Continuous method to determine the trypsin inhibitor activity in soybean flour

The determination of trypsin inhibitor (TI) activity is of importance to evaluate the nutritional value of soybean flours. An analytical method, which involves a continuous spectrophotometric rate determination for trypsin activity against the substrate N-benzoyl-DL-arginine p-nitroanilide, is proposed as an alternative to the standard discontinuous assay. Stopping the reaction with acetic acid and a centrifugation/filtration step to decrease turbidity are not required, thus reducing costs and sample preparation time. The TI activity of different flour samples, determined by both assays, demonstrated to be statistically comparable, irrespective of the TI concentration level. The coefficients of variation of the novel method did not exceed 8% at any concentration level. The curves of progress reaction showed a non-linear behavior in samples without TI. A reduction of incubation time from 10min to 2min increased the method sensitivity and extended its linear range. A more economical, faster and simpler assay was developed.

Interaction of tannase from Aspergillus niger with polycations applied to its primary recovery

The interaction of tannase (TAH) with chitosan, polyethyleneimine and Eudragit(®)E100 was studied. It was found that TAH selectively binds to these polycations (PC), probably due to the acid nature of the target protein. TAH could interact with these PC depending on the medium conditions. The effect of the interaction on the secondary and tertiary structure of TAH was assayed through circular dichroism and fluorescence spectroscopy. TAH was recovered from Aspergillus niger culture broth by means of precipitation and adsorption using chitosan.

Extraction of trypsin from bovine pancreas by applying polyethyleneglycol/sodium citrate aqueous two-phase systems

The goal of this work was to determine the optimal conditions for separating trypsin (TRP) from alpha-chymotrypsin (ChTRP) and to apply them for trypsin purification from bovine pancreas by liquid-liquid extraction with polyethyleneglycol/sodium citrate (PEG/NaCit) aqueous two-phase systems. Partitioning behaviours of TRP and ChTRP are demonstrated to be very sensitive to variables such as PEG molecular weight, pH and tie line length. Aqueous two-phase systems (ATPSs) formed by PEG of MW 3350 and NaCit pH 5.20 showed the best separation capability. The addition of NaCl up to a final concentration of 7% (w/w) and the decrease of top/bottom volume ratio to 0.1 led to the recovery of 60% of pancreatic TRP in a concentrated form in the top phase with a 3-fold purification. Biomass presence up to 25% (w/w) of the total system mass did not affect significantly yield and purification parameters.

Partition of whey milk proteins in aqueous two-phase systems of polyethylene glycol–phosphate as a starting point to isolate proteins expressed in transgenic milk

Partitioning behaviour of the bovine whey proteins (bovine serum albumin, alpha lactoalbumin and beta lactoglobulin) and alpha-1 antitrypsin in aqueous two-phase systems prepared with polyethyleneglycol (molecular masses: 1000; 1500 and 3350)-potassium phosphate was analysed. Bovine serum albumin and alpha lactoalbumin concentrated in the polyethyleneglycol rich phase with a partition coefficient of 10.0 and 27.0, respectively, while beta lactoglubulin and alpha-1 antitrypsin showed affinity for the phosphate-rich phase with a partition coefficient of 0.07 and 0.01, respectively. An increase of medium pH induced an increase of the partition coefficient of these proteins while the increase in polyethyleneglycol molecular mass induced the opposite behaviour. The system polyethyleneglycol 1500-pH 6.3 showed the best capacity for recovering the alpha-1 antitrypsin with a yield of 80% and a purification factor between 1.5 and 1.8 from an artificial mixture of the milk whey proteins and alpha-1 antitrypsin. The method appears to be suitable as a starting point to isolate proteins expressed in transgenic milk.

Thermodynamic features of the chemical and thermal denaturations of human serum albumin

The unfolding process of human serum albumin between pH 5.4 and 9.9 was studied by chemical and thermal denaturations. The experimental results showed that there is no correlation between the stability of albumin at different pH values determined by both methods. The free energy change of unfolding versus concentration of guanidine showed a close dependence on the pH, suggesting that the variation of the electrical charge of albumin influences the final state of the unfolded form of the protein. Spectroscopic techniques, such as native fluorescence of the protein and circular dichroism, demonstrated that the unfolded state of the protein obtained from both methods possesses a different helical content. The solvophobic effect and the entropy of the chains have no influence on the final unfolding state when the protein is unfolded by thermal treatment, while, when the protein is unfolded by chemical denaturants, both effects depend on the medium pH. The results indicate that guanidine and urea interact with albumin by electrostatic forces, yielding a randomly coiled conformation in its unfolded state, while thermal denaturation produces a molten globule state and the aggregation of the protein; therefore, both methods yield different structurally unfolded states of the albumin.

The binding of 3,6-disubstituted bile salts to human serum albumin induces conformational change on the molecule of this protein

The binding of 3,6-hydroxy and keto disubstituted bile salts to human serum albumin was studied using differential scanning calorimetry, fluorescence spectroscopy and circular dichroism. The bile salts assayed did not produce any modification in the shape of the albumin thermogram, its thermal unfolding process in their presence being reversible; however, an increase in the enthalpy of unfolding and in the Tm was observed in the presence of 3,6-diketo and 3-hydroxy-6-keto bile salts. These two derivatives induced a negative circular dichroism spectrum of the protein around 280-290 nm, quenched the native fluorescence of the buried tryptophan of albumin and induced energy transfer between 1 aniline-8-naphthalene sulfonate and the buried tryptophan 214 of albumin. The presence of a keto group at C6 in the steroid ring of the bile salts plays an important role in producing slight movement of the albumin domains, increasing the distance between domains I and II.

Thermodynamic features of the thermal unfolding of human serum albumin

The unfolding process of human serum albumin (HSA) was studied by thermal effect on the native fluorescence of the protein, thermal inactivation of the hydrolase activity of albumin and differential scanning calorimetry using the high sensitive calorimeter developed by Privalov. The denaturation process can be described by an approximation of the model of Eyring and Lumry: native [symbol: see text] unfolded reversible [symbol: see text] unfolded irreversible. It was found that the rate of irreversible step was very slow (at temperatures below 74 degrees C), allowing the resolution of the denaturation process as a reversible one on the basis of two states approximation. However, the presence of intramolecular cooperation in the thermal denaturation process at temperatures above 74 degrees C cannot be discarded, which might be favoring the aggregation of albumin molecules. The midpoint temperature of unfolding obtained by differential scanning calorimetry was of 63.1 degrees C +/- 0.4 at pH 7.4. This value was independent of the rate of scanning and it is in agreement with those obtained by techniques such as thermal effect on the protein fluorescence and on the hydrolase activity of albumin. The enthalpy of unfolding at pH 7.4 was 88.9 +/- 4 Kcal/mol. This value was low compared with those obtained for other proteins, suggesting the presence of a molten globule in the unfolding pathway of albumin. The neutral-basic conformational change (pH 7.4) of albumin did not modify the thermal stability and the enthalpy of denaturation of the protein. A pH below 4.3 (transition acid-neutral) the presence of a second peak in the thermogram of albumin with a TM of 46.2 degrees C +/- 0.9 would be suggesting a lost of cooperativity between the various domains of albumin in the unfolding.

Contribution of the FAD binding site residue tyrosine 308 to the stability of pea ferredoxin-NADP+ oxidoreductase

The contribution made by tyrosine 308 to the stability of pea ferredoxin-NADP+ reductase was investigated using site-directed mutagenesis. The phenol side chain of the invariant carboxyl terminal tyrosine is stacked coplanar to the isoalloxazine moiety of the FAD cofactor. Fluorescence measurements indicate that this interaction plays a significant role in FAD fluorescent quenching by the reductase apoprotein. Replacement of the tyrosine by tryptophan or phenylalanine caused only a minor increase in the quantum yields of bound FAD, whereas nonaromatic substitutions to serine and glycine resulted in a large fluorescent rise. Results from NADP+ titration experiments support a recent hypothesis [Karplus et al. (1991) Science 251, 60-66], suggesting that the phenol ring of Tyr 308 may fill the nicotinamide binding pocket in the absence of the nucleotide. The stability of the site-directed mutants, judged by thermal- and urea-induced denaturation studies, was lowered with respect to the wild-type enzyme. FNR variants harboring nonaromatic substitutions displayed more extensive destabilization. The decrease in thermodynamic stability correlated with the impairment of catalytic activities [Orellano et al. (1993) J. Biol. Chem 268, 19267-19273]. The results indicate that the presence of the electron-rich aromatic side chain adjacent to the isoalloxazine ring is essential for maximum stabilization of the FNR holoenzyme, resulting in a flavin conformation which optimizes electron flow between the prosthetic group and its redox partners.

Thermodynamic study of taurocholate binding to rat serum albumin

Taurocholate binding to rat serum albumin was studied by equilibrium dialysis. The bile salt-protein interaction was studied under different experimental conditions with respect to temperature; ionic strength; Cl- concentration; pH and the presence of butanol in the medium. The results obtained suggest the existence of two binding sites for taurocholate on the albumin molecule, and indicate that both electrostatic and hydrophobic interaction play a role in the binding process.

Fluorometric studies on the binding of aflatoxin B1 to bovine serum albumin

The quenching of dansylated BSA fluorescence by Aflatoxin B1 at the dansyl emission peak provided a useful method to study Aflatoxin B1 - BSA interaction, making evident one binding site with hydrophobic characteristics. The Ka = 4.0 X 10(4) M-1 at 18 degrees C could assing to this site a role in Aflatoxin B1 transport, but not in storage in the systemic circulation. Evidence supporting the presence of more binding sites, probably of similar characteristics as the former, was obtained from the study of the displacement of ANS bound to BSA by Aflatoxin B1, in spite of the fact that this interaction cannot be explained as a simple competition between ligands.

Interaction of unconjugated bilirubin with brush border vesicles from proximal tubules of rat kidney

An interaction between non conjugated bilirubin and brush border vesicles was found which does not change the absorbance spectrum of bilirubin, nor protects it against the action of peroxidase. It seems to reach saturation at about 20 min., and appears to be mainly a superficial binding, probably non specific and relatively weak. The bound amount of bilirubin was high and can not be neglected in any interpretation of the reabsorption of the solute by proximal tubules.