Protein-ligand interactions: interaction of nitrosamines with nicotinic acetylcholine receptor

Determination of the binding mechanism of cobalt(II) meso-tetraphenyl porphyrin with plant-esterase

Plant-esterase (EC 3.1.1.X) has received much attention because plant esterase and acetylcholinesterase (AChE) share a similar sensitivity towards organophosphorus (OP) pesticides detection with the same inhibition mechanism. To improve the analytical performance, tetraphenyl metal porphyrin, as an indicator was introduced to combine with plant-esterase. The time of reach equilibrium in PBS solution was shortened after adding plant-esterase by assaying the intensify change of the porphyrin spectrum. Meanwhile, intensify of porphyrin spectrum with plant-esterase was increased compared with that of only the porphyrin spectrum in solution. Tetraphenyl metal porphyrin, such as cobalt(II) meso-tetraphenyl porphyrin, is a mixed reversible inhibitor of plant-esterase from kinetic parameters. The combination ratio of plant-esterase and porphyrin is 2:1. On the other hand, the interaction between CoTPPCl and plant-esterase is the strongest among all tested tetraphenyl metal porphyrin. And the mixed system (CoTPPCl-plant-esterase) showed the best sensitivity towards the tested pesticide. All these results indicated that a complex system composed of tetraphenyl metal porphyrin and plant-esterase was fit for detecting pesticides. They make meaningful guidance on the further design of sensing material in monitoring pesticides.

Papaverine increases human serum albumin glycation

Glycation is a non-enzymatic reaction that is initiated by the primary addition of sugars to amino groups of proteins. In the early phase of glycation, the synthesis of intermediates leads to formation of Amadori compounds. In the last phase, advanced glycation end products (AGE) are irreversibly formed following a complex cascade of reactions. It has recently been shown that glycation also affects diabetes-related complications and Alzheimer's disease. In this study, human serum albumin at a concentration of 10 mg/ml was incubated in PBS with 40 mM of glucose and in different concentrations of papaverine (25, 100, 250, 500 μM) for 42 days at 37 °C. HSA with no additives as well as with glucose 40 mM were incubated as a control and as a glycated sample, respectively. Following the incubation, the samples were prepared for circular dichroism, fluorescence and absorbance techniques. The results showed that in presence of papaverine and glucose, the glycation of HSA increased notably compared with the glycated sample. In conclusion, in this work, we showed that papaverine affects HSA and increases its glycation level.

Haematoporphyrin enhances the peroxidase activity of haemoglobin

The effect of haematoporphyrin, a component of some of the widely used anticancer drugs, on the peroxidase activity of haemoglobin has been studied. Haematoporphyrin increases the haemoglobin-catalysed hydrogen peroxide-mediated oxidation of o-dianisidine or NADH. Spectrophotometric study reveals that an interaction occurs between haemoglobin and haematoporphyrin which leads to a conformational change of the protein. The extent of enhanced peroxidase activity as well as conformational change of the protein vary in a positive manner with the stoichiometric ratio of haematoporphyrin/haemoglobin. An increase in the peroxidase activity of haemoglobin was also observed in the presence of superoxide dismutase, which catalysed the removal of superoxide anion generated during autoxidation of haemoglobin. Possible mechanisms underlying the relation between the conformational change of haemoglobin due to its interaction with haematoporphyrin and the enhanced peroxidase activity are discussed.

Spectroscopic behavior of cationic metallophthalocyanines in the presence of anionic quantum dots

The interactions and spectroscopic properties between cationic zinc phthalocyanine derivatives (peripherally and non-peripherally tetrasubstituted and peripherally octa substituted with 2-diethylmethylaminoethylsulfanyl (betaTZnPc, alphaTZnPc and betaOZnPc)) and CdTe core quantum dots (QDs) capped with mercaptopropionic acid or thioglycolic acid (represented as CdTe@MPA and CdTe@TGA, respectively) have been studied in methanol:water mixture. Strong coupling of MPcs was deduced from the interaction since the UV-vis spectroscopic studies of the ground state complex formed on mixing both components showed loss of the phthalocyanine monomeric band with the formation of a dimeric band (spectrum of aggregated species). The dimerization constants were of the order of 10(4)M(-1).

Alprazolam induced conformational change in hemoglobin

Alprazolam (ALP) is a widely prescribed sedative and antidepressant benzodiazepine group of drugs. The wide uses of this drug lead us to investigate its possible interaction with hemoglobin (Hb). Spectrophotometric and spectofluorimetric studies showed strong binding of ALP with Hb. Circular dichroic spectra showed that alpha-helical structure of Hb-subunits has been largely changed. On ALP treatment partial pressure of O(2) is increased in the blood indicating release of O(2) from erythrocytes. Further, the binding of ALP-induced conformational changes in Hb resulting in larger Hb particle size was demonstrated by dynamic light scattering experiment. Thus, the present study unambiguously raises question of danger of random usage of ALP, which binds with and changes the function of Hb.

Effect of Glycation of Hemoglobin on its Interaction with Trifluoperazine

Trifluoperazine (TFZ), a phenothiazine drug, penetrates into human erythrocytes and releases oxygen by interaction with hemoglobin. TFZ-induced oxygen release from hyperglycemic erythrocytes isolated from diabetic patients is considerably less compared to that from the cells of normoglycemic individuals. In diabetes mellitus, hemoglobin is significantly glycated by glucose. Non-glycated hemoglobin, HbA0 and its major glycated analog, HbA1c have been separated from the blood samples of diabetic patients. TFZ releases considerable amount of oxygen from HbA0, but very little from HbA1c. Spectrofluorimetric studies reveal that TFZ forms excited state complexes with both HbA0 and HbAlc. Titration of HbA0 with TFZ in a spectrophotometric study exhibits two isosbestic points. Similar experiment with HbAlc causes gradual loss of the Soret peak without appearance of any isosbestic point indicating a possibility of heme loss during interaction, which is also supported by gel filtration experiment and SDS-PAGE experiment followed by heme staining. The results suggest that drug action on hemoglobin is influenced by glycation-induced structural modification of the protein.

Binding of porphyrin to horseradish peroxidase: effects on structure and function

Spectrofluorimetric and spectrophotometric studies were done to understand the binding of hematoporphyrin, a photosensitizer to horseradish peroxidase (EC1.11.1.7). The binding affinity constant (K) decreases as the state of aggregation of the porphyrin increases, while the number of binding sites (approximately 1) remains unchanged. The interaction appears to be mostly hydrophobic, entropy-driven and endothermic process. Hematoporphyrin potentiates horseradish peroxidase-catalyzed H2O2-mediated NADH oxidation, probably by porphyrin-influenced removal of superoxide radicals, which are generated in the system. Conformational change of the protein due to its interaction with porphyrin may be associated with potentiation of the catalytic activity of the enzyme.

Fluorescent assay for riboflavin binding to cytochrome P450 2B4

The interactions between the hemoprotein cytochrome P450 2B4 (CYP 2B4) and riboflavin - a low molecular weight component of the flavoprotein NADPH-dependent cytochrome P450 reductase - were investigated by fluorescence spectroscopy. Riboflavin fluorescence quenching by cytochrome P450 2B4 was used to probe the ligand-enzyme binding (lambda(ex)=385 nm, lambda(em)=520 nm). Fluorescence titration experiments showed formation of a complex between cytochrome P450 2B4 and riboflavin with an apparent dissociation constant value, K(d)=8.8+/-1 microM. The fluorescence intensity of riboflavin was decreased with increasing the cytochrome P450 2B4 concentration, indicating the transfer of resonance excitation energy from riboflavin (energy donor) to the cytochrome P450 2B4 heme (energy acceptor). The data obtained are suggestive of the existence of riboflavin binding site(s) on the hemeprotein molecule.

Protoporphyrin IX-induced structural and functional changes in human red blood cells, haemoglobin and myoglobin

Protoporphyrin IX and its derivatives are used as photosensitizers in the photodynamic therapy of cancer. Protoporphyrin IX penetrates into human red blood cells and releases oxygen from them. This leads to a change in the morphology of the cells. Spectrophotometric studies reveal that protoporphyrin IX interacts with haemoglobin and myoglobin forming ground state complexes. For both proteins, the binding affinity constant decreases, while the possible number of binding sites increases, as the aggregation state of the porphyrin is increased. The interactions lead to conformational changes of both haemoglobin and myoglobin as observed in circular dichroism studies. Upon binding with the proteins, protoporphyrin IX releases the heme-bound oxygen from the oxyproteins, which is dependent on the stoichiometric ratios of the porphyrin : protein. The peroxidase activities of haemoglobin and myoglobin are potentiated by the protein-porphyrin complexation. Possible mechanisms underlying the relation between the porphyrin-induced structural modifications of the heme proteins and alterations in their functional properties have been discussed. The findings may have a role in establishing efficacy of therapeutic uses of porphyrins as well as in elucidating their mechanisms of action as therapeutic agents.

Interactions of chlorpromazine with alpha-, beta- and gamma-crystallins

The binding parameters (binding affinity constant, K and number of binding sites, p) has been determined spectrofluorometrically for chlorpromazine (CPZ) binding to the lens proteins--alphaL-crystallin, betaL-crystallin and gamma-crystallin. The binding affinity constants for CPZ binding to alphaL- and gamma-crystallins are higher than the binding affinity constants for 3betaL-crystallin, although the number of CPZ binding sites for betaL-crystallin is comparatively higher than the number for the other two lens proteins. CPZ causes local conformational changes around the tryptophan moieties of the protein molecules but does not cause any gross conformational change within the protein moieties. Binding of CPZ to alphaL-crystallin does not significantly alter the anti-aggregation properties of the molecular chaperone, alphaL-crystallin against oxidation-induced aggregation of gamma-crystallin at 37 degrees C and thermal aggregation of alcohol dehydrogenase (ADH) at 48 degrees C. Therefore, CPZ induced alteration in chaperone activity of alphaL-crystallin is probably not associated with the formation of cataracts.

Hematoporphyrin interacts with myoglobin and alters its functions

The binding parameters of hematoporphyrin, a photosensitizing drug used in photodynamic therapy, interacting with myoglobin, an oxygen storage protein, have been studied spectrofluorometrically and spectrophotometrically. Two concentration ranges of hematoporphyrin, representing significantly monomeric and aggregated (dimeric) states have been used. The binding affinity constant (K) decreases and the possible number of binding sites (p) increases as the porphyrin changes from significantly monomeric state to predominantly dimeric state. Titration of the protein with hematoporphyrin in a spectrophotometric study (differential spectroscopy) exhibits an isosbestic point indicating a ground state complex formation. The interaction leads to a conformational change of the protein as observed in a circular dichroism study. The hematoporphyrin-myoglobin interaction causes oxygen release from the protein and it varies with the stoichiometric ratio of the porphyrin:protein. Hematoporphyrin also increases the myoglobin-catalysed hydrogen peroxide-mediated oxidation of o-dianisidine and NADH. These findings on the effects of hematoporphrin-myoglobin interaction should be given due consideration in therapeutic uses of the porphyrin and its derivatives.

Interactions of chlorpromazine with milk proteins

The mode and nature of the binding of chlorpromazine (CPZ), a psychotropic drug, with milk proteins--alpha-lactalbumin (with substantial amounts of alpha-helix, beta-sheet and random coil), alpha-lactoglobulin (a major beta-sheeted protein) and alphas-casein (a random coiled protein) have been studied spectrofluorometrically and spectropolarimetrically. The binding affinity of CPZ for unfolded proteins is comparatively less than that of folded proteins although the number of binding sites is smaller in the latter case, due to the greater extent of binding of CPZ for folded proteins. Thermodynamic analysis reveals that CPZ binds to alpha-lactalbumin and alphas-casein in an endothermic (deltaH degrees is positive) and hydrophobic manner but with beta-lactoglobulin in an exothermic (deltaH degrees is negative) manner. Far UV Circular dichroic studies reveal that CPZ increases the secondary structure of the major beta-sheeted protein, beta-lactoglobulin possibly by increasing the relative contact orders (non-local contacts) within the residues. On the other hand, for proteins possessing random coil, it increases the unfolded state of the protein. CPZ does not affect local contacts in alpha-helix when its interaction is compared with a major alpha-helical protein, myoglobin.

Deferiprone (L1) induced conformation change of hemoglobin: A fluorescence and CD spectroscopic study

The interaction of deferiprone (1,2-dimethyl-3-hydroxy-pyrid-4-one) L1, the first clinically available oral iron chelator, with the tetrameric allosteric protein hemoglobin from human red blood cells has been investigated spectrofluorometrically and by circular dichroism spectroscopy. The interaction is hydrogenbond like electrostatic in nature, the binding constant being 4.54 x 10(3) M(-1) in 0.15 M NaCl. Circular dichroism studies indicate a conformational change of hemoglobin in presence of deferiprone, helicity of hemoglobin being reduced in presence of increasing concentration of the drug L1.

Structural organisations of hemoglobin and myoglobin influence their binding behaviour with phenothiazines

Binding modalities of chlorpromazine and trifluoperazine, two widely used antipsychotic phenothiazine drugs with hemoglobin and myoglobin have been studied to understand how the quaternary, tertiary and secondary structural organisations of the proteins regulate the binding process. NaCl-induced alteration in the quaternary structure of hemoglobin influences its binding modality with phenothiazines. Minor alterations in the tertiary structure of thermally denatured myoglobin (denaturation temperature ranging between 30-70 degrees C) do not affect its affinity and the modality of binding with the drugs, but alterations in the secondary structure of the protein denatured at temperatures between 70-80 degrees C influence its binding.

Studies on the interaction of hematoporphyrin with hemoglobin

Spectrophotometric and spectrofluorimetric studies reveal that an interaction occurs between hemoglobin and hematoporphyrin, a photosensitizing drug used in photodynamic therapy. Two concentration ranges of hematoporphyrin, 0.4-0.9 microM and 1.8-3.6 microM, representing significantly monomeric and aggregated (dimeric) state, respectively, have been used in the binding studies. The binding affinity constant (K) decreases, while the possible number of binding sites (p) increases as the concentration range of the porphyrin is increased. The nature of interaction has been studied by fluorescence quenching titration method under different ionic strengths and temperature conditions. It appears to be predominantly electrostatic and enthalpy-driven in the lower range of porphyrin concentration. However, the interaction follows mostly hydrophobic and entropy-driven modality in the higher concentration range of the ligand. The porphyrin-hemoglobin interaction results in release of oxygen from the protein. The extent of oxygen release depends on the stoichiometric ratio of hematoporphyrin:hemoglobin.

Interaction of chlorpromazine with myoglobin and hemoglobin. A comparative study

The mode and nature of the binding of chlorpromazine (CPZ), a psychotropic drug, with myoglobin, a monomeric muscle protein, were studied spectrofluorometrically and the results compared with those from the binding of CPZ to hemoglobin, a tetrameric allosteric protein from red blood cells (RBC). CPZ interacted with myoglobin in a non-cooperative mode, with a binding constant of 8.4 x 10(3) M-1 in 0.145 M NaCl, pH 6.8, whereas in the case of hemoglobin this interaction was found to be positively cooperative with a binding constant of 4.2 x 10(3) M-1. The interaction of CPZ with myoglobin was not influenced by the NaCl molarity of the solution, whereas CPZ interaction with hemoglobin significantly decreased with increasing NaCl molarity, indicating that CPZ-hemoglobin binding is mostly electrostatic in nature, whereas that of the CPZ-myoglobin complex is of a non-electrostatic type. Thermodynamic analysis revealed that binding of CPZ to hemoglobin was exothermic (delta H degrees = -2.65 kcal/mol), whereas binding to myoglobin was endothermic (delta H degrees = + 1.39 kcal/mol) with a high entropic contribution (delta S degrees = +23 cal/degree/mol), suggesting that CPZ binding to myoglobin is hydrophobic in nature. Such contrasting binding features of this drug have been discussed in the light of a typical subunit interaction property present and absent in hemoglobin and myoglobin, respectively.