/incubation times and inhibited the respiratory chain at high concentrations/incubation times. In the presence of ROS generated by mitochondria NO gave rise to peroxynitrite, which, in turn, inhibited the respiratory chain and oxidized membrane protein-thiols to elicit a Ca(2+)-independent mitochondrial permeability transition; this process was only partially inhibited by cyclosporine-A, almost fully inhibited by the thiol reagent N-ethylmaleimide (NEM) and fully inhibited by the NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO). These actions correlated with the release of cytochrome c from isolated mitochondria as detected by Western blotting analysis. These events, typically involved in cell necrosis and/or apoptosis denote a potential specific action of (I) and analogs against tumor cells via mitochondria-mediated processes.

Understanding the binding of procyanidins to pancreatic elastase by experimental and computational methods

Human diets are rich in secondary metabolites such as polyphenols. These compounds perform a wide range of crucial functions in biological systems and are of great interest for the pharmaceutical and food industries. In this work, the binding mode of the natural polyphenolic compounds from grape seed on the porcine pancreatic elastase surface was studied by experimental and computational methods. Fluorescence quenching, circular dichroism, nephelometry, dynamic light scattering (DLS), molecular docking, and molecular dynamics simulation studies were performed. A decrease in fluorescence intensities was observed with addition of increasing polyphenol concentrations. The order of binding ability obtained was oligomeric fraction of procyanidins (OFP) > tetramer > trimer > dimer B3 procyanidins. Thus a relationship between higher molecular weight and binding ability was observed. The interaction between these molecules and the enzyme occurs by a static mechanism, as inferred from the high apparent fluorescence and bimolecular quenching constants. A blue shift in the maximal emission wavelength could be seen, which indicates that the tryptophan residues acquire a more hydrophobic character upon procyanidin binding. Molecular docking and dynamics simulations also demonstrate that the SASA (solvent-accessible surface area) values of tryptophans decrease with the binding of these compounds, preventing the accessibility of water molecules, which agrees with the referred blue shift. Circular dichroism studies indicate a decrease in alpha-helix content, followed by an increase in the beta-sheet component of secondary structures of this enzyme. DLS and nephelometry techniques also indicate a relationship between large procyanidins and aggregate formation ability.

Direct kinetic evidence for triplet state energy transfer from Escherichia coli alkaline phosphatase tryptophan 109 to bound terbium

The addition of excess Tb3+ to metal-depleted Escherichia coli alkaline phosphatase results in enhanced luminescence from enzyme-bound terbium, which increases with sample deoxygenation and exhibits a tryptophan-like excitation spectrum. Following pulsed excitation at 280 nm, the time-resolved terbium emission shows a negative prefactor associated with a submillisecond rise time, which is independent of the concentration of dissolved oxygen. The absence of a build-up phase and similarity in lifetime in the decay kinetics of directly excited (488 nm) terbium allows for the assignment of the submillisecond component in the 280 nm excited sample to bound terbium. The results of the steady state and time-resolved experiments suggest that the time evolution of alkaline phosphatase-bound terbium emission is determined by energy transfer (kET approximately 360 and 120 s-1) from the triplet state of tryptophan to terbium followed by terbium decay. This model is based on the observations that 1) the tryptophan phosphorescence lifetime (previously assigned to Trp109) corresponds to the longer component of the terbium emission and 2) the long-lived emission is enhanced, as is the Trp109 phosphorescence, by deoxygenation. An energy transfer mechanism involving the Trp109 triplet state is shown to be inconsistent with a dipole-dipole process and is best understood as a through-space electron exchange over a donor-acceptor distance of 9-10 A.

On the mechanism of energy transfer to Tb3+ ions in proteins. A time-resolved luminescence study of the Tb-elastase complex

Intraprotein energy transfer to terbium ions is widely used for probing distances of calcium sites in proteins. In this work we have performed a time-resolved study of the sensitized luminescence in elastase using a pulsed laser excitation at 265 nm. Terbium-sensitized luminescence was found to build-up within about 150 microseconds, which indicates that the protein transfers energy at a rate several orders of magnitude slower than expected for a singlet state donor. From the rise time of the signal and from its variation with the oxygen concentration, it can be deduced that 80% of the transfer originates from the first triplet excited state of one unique aromatic residue. From the comparison of protein fluorescence and sensitized terbium luminescence excitation spectra the sensitizer was identified as a tryptophan, presumably Trp141, which is situated only 0.7-0.9 nm away from the Tb site. The results are at variance with the usual assumption that energy is transferred from the first excited singlet state of aromatic residues according to a long-range dipole-dipole interaction and are more consistent with a short-distance exchange mechanism.

Ion-induced DNA structure change in nucleosomes

Physical methods have been used to study calcium binding to the nucleosome core particle. Equilibrium dialysis of Ca2+ and spectroscopic analysis of a Ca2+ analogue show that the ion binds tightly to the particles, resulting in a significant change of DNA circular dichroism. This suggests that base stacking may be altered as a result of Ca2+ binding. In the presence of Ca2+, the absorbance and fluorescence properties of methylene blue (MB), a DNA-specific intercalator, confirm that the dye binds tightly to nucleosomes by intercalation. However, secondary changes occur which suggest that the MB binding site is altered as a result of Ca2+ binding. Triplet state anisotropy decay and triplet lifetime quenching both show that in the Ca2+-nucleosome complex, methylene blue is capable of wobbling over a substantial angular range at its binding site. To explain these data, it is proposed that Ca2+ binding to nucleosomes causes DNA to fold by means of a series of sharp bends (kinks). The properties of bound MB are best explained if it is presumed that the intercalator binds tightly to such kinked sites in the nucleosome. On the basis of these observations, we discuss the possibility that multivalent ion concentration in the nucleus is high enough that the smooth to kinked helix equilibrium may be near to its midpoint. Near such a midpoint, the secondary structure of DNA in the nucleosome might prove to be sensitive to effector molecule binding and to site-specific variation of DNA or histone composition within genes.