Stabilizing effects of caprylate and acetyltryptophanate on heat-induced aggregation of bovine serum albumin

Ferguson plot analysis of multiple intermediate species of thermally unfolded bovine serum albumin

Ferguson plot was used to characterize the multiple intermediate species of bovine serum albumin (BSA) upon thermal unfolding. Differential scanning calorimetry showed an irreversible melting of BSA in Tris-HCl and phosphate buffers with a mid-transition temperature, Tm, of ∼68 °C. Thermally unfolded BSA was analyzed by agarose native gel electrophoresis stained by Coomassie blue and SYPRO Orange staining as a function of pH or protein concentration. SYPRO Orange was used to stain unfolded proteins. BSA heated at 70 and 80 °C, i.e., above the Tm, formed multiple intermediate species, which depended on the pH between 7.0 and 8.0, protein concentration and which buffer was used. These intermediate species were analyzed by Ferguson plot, which showed that BSA heated at 60 °C had a similar size to the native BSA, indicating that they are either native or native-like state consistent with no SYPRO Orange staining. The intermediate species observed at higher temperatures with the mobility less than that of the native BSA showed a steeper Ferguson plot and were stained by SYPRO Orange, indicating that these species had a larger hydrodynamic size than the native BSA and were unfolded.

Ladder observation of bovine serum albumin by high resolution agarose native gel electrophoresis

A commercially available bovine serum albumin (BSA) was examined by agarose native gel electrophoresis using two different agarose sources, UltraPure and MetaPhor agarose. While UltraPure agarose up to 5 % showed no clear separation of BSA oligomers, MetaPhor agarose clearly demonstrated oligomer bands above 4 %, indicating that the latter agarose has greater molecular sieving effects and is hence characterized to have high resolution for size differences, as probed by a greater slope of Ferguson plot. Physical properties are different between two agaroses. In general, UltraPure agarose has physical strength, while MetaPhor agarose is considerably fragile, but MetaPhor agarose solution is less viscous so that even 10 % gel can be made. Cause of oligomers was shown to be not associated with inter-chain disulfide bonds, but is due to association of native or native-like molecules.

Thermally Induced Denaturing Energetics of Human Blood Plasma Albumin by Differential Scanning Calorimetry (DSC) as an Indicator for Breast Cancer Diagnosis in Female Patients

Cancerous invasion yields unusual metabolisms providing a significant amount of peptide albuminomes that modulate albumin stability via binding. The study aimed at the investigation of the thermal stability of human plasma albumin with breast cancer of various stages by means of differential scanning calorimetry (DSC). Blood plasma was drawn from 11 female breast cancer patients and 50 healthy volunteers of homogeneous demographics. Plasma samples and their albumin-enriched fractions were subjected to DSC scanning between 37 and 90°C at 5°C/min rate. For normal blood plasma, a characteristic signature of DSC tracing was observed. And, the deconvolution of DSC thermograms revealed the recognition of thermal transition of albumin. It was found that denaturing temperature of albumin increased with increasing breast cancer staging which implied the increase in albuminome/peptide abundance produced by cancerous invasion. The analysis of albumin denaturing energetics based on rational approximation of the simple Lumry-Eyring model demonstrated that thermal transition of free albumin and albuminome-bound form attained energetic levels expressed as apparent activation energy (E_app ± s.e.) of 132.68 ± 14.21 and 46.76 ± 8.42 Kcal/mol corresponded to the schemes dominated by irreversible alteration and by reversible unfolding, respectively. Thus, E_app value may indicate the degrees of cancerous invasion. It was proposed that E_app may be used as an indicator to diagnose and assess the prognosis of breast cancer.

Assessment of the Dissociation Energetics of Some Selected Ligand Drugs Bound on Human Serum Albumin by Differential Scanning Calorimetry

Drug-protein binding may play a role in the thermal energetics of protein denaturation and could lead to the determination of its equilibrium dissociation parameter. The aim of this study was to assess the energetics of a drug that was bound to human serum albumin (HSA) during thermal denaturation. Drugs that were bound at a single high-affinity primary binding site on HSA, including diazepam and ibuprofen, were employed. Commercial HSA was treated with charcoal to remove stabilizers and adjusted to 20% w/v in a pH 7.4 buffered solution. Serial concentrations of individual drugs up to 0.16 mmole/g-protein were added to the cleaned HSA solutions whereas diazepam was added to a commercial HSA solution. Samples were subjected to differential scanning calorimetry (DSC) set to run from 37 to 90°C at 3.0°C/min. Binding of the drug slightly increased the denaturing temperature of the cleaned HSA due to a shift in the equilibrium toward the native protein bound with the drug. Diazepam depressed the denaturing temperature of the commercial HSA by competing with the stabilizers already bound to the primary site of the HSA. This yielded not only the HSA-stabilizer but also the HSA-diazepam type complexes that exhibited a different denaturation process. A rational approximation of the Lumry-Eyring protein denaturation model was used to treat the DSC endotherms. The approximated scheme: [Formula: see text] was successfully fitted to the data. It was used to determine the dissociation parameters for diazepam and ibuprofen bound to the HSA. These results were comparable to those obtained from other methods.

Synthesis, characterization and stability of BSA-encapsulated microbubbles

In this work, we present an account of experimental studies performed for the synthesis, shelf stability andin vitrostability of microbubbles made from perfluorobutane (PFB) gas and coated in a shell of Bovine Serum Albumin (BSA).

Explanatory chapter: troubleshooting protein expression: what to do when the protein is not soluble

Production of soluble protein remains a bottleneck in the biochemistry and structural biology fields. Unfortunately, there is no 'magic bullet' that solves all solubility problems. The following is a protocol to test whether a protein expressed recombinantly is soluble, and possible strategies to circumvent insolubility issues.

N-acetyl-l-methionine is a superior protectant of human serum albumin against photo-oxidation and reactive oxygen species compared to N-acetyl-L-tryptophan

BACKGROUND

Sodium octanoate (Oct) and N-acetyl-l-tryptophan (N-AcTrp) are widely used as stabilizers during pasteurization and storage of albumin products. However, exposure to light photo-degrades N-AcTrp with the formation of potentially toxic compounds. Therefore, we have examined the usefulness of N-acetyl-l-methionine (N-AcMet) in comparison with N-AcTrp for long-term stability, including photo stability, of albumin products.

METHODS

Recombinant human serum albumin (rHSA) with and without additives was photo-irradiated for 4weeks. The capability of the different stabilizers to scavenge reactive oxygen species (ROS) was examined by ESR spectrometry. Carbonyl contents were assessed by a spectrophotometric method using fluoresceinamine and Western blotting, whereas the structure of rHSA was examined by SDS-PAGE, far-UV circular dichroism and differential scanning calorimetry. Binding was determined by ultrafiltration.

RESULTS

N-AcMet was found to be a superior ROS scavenger both before and after photo-irradiation. The number of carbonyl groups formed was lowest in the presence of N-AcMet. According to SDS-PAGE, N-AcMet stabilizes the monomeric form of rHSA, whereas N-AcTrp induces degradation of rHSA during photo-irradiation. The decrease in α-helical content of rHSA was the smallest in the presence of Oct, without or with N-AcMet. Photo-irradiation did not affect the denaturation temperature or calorimetric enthalpy of rHSA, when N-AcMet was present.

CONCLUSION

The weakly bound N-AcMet is a superior protectant of albumin, because it is a better ROS-protector and structural stabilizer than N-AcTrp, and it is probable and also useful for other protein preparations.

GENERAL SIGNIFICANCE

N-AcMet is an effective stabilizer of albumin during photo-irradiation, while N-Ac-Trp promotes photo-oxidative damage to albumin.

The role of caprylate ligand ion on the stabilization of human serum albumin

Sodium caprylate was added to a pharmaceutical-grade human serum albumin (HSA) to stabilize the product. In this study we have aimed to establish how caprylate ligand protects HSA from thermal degradation. The fatty acid stabilizer was first removed from commercial HSA by charcoal treatment. Cleaned HSA was made to 10% w/v in pH 7.4 buffered solutions and doped with sodium caprylate in serial concentrations up to 0.16 mmol/g-protein. These solutions as well as a commercial HSA, human serum, and enriched-albumin fraction were subjected to differential scanning calorimetry (DSC) within the temperature range of 37-90°C at a 5.0°C/min scanning rate. The globular size of the cleaned HSA solutions was measured by dynamic light scattering. The denaturing temperatures for albumin with sodium caprylate and a commercial one were significantly higher than for albumin only. It was found that the protein globules of cleaned HSA were not as stable as that of the native one due to aggregation, and the caprylate ion may reduce the aggregation by enlarging the globules' electrical double layer. A rational approximation of the Lumry-Eyring protein denaturation model was used to treat DSC denaturing endotherms. The system turned from irreversible dominant Scheme: N (k3K)→ P to reversible dominant Scheme:N (k1)→ P with the increase in caprylate concentration from null to ~0.08 mmol/g-protein. It was postulated that the caprylate ligand may decrease the rate of reversible unfolding as it binds to the IIIA domain which is prone to reversible unfolding/refolding and causes further difficulty for irreversible denaturation which, in turn, HSA can be stabilized.

Protein aggregation and lyophilization: Protein structural descriptors as predictors of aggregation propensity

Lyophilization can induce aggregation in therapeutic proteins, but the relative importance of protein structure, formulation and processing conditions are poorly understood. To evaluate the contribution of protein structure to lyophilization-induced aggregation, fifteen proteins were co-lyophilized with each of five excipients. Extent of aggregation following lyophilization, measured using size-exclusion chromatography, was correlated with computational and biophysical protein structural descriptors via multiple linear regression. Descriptor selection was performed using exhaustive search and forward selection. The results demonstrate that, for a given excipient, extent of aggregation is highly correlated by eight to twelve structural descriptors. Leave-one-out cross validation showed that the correlations were able to successfully predict the aggregation for a protein "left out" of the data set. Selected descriptors varied with excipient, indicating both protein structure and excipient type contribute to lyophilization-induced aggregation. The results show some descriptors used to predict protein aggregation in solution are useful in predicting lyophilized protein aggregation.

Interactions of formulation excipients with proteins in solution and in the dried state

A variety of excipients are used to stabilize proteins, suppress protein aggregation, reduce surface adsorption, or to simply provide physiological osmolality. The stabilizers encompass a wide variety of molecules including sugars, salts, polymers, surfactants, and amino acids, in particular arginine. The effects of these excipients on protein stability in solution are mainly caused by their interaction with the protein and the container surface, and most importantly with water. Some excipients stabilize proteins in solution by direct binding, while others use a number of fundamentally different mechanisms that involve indirect interactions. In the dry state, any effects that the excipients confer to proteins through their interactions with water are irrelevant, as water is no longer present. Rather, the excipients stabilize proteins through direct binding and their effects on the physical properties of the dried powder. This review will describe a number of mechanisms by which the excipients interact with proteins in solution and with various interfaces, and their effects on the physical properties of the dried protein structure, and explain how the various interaction forces are related to their observed effects on protein stability.

Formulation Development for Hydrophobic Therapeutic Proteins

The major challenges in formulation development for hydrophobic proteins are low solubility often combined with a strong tendency for adsorption. Human serum albumin (HSA) is frequently used as excipient to overcome these problems. Due to several drawbacks with HSA, new ways need to be found to circumvent the use of this excipient in protein formulations. One possible approach is to select an appropriate formulation pH and ionic strength in combination with excipients that provide sufficient stability and solubility for the hydrophobic protein. A reduction in adsorption can be achieved by adding surfactants or using special containers.

Formulation of maltodextrin entrapped in polycaprolactone microparticles for protein and vaccine delivery: effect of size determining formulation process variables of microparticles on the hydrodynamic diameter of BSA

Size of the microparticle and integrity of the released protein are two crucial factors which dictate the success of any protein or vaccine delivery system. The primary objective was to optimize bovine serum albumin (BSA) loaded polycaprolactone/maltodextrin (PCL/MD) microparticles in terms of its size and the hydrodynamic diameter of the released protein. The effect of size determining formulation process variables (SDFPV) of microparticles on the hydrodynamic diameter of protein antigen was determined. The SDFPV were optimized by a compromise between the microparticle size and the relative hydrodynamic stability of protein released from it. Percentage of secondary structure of the protein released from the optimized formulation as determined by circular dichroism spectra along with SELCON software was also similar to that of native BSA suggesting the potential of PCL /MD microparticles for protein or vaccine delivery.

Isolation of esterified fatty acids bound to serum albumin purified from human plasma and characterised by MALDI mass spectrometry

Human serum albumin (HSA) is the most abundant protein in plasma. It is known to transport drugs as well as endogenous ligands, like free fatty acids (FFA). A mass spectrometry based method was applied to analyze the albumin bound lipid ligands. HSA was isolated from a human plasma pool by cold ethanol fractionation and ion exchange chromatography. HSA was defatted using a solvent extraction method to release the copurified lipids bound to the protein. The extracts were then analyzed by matrix-assisted laser desorption ionisation (MALDI) mass spectrometry (MS). Using this method, phospholipids and acylglycerols were detected. The phospholipids were identified to be lyso-phosphatidylcholine (lyso-PC) with distribution of different fatty acids (palmitic, stearic, oleic, and linoleic acids). An abundant species in the HSA lipid extract was found to be a diacylglycerol, composed of two linoleic and/or oleic acid chains. The identified motifs reflect structures that are known to be present in plasma. The binding of lysophospholipids has already been described but it is the first ever-reported evidence of native diacylglycerol ligands bound to HSA. Besides the native ligands from plasma a triacylglycerol was detected that has been added during the albumin preparation steps.

Physicochemical characterization of the freezing behavior of mannitol-human serum albumin formulations

The goal of the study was to analyze the impact of human serum albumin (HSA) quality (stabilized or nonstabilized HSA), the addition of NaCl, and the HSA stabilizers Na-octanoate and Na-N-acetyltryptophanate on the freezing behavior of mannitol-HSA formulations. The focus was on crystallization, Tg' (glass transition temperature of the maximally freeze-concentrated phase), and Tc (collapse temperature). Differential scanning calorimetry (DSC), cryomicroscopy, and low-temperature x-ray powder diffraction (LTXRD) were used to study the frozen state. In mannitol-HSA formulations, mannitol crystallization was inhibited and Tg' lowered to a greater extent by stabilized HSA (containing Na-octanoate, Na-N-acetyltryptophanate, and NaCl) than by unstabilized HSA. Detailed DSC and LTXRD studies showed that in the concentrations used for stabilizing HSA, NaCl led to changes in the freezing behavior, an effect that was less pronounced for the other stabilizers. NaCl further lowered the Tc, which was determined by cryomicroscopy. As the freezing behavior governs the lyophilization process, the changes have to be taken into consideration for the development of a lyophilization cycle, to avoid collapse and instabilities.

Physico-chemical lyophilization behavior of mannitol, human serum albumin formulations

The lyophilization behavior is influenced by the presence of salts. The impact of NaCl on drying-time and the morphology after lyophilization was studied for formulations with mannitol and HSA, using different freezing-protocols. The drying-process was monitored by thermocouples and microbalance technique. Karl-Fischer titration, DSC, XRD, SEM and turbiditimetry and HP-SEC were used to study the products after lyophilization and 6 months storage at 2-8 degrees C, 25 degrees C/60% RH and 40 degrees C/75% RH. NaCl decreases the drying-time of mannitol-HSA formulations, indicating morphological changes, which was confirmed by XRD and SEM. Without NaCl exclusively delta-mannitol is formed, which remains physico-chemically stable upon storage for 6 months. With increasing NaCl concentrations more beta-mannitol and finally amorphous products are formed, whereby the freezing-protocol determines how much NaCl is needed to achieve an amorphous product. Upon storage, the amorphous state cannot be preserved and mannitol and NaCl crystallize, which can damage proteins in the formulations. However, regarding the stability of HSA in the lyophilized products, monitored by turbidimetry and HP-SEC, the addition of NaCl can prevent increases in turbidity upon storage. Here the benefit of NaCl on HSA stability outbalances the potential drawbacks that morphological changes, like crystallization impose on protein stability in the lyophilized products.

Effect of genetic variation on the thermal stability of human serum albumin

Reversible thermal denaturation of 33 genetic variants of human serum albumin (HSA) appeared to be a two-state process when studied by circular dichroism (CD). Fourteen single-residue variants have Tm values (midpoint of denaturation) higher than, and nine have Tm values lower than, their endogenous, wild-type counterpart. Nine single-residue variants have DeltaHv values (van't Hoff enthalpy) higher than, and 14 have DeltaHv values lower than, normal albumin. All types of combinations of positive and negative DeltaTm values and Delta(DeltaHv) values were found. Good linear correlations between mutation-induced changes of alpha-helical content and Delta(DeltaHv) values, but not DeltaTm values, were found especially for the variants mutated in domains I and III. The effect of altered chain length and glycosylation on Tm and DeltaHv was also studied. For all variants, no clear relationship was found between the changes in the thermodynamic parameters and the type of substitution, changes in protein charge or hydrophobicity. However, the protein changes taking place in domain I have a rather uniform effect (almost all of the nine variants have positive DeltaTm values and negative Delta(DeltaHv) values, i.e., they denature more easily than normal albumin but they do so at a higher temperature). The present results can be of both protein chemical relevance and of clinical interest, because they could be useful when designing stable, recombinant HSAs for clinical applications.

Stabilizing mechanisms in commercial albumin preparations: octanoate and N-acetyl-l-tryptophanate protect human serum albumin against heat and oxidative stress

The capability of octanoate, N-acetyl-L-tryptophanate (N-AcTrp) and other ions of fatty acids and amino acids to stabilize human serum albumin (HSA) against thermal and oxidative stress was studied. Native-PAGE showed that octanoate, and more hydrophobic fatty acids anions, stabilizes the monomeric form of HSA during heating at 60 degrees C for 30 min. Heating in the presence of octanoate did not change the far-UV CD-spectrum. The stabilizing role of octanoate also showed as an increase in denaturation temperature and calorimetric enthalpy, determined by differential scanning calorimetry (DSC). N-AcTrp, which was found to compete with octanoate for a common high-affinity site, has only a minor stabilizing effect. By contrast, no effect was found for l-tryptophanate or N-acetyl-L-cysteinate. Any ligand effect on oxidation was examined by using 2,2' -azobis(2-amidino-propane)dihydrochloride (AAPH) as oxidizing agent. One hour of incubation resulted in the formation of the same number of carbonyl groups, whether octanoate or one of the above mentioned amino acids was present or not. However, the number of groups formed after 24 h of incubation was significantly decreased in the presence of L-tryptophanate and, especially, N-AcTrp. The effect of 1-h incubation with AAPH on the oxidative status of 34-Cys was studied by the HPLC technique. It was found that N-AcTrp, but not octanoate, has a large protecting effect on the sulfhydryl group. Thus, octanoate has the greatest stabilizing effect against heat, whereas the presence of N-AcTrp diminishes oxidation of HSA.

Ultrasound-mediated cavitation thresholds of liquid perfluorocarbon droplets in vitro

This study was undertaken to measure the ultrasound (US)-mediated cavitation threshold of microdroplets as a function of its content and US parameters (frequency, amplitude and burst length). Albumin-coated droplets were prepared with perfluoropropane, perfluorohexane or perfluoromethylcyclohexane contents. The filtered suspensions were diluted to 1:1000 (v) and compared with Optison. The formulations were injected into an acoustically transparent vessel and sonicated with a single focused transducer. The frequencies employed were 0.74, 1.1, 2.18 and 3.3 MHz and the burst length and acoustic pressure were varied. The inertial cavitation threshold for each experiment was monitored through passive acoustic detection. The formation of droplet emulsion of the perfluorocarbon increased the natural boiling point of the perfluorocarbon. However, perfluorocarbon droplets having contents with higher molecular weights and boiling points did not have detectably higher inertial cavitation thresholds and, thus, the droplets do not need to be in a superheated state to be cavitated by US bursts. Therefore, higher boiling point perfluorocarbons should be investigated for this purpose and may prove to be useful for both imaging and therapy. The inertial cavitation threshold of perfluorocarbon droplets increases with frequency, and was approximately 0.7 MPa at 0.74 MHz and 1.75 MPa at 3.3 MHz. Optison, already in a gaseous state, has the lowest cavitation threshold of all formulations studied. Results show that, for the frequencies tested, there is no dependence between inertial cavitation threshold and burst lengths between 20 and 100 ms. As a conclusion, the inertial cavitation threshold of albumin-coated microdroplets of several perfluorocarbons was determined in vitro. The results indicate that the physical properties of these droplets are such that they may be useful for localized US therapies.