Absence Of Micelles Research Articles

Understanding the effect of the membrane-mimetic micelles on the interplay between α-synuclein and Cu(II)/Cu(I) cations

α-Synuclein (αS) is a presynaptic protein whose aggregates are considered as a hallmark of Parkinson's disease (PD). Although its physiological function is still under debate, it is widely accepted that its functions are always mediated by its interaction with membranes. The association of αS with phospholipid membranes occurs concomitant to its folding from its monomeric, unfolded state towards an antiparallel amphipathic α-helix. Besides this, copper ions can also bind αS and modify its aggregation propensity. The effect of Cu(II) and Cu(I) on the lipid-αS affinity and on the structure of the membrane-bound αS have not yet been studied. This knowledge is relevant to understand the molecular pathogenesis of PD. Therefore, we have here studied the affinities between Cu(II) and Cu(I) and the micelle-bound αS, as well as the effect of these cations on the structure of micelle-bound αS. Cu(II) or Cu(I) did not affect the α-helical structure of the micelle-bound αS. However, while Cu(I) binds at the same sites of αS in the presence or in the absence of micelles, the micelle-bound αS displays different Cu(II) binding sites than unbound αS. In any case, sodium docecyl sulphate -micelles reduce the stability of the αS complexes with both Cu(II) and Cu(I). Finally, we have observed that the micelle-bound αS is still able to prevent the Cu(II)-catalysed oxidation of neuronal metabolites (e.g. ascorbic acid) and the formation of reactive oxygen species, thus this binding does not impair its biological function as part of the antioxidant machinery.

First Observation of Protomicelles in the System with a Non-Colloidal Surfactant

A spectrophotometric study of the system heptanol—Nile red (NR)—water was carried out, where, for the first time for such studies, a non-colloidal surfactant that does not form micelles was taken as a surfactant. The dependence of the solubility of NR on the concentration of heptanol in an aqueous solution was studied. The experiments were carried out at a given chemical potential of NR, which was provided by an excess of the solid phase of NR. The existence of a solubilization effect has been theoretically and experimentally established: An increase in the solubility of NR with an increase in the concentration of heptanol in solution. It was found that heptanol protomicelles with a solubilization core as an NR molecule are formed in such a system, so that in the absence of micelles, the protomicelles take on the entire solubilization load. From the experimental data, the concentration of protomicelle formation was calculated, which can also be taken as the concentration of NR monomerization in an aqueous solution, since the formation of protomicelles prevents the dye aggregation. Based on the results obtained, the following generalizations were made: (1) non-colloidal surfactants, although they do not give micelles, are capable of forming protomicelles; and (2) non-colloidal surfactants can serve as a practical means of dye monomerization.

Epoxidation kinetics of vegetable oil in micellar medium by reaction calorimetry

The kinetics of vegetable oil epoxidation in the presence of different families of surfactants was conducted in an RC1 calorimetric reactor, to explore the micellar effect on the overall rate and kinetic parameters of the reaction. The reaction time was reduced by 2 to 3 hours compared to that obtained in the absence of micelles. A kinetic model is proposed whose parameters are estimated using a method based on the reconstruction of the thermal power profile released by the synthesis. The activation energy of the epoxidation reaction is 44.08 kJ. Safety parameters (MTSR, MTT, TD24) are also determined to study the possibility and severity of thermal runaway.

Transepithelial transfer of phenanthrene, but not of benzo[a]pyrene, is inhibited by fatty acids in the proximal intestine of rainbow trout (Oncorhynchus mykiss)

The inclusion of vegetable oils in aquafeeds introduces contaminating polycyclic aromatic hydrocarbons (PAHs) in salmonids. Since lipophilic PAHs solubilize in micelles composed of lipids, bile salts and fatty acids, dietary lipid composition can alter intestinal transepithelial PAH transfer. We studied the uptake of two PAHs, viz. benzo[a]pyrene (BaP) and phenanthrene (PHE), in rainbow trout (Oncorhynchus mykiss) intestine. We also investigated the effects of two fatty acids, viz. fish oil-derived eicosapentaenoic acid (EPA, 20:5n-3) and vegetable oil-derived oleic acid (18:1n-9) on intestinal uptake. Radiolabeled PAHs were solubilized in micelles composed of tritiated EPA and oleic acid, respectively, and administrated to intestinal segments mounted in Ussing chambers.In the absence of micelles, PHE accumulation was two times higher than BaP in the mucosal and serosal layers of proximal and distal intestine. Administration of PHE in micelles composed of oleic acid resulted in a 50% lower accumulation of PHE in the mucosal layers of the proximal intestine compared to EPA-composed micelles. Accumulation of EPA and oleic acid in the proximal intestinal mucosa correlated negatively with the transepithelial transfer of these fatty acids across the proximal intestinal epithelium. Transepithelial PHE transfer across the proximal intestine was reduced by 30% in co-exposure with EPA-composed micelles compared to 80% with oleic acid micelles. BaP was not transferred across the intestine.We conclude that the lipid composition of an aquafeed is an important determinant of PAH bioavailability. Therefore, lipid composition should be an important consideration in choosing vegetable oils as alternatives for fish oil in aquafeeds.

A fluorinated detergent for membrane-protein applications.

Surfactants carrying fluorocarbon chains hold great promise as gentle alternatives to conventional hydrocarbon-based detergents for the solubilization and handling of integral membrane proteins. However, their inertness towards lipid bilayer membranes has limited the usefulness of fluorinated surfactants in situations where detergent-like activity is required. We demonstrate that fluorination does not necessarily preclude detergency, as exemplified by a fluorinated octyl maltoside derivative termed F6 OM. This nonionic compound readily interacts with and completely solubilizes phospholipid vesicles in a manner reminiscent of conventional detergents without, however, compromising membrane order at subsolubilizing concentrations. Owing to this mild and unusual mode of detergency, F6 OM outperforms a lipophobic fluorinated surfactant in chaperoning the functional refolding of an integral membrane enzyme by promoting bilayer insertion in the absence of micelles.

Micellization behavior of Aerosol OT in alcohol/water systems.

This paper examines the effects of solvent composition on the micellization behavior of the surfactant Aerosol OT (AOT). The critical micelle concentrations of AOT in the pure solvents methanol, ethanol, propanol, and isopropanol were measured using conductiometric techniques. These solvents were then mixed with water to create solvent spectra from pure alcohol to pure water in 12 increments. Critical micelle concentrations were measured at each solvent composition. Dynamic light scattering was used to verify the presence or absence of micelles in the solvent mixtures. It was found that inverse micelles exist over a range of solvent compositions where εeff < 48 with CMCs increasing with increasing solvent polarity. Micellization was found not to occur when 48 < εeff < 80. Regular micelles formed in pure water, with the measured CMC agreeing with the literature value of 2.25 mM.

Antiproliferative effect of alkylglycerols as vehicles of butyric acid on colon cancer cells

The anticarcinogenic activity of synthetic 1-O-octadecyl-2,3-dibutyroilglycerol (D-SCAKG) in tumor-cell line of colonocytes (SW620) was performed. The effect of the previously digested D-SCAKG under in vitro intestinal conditions was compared to the bioactivity of non-digested D-SCAKG. Antiproliferative activity of each individual product from digestion (1-O-octadecyl-2-butyroilglycerol; 1-O-octadecyl glycerol; butyric acid) was also performed. The impact of solubilization of lipid products within micellar structures was also tested. The 1-O-octadecyl glycerol was the most active compound, followed by 1-O-octadecyl-2-butyroilglycerol, D-SCAKG and butyric acid. The 1-O-octadecyl glycerol and butyric acid were the only molecules that showed antiproliferative effect in absence of micelles. Digested D-SCAKG was 4-fold more effective than non-digested D-SCAKG. A synergism between 1-O-octadecyl-2-butyroilglycerol and 1-O-octadecyl glycerol was evidenced. As summary, the synthetic D-SCAKG seems to be an interesting antitumoral lipid against colonocytes, especially after previous intestinal digestion, and mainly due to the synergism of the major products, namely 1-O-octadecyl-2-butyroilglycerol and 1-O-octadecyl glycerol. At the same time, 1-O-octadecyl-2-butyroilglycerol would constitute a stable esterified form of butyric acid for its vehiculization.

Measuring Local Equilibrium Flavor Distributions in SDS Solution Using Headspace Solid-Phase Microextraction

Solid-phase microextraction (SPME) sampling of the headspace above an aqueous micellar solution of sodium dodecyl sulfate (SDS) was shown to be effective for quantifying the equilibrium partitioning of limonene solute between water and SDS micellar aggregates. Concentrations in the headspace were determined from the amount absorbed by the SPME fiber during 1 min extractions, with the quantity on the fiber determined using gas chromatography/mass spectrometry (GC/MS). Headspace concentrations as a function of surfactant concentration were fit to a mass balance to yield the partition coefficient and critical micelle concentration. When the total limonene in the system was low enough that it could be completely dissolved by water in the absence of micelles, a constant value for the partition coefficient of 1700 M(-1) was obtained, independent of the limonene concentration. However, at higher total limonene concentrations, the partition coefficient became a function of the amount of limonene in the micelles, as confirmed by separate experiments in which either limonene or SDS concentration was varied. The observed increase in partition coefficient with increasing limonene likely signals a shift from micelles to swollen micelles and ultimately to microemulsion droplets. The effect of SDS concentration on the aqueous solubility limit of limonene could also be observed in HS-SPME experiments where either SDS or limonene was varied.

Micellar effect on the kinetics of oxidation of methyl blue by Ce(IV) in sulfuric acid medium

The kinetics of oxidation of methyl blue (MB) by Ce(IV) in aqueous and surfactant media has been carried out to explore the micellar effect on the rate and kinetic parameters of the reaction. The reaction was found to be first order with respect to both oxidant and substrate and fractional order with respect to H+. The active kinetic species of the oxidant was found to be Ce(SO4)+2 based on the effect of ionic strength and sulfate ion on the rate of the reaction. The presence of micelles was found to inhibit the reaction and this effect has been explained by the association of one of the reactants with the micelles leaving the other reactant in the bulk solution. The binding constant and first order rate constant in micellar medium has been obtained by the application of pseudo-phase model to the experimental data. Interestingly, the temperature dependence of the reaction reveals that the reaction has negative activation energy in the absence of micelles, which turns to a positive value in the presence of micelles.

Influence of surfactant structure on the contribution of micelles to Ostwald ripening in oil-in-water emulsions

The rate of Ostwald ripening was measured, using light scattering, in 2 wt.% and 10 wt.% decane-in-water and dodecane-in-water emulsions. Sodium dodecyl sulfate and several nonionic ethylene oxide dodecyl ethers—surfactants with tails containing 12 carbons, but with various headgroups—were used to form the emulsions. Emulsions were formed with sufficient quantities of the surfactant to saturate the droplet interfaces. The influence of surfactant micelles in the continuous phase was then explored by adding 1–5 wt.% surfactant to the water. The increase in the average droplet radius in the absence of micelles was found to agree qualitatively with Lifshitz–Slyozov–Wagner theory for the different surfactant types. The addition of micelles increased the rate of Ostwald ripening, by factors between 2 and 50, depending on the type and concentration of surfactant. However, there was no systematic correspondence between the increased rate and the equilibrium solubilization capacity of the micelles, nor was the rate decreased with increased strength of repulsive interactions between micelle and the droplet interface. It is proposed that the complex influence of surfactant on Ostwald ripening kinetics may depend on the ability of micelles to become supersaturated with oil—i.e., to incorporate solute temporarily above their equilibrium solubilization capacity.

Thermodynamics of mixed anionic/nonionic surfactant adsorption on alumina

The adsorption of sodium dodecyl sulfate and a polyethoxylated nonylphenol, and well defined mixtures thereof, was measured on γ-alumina. A pseudo-phase separation model to describe mixed anionic/nonionic admicelle (adsorbed surfactant aggregate) formation was developed, analogous to the pseudo-phase separation model frequently used to describe mixed micelle formation. In this model, regular solution theory was used to describe the anionic/nonionic surfactant interactions in the mixed admicelle and a patch-wise adsorption model was used to describe surfactant adsorption on a heterogeneous solid surface. The formation of mixed anionic/nonionic admicelles in the absence of micelles was accurately described by regular solution theory; mixed admicelle formation exhibited stronger negative deviations from ideality than mixed micelle formation. An adequate description of mixed anionic/nonionic admicelle formation in the presence of mixed micelles was obtained through a simultaneous solution of the pseudo-phase separation models for mixed admicelle and mixed micelle formation, and the appropriate mass balance equations. Anionic/nonionic mixed adsorption in the presence of mixed micelles was shown to correspond to an admicelle composition of approximately a 1:1 anionic/nonionic mole ratio throughout Regions II and III of the adsorption isotherm.

Quenching and Dequenching of Pyrene Fluorescence by Nucleotide Monophosphates in Cationic Micelles

The fluorescence behavior of pyrene solubilized in the hexadecyltrimethylammonium bromide aqueous micellar solution in the presence of adenosine 5'-monophosphate (AMP) and uridine 5'-monophosphate (UMP) was investigated. AMP and UMP were found to influence oppositely the fluorescence of micellized pyrene. UMP acts as quencher, while AMP acts as dequencher. Both effects saturate at high nucleotide concentration (about 40 mM). Dequenching of micellized pyrene fluorescence is induced also by addition of disodium hydrogen orthophosphate (Na 2HPO 4), while loading with sodium bromide (NaBr) quenches the fluorescence. Furthermore, in absence of micelles, pyrene fluorescence depends on the UMP, according to the Stern-Volmer relation, but is unaffected by AMP. Dynamic light scattering experiments showed that the size and shape of aggregates is not affected by different types of nucleotide loaded into the solution; thus, we conclude that the opposite photophysical effect exploited by AMP and UMP are uncorrelated to any change in micellar microstructure. The whole fluorescence data set was successfully accounted for by assuming that the anionic nucleotides compete with the surfactant counterion (bromide) for the surface of the micelle. Accordingly, substitution of bromide with the more effective quencher UMP results in a strong decrease of the pyrene fluorescence, while the substitution of bromide with the nonquencher AMP results in an increase in the pyrene fluorescence.

The influence of the structural characteristics of the substrate and the medium on the stability of triflusal and acetylsalicylic acid in micellar systems

A spectrophotometric study of the alkaline hydrolysis of two salicylic acid (SA) derived drugs, performed on acetylsalicylic acid (ASA) and triflusal, both in the absence and presence of cationic micelles. In the absence of micelles, a catalytic effect is produced by the favoured OH − ion in the molecule of triflusal, due to the presence of the trifluoromethyl group. The surfactants used were: hexadecyltrimethylammonium hydroxide (HDTAOH), hexadecyltrimethylammonium chloride (HDTACl), hexadecyltrimethylammonium bromide (HDTABr), hexadecylethyldimethylammonium bromide (HDEDABr) and tetradecyltrimethylammonium bromide (TDTABr). In micelles with reactive counterions, the pseudo-first-order rate constant ( k obs) increases with the increase in surfactant concentration, while in micelles with non-reactive counterions, the rate constant increased with surfactant concentration at low concentration, reaching a maximum, and decreased at high surfactant concentration, even to below the value found in the absence of micelle. The micellar binding constant of both drugs ( K S), the micellar rate constant ( k M) and the ion-exchange constant ( K X OH) were determined according to variation in k obs in relation to surfactant concentration, through the application of the pseudophase ion-exchange model (PPIE) proposed. The empirical parameters obtained were found to depend on the substrate and the surfactant structure, these parameters were: the counterion of the micelle, the size of the headgroup and the chain length of the hydrophobic tail of the surfactant.

α-Synuclein Multistate Folding Thermodynamics: Implications for Protein Misfolding and Aggregation

Alpha-synuclein aggregation has been tightly linked with the pathogenesis of Parkinson's disease and other neurodegenerative disorders. Despite the protein's putative function in presynaptic vesicle regulation, the roles of lipid binding in modulating alpha-synuclein conformations and the aggregation process remain to be fully understood. This study focuses on a detailed thermodynamic characterization of monomeric alpha-synuclein folding in the presence of SDS, a well-studied lipid mimetic. Far-UV CD spectroscopy was employed for detection of conformational transitions induced by SDS, temperature, and pH. The data we present here clearly demonstrate the multistate nature of alpha-synuclein folding, which involves two predominantly alpha-helical partially folded thermodynamic intermediates that we designate as F (most folded) and I (intermediately folded) states. Likely structures of these alpha-synuclein conformational states are also discussed. These partially folded forms can exist in the presence of either monomeric or micellar forms of SDS, which suggests that alpha-synuclein has an intrinsic propensity for adopting multiple alpha-helical structures even in the absence of micelle or membrane binding, a feature that may have implications for its biological activity and toxicity. Additionally, we discuss the relation between alpha-synuclein three-state folding and its aggregation, within the context of isothermal titration calorimetry and transmission electron microscopy measurements of SDS-initiated oligomer formation.

Partitioning of hydrogen peroxide between the gas and liquid phases in the presence of surfactant

Gas-liquid phase partitioning is a key physical property that can predict the environmental fate of a compound between two phases. Several environmental factors have been known to affect the gas-liquid phase partitioning. We investigated the influence of surfactant on the gas-liquid phase partitioning of hydrogen peroxide (H(2)O(2)). The surfactant used was ammonium perfluorooctanoate (APFO). H(2)O(2) solution containing the surfactant was equilibrated in a closed system and gas phase H(2)O(2) concentration was measured by the peroxyoxalate chemiluminescence (PO-CL) method. Gas phase H(2)O(2) concentrations remained constant below the critical micelle concentration (CMC) and increased linearly with surfactant concentration above the CMC, which indicated that surfactant micelles influenced the gas-liquid phase partitioning of H(2)O(2). This result showed that H(2)O(2)-micelle interactions are less favorable than H(2)O(2)-H(2)O interactions. Surfactant monomers did not affect the gas-liquid phase partitioning of H(2)O(2) due to the absence of micelles. Solvent (methanol) effect was also investigated and showed that gas phase H(2)O(2) concentrations increased with the addition of solvent. This indicated the unfavorable interaction of H(2)O(2) with hydrophobic medium compared to hydrophilic one. It is consistent with the result that H(2)O(2)-micelles has a weaker interaction than H(2)O(2)-water because surfactant micelles are hydrocarbon-like organic phase rather than aqueous phase.

Kinetics of Oxidation of Glutathione by Micelle Trapped Tris(benzhydroxamato)iron(III)

Kinetics of the oxidation of glutathione (GSH) by micelle Sodium dodecyl sulphate (SDS) trapped tris(benzhydroxamato)iron(III) (TBH) at pH 3.3 and at 298 K has been described. The reaction is first order in TBS and GSH. Increase in pH decreased the rate. Order in H+ is one The reaction between TBH and GSH on micellar surface (Stern layer) differs from that occurring in bulk aqueous phase in absence of micelle (SDS). Reactive FeL3 species on micellar surface is FeL2+.

Interactions of Hydrophobically Modified Polyvinylamine with Pluronic Triblock Copolymer Micelles

The heats associated with the addition of triblock copolymers of ethylene oxide and propylene oxide (PEO-PPO-PEO pluronics) to solutions of linear polyvinylamine, with N-substituted pendant octyl groups (HMPVAm), were measured as functions of pH and pluronic structure. The interactions were exothermic with the overall enthalpy decreasing with increasing pH from 5 to 10. Surprisingly, the heat effect increased with increasing pluronic hydrophilicity; however, no enthalpy change was observed in the absence of micelles. The results were quantitatively modeled by assuming two competing processes-micellar dissolution and HMPVAm coating of micelles, preventing dissolution.

Surfactant Adsorption and Marangoni Flow in Liquid Jets. 3. Modeling in the Presence of Micellar Surfactant

We present a hybrid computational fluid dynamics (CFD) model of surfactant adsorption and Marangoni flow at the rapidly expanding surface of a gravity-driven, laminar liquid jet that takes account of the presence of micelles. Micellar diffusion and finite rates of micellar formation and degradation are included in the model. This hybrid CFD model, which is developed within the CFD code FIDAP, is an extension of one recently published hybrid CFD model for adsorption and Marangoni flow in the absence of micelles. A boundary-layer treatment of the jet flow near the nozzle exit-which assumes that the stress at the nozzle exit is not exceeded at any point on the jet surface-shows that the adsorption of surfactant causes the surface velocity, and also the surface concentration, to increase linearly from the point where the jet detaches from the nozzle. The two major results of this theory are used in the hybrid CFD model; these are that (i) the rate of surface expansion of the jet remains finite at the point of detachment and (ii) there is a finite surface concentration at the nozzle exit. The analytical solution for transport of surfactant monomers to the jet surface in the presence of micelles assumes infinitely fast micellar breakdown. In the numerical model, finite micellar breakdown rates can be accommodated. Only one micellar species is considered. The hybrid CFD model is used to establish the order of magnitude of the micellar disintegration rate constant and to study the effect of variations in the mean aggregation number of the micellar species. Computed results are presented for a micellar solution of aqueous C 16 TAB. These results are compared with experiments. Good agreement between the model and the experiments can be obtained when the rate constant for micellar breakdown takes a value O(10 2 ) s -1 , for an aggregation number of 90.

Remediation of a Diesel Contaminated, Sandy-Loam Soil Using Low Concentrated Surfactant Solutions (5 pp)

Surfactant enhanced ex-situ soil washing can be used to remediate diesel contaminated soils. Surfactants enhance the diesel removal from soils by two processes: mobilization and solubilization. Mobilization occurs at surfactant concentrations below the critical micelle concentration (CMC), while solubilization occurs at surfactant concentrations beyond the CMC of the surfactant. In this paper, the leaching of diesel from a self contaminated (10 000 mg diesel/kg) sandy-loam soil was studied using low concentrated non-ionic surfactant solutions of Tergitol NP-10 (10-6 to 10-3 mol/L). The surface tension of the supernatant solutions of soil suspensions in non-ionic surfactant (Tergitol NP-10) solutions (10-6 to 10-3 mol/L) was measured to determine the total surfactant concentration (surfactant sorbed onto the soil + surfactant in solution) at which micelles are present in the water phase of the soil suspension, or 'effective critical micelle concentration' (ECMC), and to calculate the sub-CMC sorption isotherm of Tergitol NP-10 onto the soil. The diesel removal was measured by soil leaching experiments with Tergitol solutions ranging from 10-6 to 10-3 mol/L. The sub-CMC sorption isotherm of Tergitol onto the soil could be described by the Freundlich equation. Even at very low surfactant concentrations (10-6 mol/L) the surfactant enhanced the diesel removal from the soil. Up to the surfactants' ECMC the diesel removal increased in a linear way with an increasing surfactant concentration in the wash water. A significant part (20%) of the diesel oil was removed in the presence of surfactant, but in absence of micelles. Beyond the surfactants' ECMC, the increase in diesel removal efficiency with increasing surfactant concentration was less pronounced. When the added surfactant concentration was increased to 10-3 mol/L, which corresponds to an equilibrium surfactant concentration in the supernatant solution (6.77.10-4 mol/L) above the CMC, the diesel oil removal was increased up to 50%. At this surfactant concentration emulsion formation was observed. Surfactant aided leaching of diesel from a self-contaminated, sandy-loam soil with surfactant solutions below their ECMC was able to remove in one step 20% of diesel from the soil by the mobilization mechanism. Succesive treatments or continuous leaching with surfactant solutions below their ECMC was not studied but is likely to reduce the diesel contamination further and at the same time avoid problems with emulsion formation.

Effect of urea on the enzymatic activity of a lipase entrapped in AOT–heptane–water reverse micellar solutions

A study has been made of the effect of urea upon the hydrolysis of 2-naphthyl acetate (2-NA) catalyzed by lipase from Rhizopus arrhizus in AOT–heptane–water reverse micellar solutions at pH 7. The partition constants, K, of 2-NA between n-heptane and aqueous urea solutions in the absence of micelles were also determined. It was found that K decreases when the concentration of urea increases. In aqueous solution the rate of hydrolysis of 2-NA catalyzed by lipase is dependent on the concentration of urea (at a given 2-NA concentration). This result can be due to a decrease in the magnitude of the association of lipase with 2-NA and/or to changes in the reaction rate of the lipase–2-NA complex. The modifications of the enzymatic activities elicited by addition of urea show a lineal correlation with K, emphasizing the relevance of hydrophobic effects in the loss of activity. Nevertheless, the slope of the line is higher than one, suggesting that changes in the conformation of the enzyme would be also important. Addition of urea to the micellar solutions provokes a decrease of the enzyme activity. From the dependence of the reaction rate with AOT concentration, the partition constant of 2-NA between n-heptane and the micelles, K p , was obtained. In the presence of 2 M urea a value of K p = 0.33 M −1 was derived. This value is lower than that measured in the absence of urea (Aguilar et al., Arch. Biochem. Biophys. 388 (2001) 231), indicating that incorporation of urea to the micellar interface produces a decrease of the association of 2-NA with the micelles. From a comparison of the results obtained in the micellar solution and in aqueous solution, it is concluded that the enzyme is more resistant to denaturation by urea in the micellar solution than in aqueous solution. Furthermore, at intermediate urea concentrations (2 M), the additive produces an increase in the Michaelis constant ( K M ) without a significant decrease (or even a small increase) in the catalytic rate constant ( k cat ).