Trypsin-SBTI interaction in reverse micelles. A slow intermicellar exchange-dependent binding

Circular dichroic properties and average dimensions of DNA-containing reverse micellar aggregates

With the aim of investigating the compartmentation of nucleic acids and surfactant aggregates, we have studied the circular dichroic properties of DNA solubilized in reverse micelles. DNA incorporated in AOT/isooctane reverse micelles (AOT=bis-2-ethyl-hexyl sodium sulfosuccinate) assumes an anomalous circular dichroism (CD) spectrum with the characteristic features of a psi spectrum. Older literature observations could therefore be confirmed that attribute these spectral changes to the fact that the reverse micelles induce the formation of a condensed form of DNA. A dynamic light scattering (DLS) characterization of the DNA-containing micellar solutions was carried out, and three populations of aggregates in a polar solvent are observed, with an average radius centered at 5, 100 and 1000 nm, respectively, all three containing DNA. Several forms of DNA, including a plasmid, have been investigated. The formation of 1 microm-large aggregates depends on the DNA concentration and such aggregates disappear in the course of a few hours. Conversely, the 100 nm aggregates are stable for at least 1 day and contain DNA in a normal spectral state at low concentration and in a condensed form-it is the characteristic psi spectrum-in a higher concentration range. The solubilization of DNA in reverse micelles brings about unexpected larger structures in hydrocarbon solution, and whereas the very large component can be with all likelihood be attributed to clusters of smaller reverse micelles, the components at 100 nm radius appear to be a quite stable and characteristic feature of DNA-containing reverse micelles.

A novel N-acyl phosphatidylethanolamine-containing delivery vehicle for spermine-condensed plasmid DNA

A unique method for formulation of plasmid DNA with phospholipids has been devised for the purpose of producing vehicles that can mediate gene delivery and transfection of living cells. The polycation, spermine, was used to condense plasmid DNA within a water-in-chloroform emulsion stabilized by phospholipids. After organic solvent removal, the particles formed could be extruded to a number average size of about 200 nm and retained DNA that was protected from nuclease digestion. This resulted in a relatively high protected DNA-to-lipid ratio of approximately 1 microg DNA/micromol lipid. The size distribution of the preparation was relatively homogeneous as judged by light microscopy and quasi-elastic light scattering. Electron microscopic studies showed structural heterogeneity, but suggested that at least some of the plasmid DNA in this preparation was in the form of the previously observed spermine-condensed bent rods and toroids and was encapsulated within liposomal membranes. Preparations with the fusogenic phospholipid composition, 1, 2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-dodecanoyl/ 1, 2-dioleoyl-sn-glycero-3-phosphocholine, showed transfection activity for several cells lines, particularly OVCAR-3 cells. The transfection activity sedimented with the lipid during centrifugation, confirming the association of active plasmid DNA with phospholipids. Transfection efficiency in culture was found to be of the same order of magnitude as cationic lipoplexes but much less toxic to the cells. Significant transfection of OVCAR-3 cells in tissue culture could also be observed, even in the presence of the intraperitoneal fluid from a mouse with an OVCAR-3 ascites tumor. These data indicate a new type of liposomal gene delivery system devoid of cationic lipids, phosphatidylethanolamine, cationic polymers and viral components.

Enzymes in low water systems

Water is fundamental for enzyme action and for formation of the three-dimensional structure of proteins. Hence, it may be assumed that studies on the interplay between water and enzymes can yield insight into enzyme function and formation. This has proven correct, because the numerous studies that have been made on the behavior of water-soluble and membrane enzymes in systems with a low water content (reverse micelles or enzymes suspended in nonpolar organic solvents) have revealed properties of enzymes that are not easily appreciated in aqueous solutions. In the low water systems, it has been possible to probe the relation between solvent and enzyme kinetics, as well as some of the factors that affect enzyme thermostability and catalysis. Furthermore, the studies show that low water environments can be used to stabilize conformers that exhibit unsuspected catalytic properties, as well as intermediates of enzyme function and formation that in aqueous media have relatively short life-times. The structure of enzymes in these unnatural conditions is actively being explored.

Kinetic models in reverse micelles

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Biocatalysis in reverse self-assembling structures: reverse micelles and reverse vesicles

The use of two reverse self-assembling systems, such as reverse micelles and reverse vesicles, to model the enzymatic function of biological membranes is discussed. They permit direct measurement of enzyme kinetics since these ternary systems form optically transparent solutions. The physicochemical characteristics of both systems are differentiated since they clearly affect enzyme behavior. The four enzymatic profiles that have been described in reverse micelles as a function of micelle size (omega 0) and the kinetic models developed to explain them are discussed. Reverse vesicles, first described in 1991, are also presented as a new system that shares properties with reverse micelles and liposomes, and in which enzymes show unexpected behavior. Finally, the potential use of these systems in protein extraction, hydrophobic protein stabilization, and biotechnology are noted, although a better physicochemical characterization is needed in order to explore their full potential.

Limited proteolysis of myelin basic protein in a system mimetic of the myelin interlamellar aqueous space

We have investigated the early steps of myelin basic protein (MBP) degradation in a membrane mimetic system (reverse micelles), resembling the interlamellar aqueous spaces where the protein is located in the myelin sheath. MBP, unfolded in buffer, refolds on incorporation into the micelles, resulting in reduced accessibility to three proteolytic enzymes, trypsin, cathepsin D, and Staphylococcus aureus V8 protease, in comparison with aqueous solution. Eleven cleavage sites seen in buffer are removed from proteolytic attack in micellar solution. These sites delineate a protected protein domain displaying a potential beta-sheet structure capable of interacting with the myelin membrane. An additional site not seen in buffer is attacked in the micelles. Experiments with a structure inducer, 15% 1-propanol in buffer, reveal that the refolding pattern of MBP in reverse micelles is specific to the membrane biomimetic system and is not produced by organic solvent per se. Micellar digestions of MBP generate long peptides, two of which, isolated after tryptic digestion, have been found to be immunodominant in multiple sclerosis patients. The findings suggest the structure induced in MBP by the micelles resembles that leading to production of the self-peptides recognized by T cells during proteolytic breakdown of MBP in autoimmune demyelinating diseases.

Protein-protein interactions in reverse micelles: trypsin shows superactivity towards a protein substrate alpha-chymotrypsinogen A in reverse micelles of sodium bis(2-ethylhexyl)sulfosuccinate (AOT) in isooctane

Protein-protein interactions in reverse micelles of sodium bis(2-ethylhexyl)sulfosuccinate (AOT), in isooctane containing varying amounts of Tris buffer are studied by using activation of alpha-chymotrypsinogen A by trypsin (EC 3.4.21.4) to alpha-chymotrypsin (EC 3.4.21.1) as a model reaction. It has been found that protein-protein interactions are strongly dependent on the water content of the medium. At an optimum water content the activation reaction in reverse micelles is faster than reaction in water by a factor of 21.3. At lower water content both reaction rates and the conversion of alpha-chymotrypsinogen A into alpha-chymotrypsin decrease with decrease in water content of the reaction medium.

Application of active-phase plot to the kinetic analysis of lipoxygenase in reverse micelles

A new plot for explaining the complex expression of the enzymic activity in reverse micelles has been developed as an extension of the theoretical model described by our group [Bru, Sánchez-Ferrer & García-Carmona (1990) Biochem. J. 268, 679-684]. The plot describes the changes in the relative volume, amount of enzyme (mumoles), enzyme concentration (microM) and substrate concentration (microM) in the phase where the enzyme is active. To illustrate the usefulness of this plot, the complex activity of soya bean lipoxygenase in reverse micelles acting on its interfacial substrate, octadecadienoic acid, was studied. It showed the key parameters ruling the activity profiles of lipoxygenase with respect to micelle size (omega 0), micelle concentration (theta) and the substrate/surfactant molar ratio (rho), which have never been described before.

Dimerization and reactivation of triosephosphate isomerase in reverse micelles

The reactivation of the homodimeric enzyme triosephosphate isomerase (TPI) was studied in reverse micelles. The enzyme was denatured in conventional aqueous mixtures with guanidine hydrochloride and transferred to reverse micelles formed with cetyltrimethylammonium bromide, hexanol, n-octane and water. In the transfer step, denatured TPI monomers distributed in single micelles, and guanidine hydrochloride was diluted more than 100 times. Under optimal reactivation conditions, 100% of the enzyme activity could be recovered. The rate of appearance of the catalytic activity increased with the concentration of protein, which indicated that catalysis required the formation of the dimer. The rate of TPI reactivation also increased with increasing protein concentration in the system with denatured TPI covalently derivatized at the catalytic site with the substrate analogue 3-chloroacetol phosphate. Thus, reactivation could take place via the formation of dimers composed of an inactive and an active subunit. Reactivation critically depended on the amount of water in the reverse micelles. The plot of the extent of reactivation versus the amount of water (2.5-7.0%) was markedly sigmoidal. Less than 20% reactivation took place with water concentrations below 3.5%, due to the formation (in less than 30 s) of stable inactive structures. The results indicate that reverse micelles provide a useful system to probe the events involved in the transformation of unfolded monomers to polymeric enzymes.