Application of reverse micelles for the extraction of proteins

New phases found in reverse micelle systems with high concentrations of AOT

This paper discusses the phase behavior, rheology, and structure of self-assembled sodium bis(2-ethylhexyl) sulfosuccinate (AOT) reverse micelle systems at high AOT concentrations. When the amount of AOT and w(o) (the molar ratio of water to AOT) were changed, many different phases were found, a fact which is not discussed in the literature. Opaque gel-like phase (phase separation) occurred with high concentrations of AOT in organic solvents without water. When the AOT concentration and w(o) were increased to 18-72 m and 2, respectively, the samples were gel-like and translucent. Dynamic rheological results indicate that the viscoelastic transition agreed with a multirelaxation time model. Small-angle X-ray scattering (SAXS) results imply that these samples showed a hexagonally close-packed cylindrical structure in which the diameter of a cylinder was ~2.5-3.0 nm, depending on the water contents. Moreover, these AOT cylinders self-assembled into fiber bundles with a diameter of 1-10 μm, as determined using a polarized optical microscope. As w(o) was increased to 2-6 in 72 m AOT samples, similar rheological and SAXS results were obtained. However, a different type of viscoelastic transition occurred, from multirelaxation to single-relaxation, when w(o) was increased to 7-11. The samples were in the transparent gel-like phase, and the structures determined by SAXS were a combination of hexagonally packed cylindrical and lamellar structure. The close-packed cylindrical structures had larger radii and shorter lengths with increasing w(o). Furthermore, when w(o) was increased to 12, the gel-like phase disappeared and a highly viscous solution was observed. This is because all the cylindrical structures collapsed and transformed into lamellar structures when the amount of water was further increased.

Electrochemical aspects of the reverse micelle extraction of proteins

The mechanism of the solvent extraction of cytochrome c (Cyt c) via reverse micelle formation was studied from an electrochemical point of view. Potentiometric measurements showed that the Galvani potential difference of the oil/water (O/W) interface played a crucial role in the spontaneous extraction of Cyt c with bis(2-ethylhexyl)sulfosuccinate (AOT). However, the dependence of the extraction efficiency on the concentration of an aqueous electrolyte (KCl) could be explained not by the effect of the interfacial potential, but by the change in the interfacial tension (gamma). Electrocapillary measurements showed that the adsorption of AOT anions to the O/W interface resulted in a significant decrease of gamma in a higher potential range, where reverse micelles were formed. The bottom level of gamma in the higher potential range was increased with [KCl]. The lower extraction efficiency for higher [KCl]'s was elucidated by a "size exclusion effect". This was also supported by water-content measurements by the Karl Fisher method.

Benzodioxole derivatives as negative effectors of plant proteases

Previous work demonstrated that a commercial formulation of piperonyl butoxide (PBO) did inhibit the activity of some plant proteolytic enzymes. In this paper, the effect of pure PBO and nine pure PBO homologues (PBOH) appropriately synthesized toward bromelain and papain was studied in hydrocarbon solution using the bis(2-ethylhexyl)sodium sulfosuccinate (AOT) reverse micellar system. This study establishes that the majority of these compounds show, in vitro, interesting protease inhibition activities. The benzodioxole and dihydrobenzofuran structures, in particular, 5-[2-(2-butoxyethoxy)ethoxymethyl]-benzo[1,3]dioxole (EN 1-40) and 6-[2-(2-butoxyethoxy)ethoxymethyl]-5-propyl-2,3-dihydrobenzofuran (EN 16-5), respectively, appear to be responsible for protease inhibition. Measures of octanol/water partition coefficients on PBO and PBOH have demonstrated that water solubility plays a fundamental role in the expression of protease inhibition activity.

Development of new reverse micellar microencapsulation technique to load water-soluble drug into PLGA microspheres

The objective of this study was to develop a new reverse micelle-based microencapsulation technique to load tetracycline hydrochloride into PLGA microspheres. To do so, a reverse micellar system was formulated to dissolve tetracycline hydrochloride and water in ethyl formate with the aid of cetyltrimethylammonium bromide. The resultant micellar solution was used to dissolve 0.3 to 0.75 g of PLGA, and microspheres were prepared following a modified solvent quenching technique. As a control experiment, the drug was encapsulated into PLGA microspheres via a conventional methylene chloride-based emulsion procedure. The microspheres were then characterized with regard to drug loading efficiency, their size distribution and morphology. The reverse micellar procedure led to the formation of free-flowing, spherical microspheres with the size mode of 88 microm. When PLGA microspheres were prepared following the conventional methylene chloride-based procedure, most of tetracycline hydrochloride leached to the aqueous external phase: A maximal loading efficiency observed our experimental conditions was below 5%. Their surfaces had numerous pores, while their internal architecture was honey-combed. In sharp contrast, the new reverse micellar encapsulation technique permitted the attainment of a maximal loading efficiency of 63.19 +/- 0.64%. Also, the microspheres had smooth and pore-free surfaces, and hollow cavities were absent from their internal matrices. The results of this study demonstrated that PLGA microspheres could be successfully prepared following the new reverse micellar encapsulation technique.

Extraction of immunoglobulin-G from colostral whey by reverse micelles

Separation of immunoglobulin G (IgG) from the other colostral whey proteins was carried out by reversed micellar extraction. The colostral whey was diluted to 5 times its original volume with 50 mM phosphate buffer at pH 6.35 containing 100 mM of sodium chloride. The aqueous solution was then mixed with an equal volume of isooctane containing 50 mM bis-(2-ethylhexyl) sodium sulfosuccinate (AOT), and shaken at 200 rpm and 25 degrees C for 10 min. After extraction, the mixture was separated to the aqueous phase and the reversed micellar phase by centrifugation. This procedure extracted most of the non-IgG proteins to the reversed micellar phase and recovered more than 90% of the IgG in the aqueous phase. The IgG in the aqueous phase had a purity of 90%, and still possessed immunological activity. AOT was not detectable in the aqueous phase.

Reverse micelles and protein biotechnology

Reverse micelles are nanometer-sized (1-10 nm) water droplets dispersed in organic media obtained by the action of surfactants. Surfactant molecules organize with the polar part to the inner side able to solubilize water and the apolar part in contact with the organic solvent. Proteins can be solubilized in the water pool of reverse micelles. Studies on the structure-function relationships of proteins in reverse micelles are very important since the microenvironment in which the protein is solubilized has physico-chemical properties distinct from a bulk aqueous solution. Some of the unique characteristics of reverse micelles make them very useful for biotechnological applications. Charge and hydrophilic/hydrophobic characteristics of the protein and the selection of surfactant can be used to achieve selective solubilization of proteins. This has been used to extend the classical liquid-liquid extraction with solvents to protein bioseparation. For biocatalysis the presence of a bulk organic solvent allow synthetic reactions to be performed via the control of water content and the solubilization of hydrophobic substrates. This is accomplished with a higher interfacial area (about 100 m2/mL) than the conventional biphasic systems, minimizing mass transfer problems.

Kinetic models in reverse micelles

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Characterization of acylating and deacylating activities of an extracellular phospholipase A2 in a water-restricted environment

The behavior of porcine pancreatic phospholipase A2 (ppPLA2) in monophasic low-water media has been explored, for the first time, in a systematic manner. It has been investigated how a number of variables can modulate both acylating and deacylating activities of the enzyme, and several interesting, unexpected results are presented. Among the most relevant, when placing ppPLA2 in the water-restricted environment, are the following: (i) it displays a remarkable alteration of its specificity toward the substrate polar head relative to all-water medium; (ii) it is quite severely inhibited by lysophosphatidylcholine (LPC), which has important implications, particularly concerning its acylation activity; and (iii) it exquisitely discriminates between saturated and unsaturated long-chain fatty acids when esterifying them with LPC. Finally, it is also illustrated how these results can be exploited to optimize the catalytic performance of the enzyme in nonaqueous medium and obtain a nearly 30-fold increase in the yield of phosphatidylcholine synthesis with respect to previously reported data.

Reverse micelles as a water-property-control system to investigate the hydration/activity relationship of alpha-chymotrypsin

alpha-Chymotrypsin, solubilized in hydrated reverse micelles of sodium bis(2-ethylhexyl)sulfosuccinate (AOT) in n-octane, was used as a model system for studying the involvement of different water structures (strongly bound water, disordered water, water clusters and bulk water) in the development of the catalytically active conformation of the enzyme. Results presented in this study indicate a characteristic dependence of the stability/activity profile on the water content of the reverse-micellar system for values of wo of approximately 5 (wo is defined as [H2O]/[AOT]). The results are consistent with heat-capacity measurements for proteins. At very low wo values, the conformation of alpha-chymotrypsin changes to a very rigid structure in comparison to the structure observed in water. This is demonstrated by the overall center of gravity of the tryptophan fluorescence spectrum of the enzyme at wo = 0.65, which is blue shifted in comparison to the spectrum in bulk water indicating that the enzyme is in an apolar environment. In the absence of a hydration shell, the protein is to a great extent frozen and inactive. A small increase in the level of enzyme hydration (up to wo = 2.3) causes an increase in the amount of strongly bound water associated with the enzyme and the enzyme displays a high catalytic activity. Upon further addition of water, a new unstable water structure with unfavourable enthalpy is developed and the enzyme activity declines, reaching a minimum at wo = 5.1. A new increase of water content within a relatively small range, wo = 5-8, causes a dramatic increase in enzymic activity, reminiscent of a cooperative hydration dependence. In the range wo = 10-29, the effect of hydration on the activity is complete which shows that the enzyme activity depends on the amount of water in contact with the enzyme and not on the total amount of bulk water in the system. The experimental results on enzyme incubation at different wo values followed by dilution to constant high wo, are indicative of inactive conformational substates of alpha-chymotrypsin. It is demonstrated that highly active enzyme conformations at very low, wo values occur via an induced fit mechanism of substrate binding.

Beta-hydroxysteroid dehydrogenase: activity in microemulsion and extraction from Pseudomonas testosteroni cells with microemulsion

The stability of purified beta-hydroxysteroid dehydrogenase activity measured as a function of time was good in buffered cationic and non-ionic microemulsions. The use of 1-pentanol and 1-hexanol in place of 1-butanol as cosurfactant gave increased activity and stability. The NAD+ Michaelis constant was 0.22 mM in buffer and 3.5 mM in waterpool concentration in microemulsion. Proteins, among them beta-hydroxysteroid dehydrogenase, were extracted from Pseudomonas testosteroni with cationic microemulsion, thus indicating that microemulsions may be utilized in protein release from cells.

The behavior of proteases in lecithin reverse micelles

Reverse micelles, formed in isooctane/alcohol by phosphatidylcholines of variable chain length (i.e. 6, 7 or 8 C atoms in the fatty acid moiety) have been studied, mostly in relation to their capability of solubilizing trypsin and alpha-chymotrypsin. It has been found that the capability of the lecithin reverse micellar systems to solubilize water is strongly affected by the chain length of the alkyl group and by the alcohol used as co-surfactant. The C8-lecithin system, i.e. 1,2-dioctanoyl-sn-glycero-3-phosphocholine, in isooctane/hexanol is the system which affords the maximal solubilization of water (up to wo 60, where wo = [H2O]/[lecithin]) and of the enzymes. The water of the water pool of lecithin reverse micelles has been investigated by 1H-NMR; the proton chemical shift as a function of wo was found to be similar to the case of reverse micelles formed by the well known negatively charged surfactant sodium bis(2-ethylhexyl sulfosuccinate). 31P-NMR studies show that the ionization behavior of phosphate groups is similar to that in bulk water, suggesting no anomaly in the pH behavior of this water pool. The stability of trypsin and alpha-chymotrypsin in the various lecithin reverse micellar system is similar and occasionally better than that in aqueous solution. The same holds for the kinetic behavior (kcat and Km have been determined for a few systems). The bell-shaped curve of the pH/activity profile in lecithin reverse micelles is, for both enzymes, shifted towards more alkaline values with respect to water. Bell-shaped curves are also obtained when studying the influence of wo on the enzyme activity, with an optimal wo which is in the range 7-10, a surprisingly small value considering that we are dealing with hydrolases. Circular dichroic studies have been carried out in order to correlate the activity with the protein conformation: for both enzymes, generally no marked perturbations appear as a consequence of the solubilization in the lecithin reverse micelles, but conditions can be found under which significant alterations are present. Certain properties of the two enzymes, which in water solution are very similar, become sharply different in reverse micelles, showing that occasionally the micellization is able to enhance the relatively small structural differences between the two proteins.