Effects of pH on structural and functional properties of porin from the outer membrane of Yersinia pseudotuberculosis. II. Characterization of pH-induced conformational intermediates of yersinin

pH-Induced intermediates of Omp F-like porin from the outer membrane of Yersinia pseudotuberculosis (yersinin) were characterized by fluorescence and fluorescent probe spectroscopy and circular dichroism. The most dramatic changes in the intrinsic fluorescence of the protein induced by pH titration correlated with different conformational states of the porin molecule. pH-induced conformational transitions of yersinin can be described in terms of a three-state model: (1) disordering of porin associates and formation of porin trimers structurally similar to the native protein; (2) unfolding of individual porin domains followed by cooperative dissociation of trimers into monomers; (3) formation of two loosely structured forms of monomer intermediates. It is assumed that one of these monomeric forms (at pH 3.0) corresponds to the molten-globule state of porin with native secondary structure, while the other one (at 2.0) represents a partly denatured (misfolded) monomer, which retains no more than 50% of the regular secondary structure. The putative mechanism of low pH-induced β-barrel unfolding is discussed in terms of a theoretical model of yersinin spatial structure.     

Empirical evaluation of the influence of side chains on the conformational entropy of the polypeptide backbone

Changes in amino acid side chains have long been recognized to alterthe range and distribution of ?, ψ angles found in the main chain of polypeptides. Altering the range and distribution of ?, ψ angles also alters the conformational entropy of the flexible denatured state and may thus stabilize or destabilize it relative to the comparatively conformationally rigid native state. A database of 12,320 residues from 61 nonhomologous, high resolution crystal structures was examined to determine the ?, ψ conformational preferences of each of the 20 amino acids. These observed distributions in the native state of proteins are assumed to also reflect the distributions found in the denatured state. The distributionswere used to approximate the energy surface for each residue, allowing the calculation of relative conformational entropies for each residue relative to glycine. In the most extreme case, replacement of glycine by proline, conformational entropy changes will stabilize the native state relative to the denatured state by ?0.82 ± 0.08 kcal/mol at 20°C. Surprisingly, alanine is found to be the most ordered residue other than proline. This unexpected result is a result of the high percentage of alanines found in helical conformations. This either indicates that the observed distributions in the native state do not reflect the distributions in the denatured state, or that alanine is much more likely to adopt a helical conformation in the denatured state than residues with longer side chains. Among those residues with ?, ψ angles compatible with helix incorporation the percentage of alanines actually in helices is very similar to other residues. This and the consistent ordering of alanine relative to other residues regardless of secondary structure are evidence that ?, ψ distributions in native states reflect those in the denatured states. © 1995 Wiley-Liss, Inc.     

Thermodynamics of protein denatured states

Recent work on the thermodynamics of protein denatured states is providing insight into the stability of residual structure and the conformational constraints that affect the disordered states of proteins. Current data from native state hydrogen exchange and the pH dependence of protein stability indicate that residual structure can modulate the stability of the denatured state by up to 4 kcal mol(-1). NMR structural data have emphasized the role of hydrophobic clusters in stabilizing denatured state residual structures, however recent results indicate that electrostatic interactions, both favorable and unfavorable, are also important modulators of the stability of the denatured state. Thermodynamics methods that take advantage of histidine-heme ligation chemistry have also been developed to probe the conformational constraints that act on denatured states. These methods have provided insights into the role of excluded volume, chain stiffness, and loop persistence in modulating the conformational preferences of highly disordered proteins. New insights into protein folding and novel methods to manipulate protein stability are emerging from this work.     

Flexibility and viscometric studies of globular proteins ovalbumin and ovotransferrin

The ultrasonic velocity, density and viscosity of two egg proteins, ovalbumin and ovotransferrin in phosphate buffer have been studied at the physiological pH values. The thermodynamic functions for unfolding, ellipticity, surface amino acid residues and compressibility have been obtained for thermal and chemical denaturation in these food proteins. The computed values of Huggin's constant and shape factor, at a fixed ionic strength 0.16 M are found to be in agreement with the reported values for globular proteins. The slow increase in free-energy of unfolding with temperature at a fixed pH 7 suggests uncoiling and in turn, disappearance of biological activity. It has been observed that the effects of temperature and chemical denaturant on the native protein may give rise to different conformational states. In the presence of urea and sodium dodecyl sulphate (SDS), the proteins gave the excessively denatured states at 25 degrees C and pH 7, in comparison to the thermal denatured state. The positive values of partial adiabatic compressibility (see symbol in text) beta s over the temperature range 45-75 degrees C suggest the possibility of large internal flexibility in ovotransferrin than in ovalbumin.     

Effect of succinylation (3-carboxypropionylation) on the conformation and immunological activity of ovalbumin.

The epsilon-amino groups of ovalbumin were modified with succinic anhydride; as many as 16 lysine residues were succinylated (3-carboxypropionylated). The five succinylated derivatives thus prepared were homogeneous with respect to the extent of chemical modification as shown by electrophoretic and immunological data. Succinylation of the amino groups altered electrophoretic mobility and isoionic pH of ovalbumin in the expected direction. U.v.-absorption and fluorescence spectra suggested changes in the microenvironment of the chromophores in the modified proteins. The difference-spectral results showed greater exposure of tyrosine and tryptophan residues in the succinylated ovalbumin. Increase in susceptibility to tryptic digestion, Stokes radius and intrinsic viscosity of native ovalbumin, which was observed on successive increase in the chemical modification, demonstrated a conformational change that was proportional to the extent of modification. The loss of immunological reactivity caused by chemical modification also indicated a conformational change in succinylated ovalbumin. The fact that the intrinsic viscosity of maximally modified ovalbumin was less than one-third of that for the completely denatured protein in 6M-guanidinium chloride suggested that the modified protein contained significant residual native structure. The latter presumably accommodates some antigenic determinants accounting for 37% residual immunological activity observed with maximally succinylated ovalbumin.     

Chemical modification of the anion selectivity of the PhoE porin from the Escherichia coli outer membrane

The PhoE porin of Escherichia coli is induced by phosphate deprivation and when purified, forms moderately anion-selective channels in lipid bilayer membranes. To further investigate the basis of anion selectivity, PhoE was chemically acetylated with acetic anhydride. Acetylation modified the mobility and staining characteristics of the PhoE porin on SDS-polyacrylamide gel electrophoresis but the acetylated protein was still found in its normal trimeric state after solubilization in SDS at low temperatures. Furthermore, the acetylated PhoE porin retained its ability to reconstitute into lipid bilayer membranes and the single channel conductance in 1 M KCl was unaltered. Zero-current potential measurements demonstrated that whereas the native PhoE porin was anion-selective, a 30-40-fold increase in preference for cations upon acetylation resulted in the acetylated PhoE porin being cation-selective. Increasing the pH of KCl solutions bathing lipid bilayer membranes from pH 3 to pH 6 caused symmetrical 4-fold increases in the selectivity of both the native and acetylated PhoE proteins for cations. In contrast, increasing the pH from 7 to 9 caused a 2.5-fold increase in selectivity only for the native PhoE porin. These results suggest that the basis of anion selectivity in the native PhoE porin is fixed protonated amino groups (possibly on lysines) in or near the channel, and furthermore indicate that deprotonated carboxyl groups have a strong influence on ion selectivity.     

Compact denatured state of a staphylococcal nuclease mutant by guanidinium as determined by resonance energy transfer.

The protein from a mutant clone of staphylococcal nuclease with a cysteine substituting for a lysine at position 78 was prepared and labeled with a cysteine-specific fluorescent probe 5-[[2-[(iodoacetyl)-amino]ethyl]amino]naphthalene-1-sulfonic acid (IAEDANS). Time-resolved nonradiative energy-transfer studies were done using the single tryptophan at position 140 as the energy donor and the IAEDANS as the receptor. Changes in distance and distance distributions were observed as a function of increasing guanidinium (GuHCl) concentration (0-2 M) and in the presence or absence of Ca2+ and inhibitor 2'-deoxythymidine 3',5'-diphosphate (pdTp). In the native state, both the ternary complex and the noncomplexed protein are best fit with one population having an average donor-acceptor distance of approximately 23 A and an "apparent" full width at half-maximum (fwhm) of distance distribution of approximately 18 A. Besides the contribution of linker arm of the acceptor, it appears that there are some conformational heterogeneties either due to the disordering of the tryptophan region or due to the whole protein in the native state. During GuHCl unfolding, the average distance remains relatively constant up to GuHCl concentrations where both the ternary complex and the ligand-free protein are denatured (1-1.3 M). The compact denatured states persist up to 2 M GuHCl. At 2 M GuHCl, the heterogeneity of the denatured state in the ternary complex is much larger than that of the ligand-free nuclease. The results show that the denatured states of staphylococcal nuclease mutant K78C by GuHCl are compact and these compact denatured states are likely due to residual structures or incompletely disrupted hydrophobic cores under these conditions.     

Protein aggregation and amyloid fibril formation by an SH3 domain probed by limited proteolysis

The SH3 domains are small protein modules of 60-85 amino acid residues that are found in many proteins involved in intracellular signal transduction. The SH3 domain of the p85alpha subunit of bovine phosphatidylinositol 3'-kinase (PI3-SH3) under acidic solution adopts a compact denatured state from which amyloid fibrils are readily formed. This aggregation process has been found to be modulated substantially by solution conditions. Here, we have analyzed the conformational features of the native and acid denatured states of PI3-SH3 by limited proteolysis experiments using proteinase K and pepsin, respectively. Moreover, we have analyzed the propensity of PI3-SH3 to be hydrolyzed by pepsin at different stages in the process of aggregation and amyloid formation at pH 1.2 and 2.0 and compared the sites of proteolysis under these conditions with the conformational features of both native and aggregated PI3-SH3. The results demonstrate that the denatured state of PI3-SH3 formed at low pH is relatively resistant to proteolysis, indicating that it is partially folded. The long loop connecting beta-strands b and c in the native protein is the region in this structure most susceptible to proteolysis. Remarkably, aggregates of PI3-SH3 that are formed initially from this denatured state in acid solution display enhanced susceptibility to proteolysis of the long loop, suggesting that the protein becomes more unfolded in the early stages of aggregation. By contrast, the more defined amyloid fibrils that are formed over longer periods of time are completely resistant to proteolysis. We suggest that the protein aggregates formed initially are relatively dynamic species that are able readily to reorganize their interactions to enable formation of very well ordered fibrillar structures. In addition, the disordered and non-native character of the polypeptide chains in the early aggregates could be important in determining the high cytotoxicity that has been revealed in previous studies of these species.     

Transient conformational states in proteins followed by differential labeling

Refolding of previously denatured and reduced elastase has been followed by titration of chemical reactivities of amino acid side chains to study the topography of the protein in the native state, and the microenvironment variations of protein side chains during the structural transition. Groups accessible to chemical reagents in the denatured form and buried in the "native" form were used as a local conformational probe. Times of labeling, depending on the reagent used, ranged from 100 to 800 ms. The reaction was stopped by isotopic dilution with an excess of unlabeled reagent under denaturing conditions to obtain a chemically homogeneous but heterogeneously labeled material. Peptide fractionation after degradation of the labeled proteins allowed the determination of the amount of radioactive label incorporated by the individual side chains during the refolding. Refolding rates, determined by physicochemical, enzymatic or immunochemical criteria, were compared with the conformational states of protein areas and evaluated by the variation of chemical reactivity at various denaturant concentrations. The importance of the last folding stages is emphasized by the results obtained which indicate that early during the refolding, two domain substructures (H-40 to H-71 and M-180 to H-200)( are stabilized, while the protein remains inactive at the time ranges of the labeling reactions.     

Photosensitized oxidation of elastase. Selective modification of the tryptophyl residues

Spectral studies demonstrated that acidic pH values induce a two-step denaturation of porcine elastase, the first conformational transition occuring over the pH range 4.2–3.8, the second between pH 3.3 and 2.9. The proflavine-sensitized photooxidation of elastase in its native state, as well as in its denatured conformations, allowed us to isolate elastase derivatives selectively modified at given tryptophyl residues, hence to draw reliable conclusions about their degree of burial inside the protein matrix and their functional and conformational role. In particular, tryptophan-26 and -164 are located at the surface of the protein molecule, and their oxidation to N-formylkynurenine has no appreciable effect on the elastolytic activity and three-dimensional geometry of elastase. Tryptophan-83 is partially shielded from the aqueous environment; its modification affects only slightly the enzymic efficiency, while the tertiary structure of the protein perhaps increases its rigidity. Tryptophan-12 must be largely buried in internal regions, since its photooxidation is possible only after the native elastase structure has been extensively randomized; its indole ring appears to be of critical importance for the enzymic activity and the conformational stability of elastase. Finally, our data suggest that tryptophan-39, -132, and -232 are deeply buried; consequently, we failed to achieve the specific or preferential modification of these residues.     

Structural and drug-binding properties of alpha(1)-acid glycoprotein in reverse micelles

Alpha(1)-acid glycoprotein (AGP) is a glycoprotein that consists of 183 amino acid residues and five carbohydrate chains and binds to neutral and basic drugs. We examined the structural properties and ligand-binding capacity of AGP in interactions with reverse micelles. Also, detailed information was obtained by comparing several different states of AGP. Interaction with reverse micelles induced a unique conformational transition (beta-sheet to alpha-helices) in AGP and decreased the binding capacity for the basic drug, chlorpromazine and the steroid hormone, progesterone to AGP. These structural conformations are very similar to those observed under conditions of acidity and high ionic strength (pH 2.0, 1.5 M NaCl). This structure seems to be an intermediate between the native state and the denatured state, possibly a molten globule. The present results suggest that when AGP interacts with the biomembrane, it undergoes a structural transition to a unique structure that differs from the native and denatured states and has a reduced ligand-binding capacity.     

Acid- and base-induced conformational transitions of equinatoxin II

We have investigated the acid- and base-induced conformational transitions of equinatoxin II (EqTxII), a pore-forming protein, by a combination of CD-spectroscopy, ultrasonic velocimetry, high precision densimetry, viscometry, gel electrophoresis, and hemolytic activity assays. Between pH 7 and 2, EqTxII does not exhibit any significant structural changes. Below pH 2, EqTxII undergoes a native-to-partially unfolded transition with a concomitant loss of its rigid tertiary structure and the formation of a non-native secondary structure containing additional alpha-helix. The acid-induced denatured state of EqTxII exhibits a higher intrinsic viscosity and a lower adiabatic compressibility than the native state. Above 50 degrees C, the acid-induced denatured state of EqTxII reversibly denatures to a more unfolded state as judged by the far UV CD spectrum of the protein. At alkaline pH, EqTxII undergoes two base-induced conformational transitions. The first transition occurs between pH 7 and 10 and results in a partial disruption of tertiary structure, while the secondary structure remains largely preserved. The second transition occurs between pH II and 13 and results in the complete loss of tertiary structure and the formation of a non-native, more alpha-helical secondary structure. The acid- and base-induced partially unfolded states of EqTxII form water-soluble oligomers at low salt, while at high salt (> 350 mM NaCl), the acid-induced denatured state precipitates. The hemolytic activity assay shows that the acid- and base-induced denatured states of EqTxII exhibit significantly reduced activity compared to the native state.     

Side-chain conformations in an unfolded protein: chi1 distributions in denatured hen lysozyme determined by heteronuclear 13C, 15N NMR spectroscopy.

Using a 13C and 15N-labelled sample, multi-dimensional heteronuclear NMR techniques have been carried out to characterise hen lysozyme denatured in 8 M urea at pH 2.0. The measurement of 3J(C',Cgamma) and 3J(N,Cgamma) coupling constants has enabled side-chain chi1 torsion angle populations to be probed in the denatured polypeptide chain. Analysis of the coupling constant data has allowed the relative populations of the three staggered rotamers about chi1 to be defined for 51 residues. The amino acids can broadly be divided into five classes that show differing side-chain conformational preferences in the denatured state. These range from a strong preference for the -60 degrees chi1 rotamer for methionine and leucine (74-79 % population) to a favouring of the +60 degrees chi1 rotamer for threonine (67 % population). The differences in behaviour reflect the steric and electrostatic characteristics of the side-chains concerned. A close agreement is seen between the chi1 populations calculated from the experimental coupling constant data and predictions from the statistical model for a random coil that uses the chi1 torsion angle distributions in a data base of native protein structures. Short-range interactions therefore dominate in determining the local conformational properties of side-chains in a denatured protein. Deviations are, however, observed for many of the aromatic residues involved in hydrophobic clusters within the denatured protein. For these residues the effects of additional non-local interactions in the clusters presumably play a major role in determining the chi1 preferences.     

Digestion of native collagen, denatured collagen, and collagen fragments by extracts of rat liver lysosomes.

Extracts of highly purified lysosomes from rat liver were examined for their ability to degrade native collagen and thermally denatured collagen at pH values between 3.5 and 7.0. After a 24-h digestion at 36 degrees with the lysosomal extract at a pH of 5.5 or lower (collagen/lysosomal protein; 2/1 or 8/1), both native and denatured collagen were degraded to an extent equivalent to 60 to 70% of that observed upon total acid hydrolysis in 6 N HCl as measured by the ninhydrin reaction (570 nm). At a pH of 6.0, native collagen and denatured collagen were degraded by the mixture of lysosomal proteinases to 11% and 40% of total acid hydrolysis, respectively. At pH 6.5 AND 7.0, the corresponding values were 3% versus 33% and 0.3% versus 11%, respectively. Fragments of collagen (TCA and TCB) are produced when mammalian collagenase degrades native collagen at 25 degrees. These fragments were degraded by the lysosomal extract at 36 degrees to an extent equivalent to 28% and 8% of total acid hydrolysis at pH 6.5 and 7.0, respectively. The experiments at pH 6.5 and 7.0 were done using a collagen/lysosomal protein ratio of 2/1. At pH 5.0 (a pH which is found within secondary lysosomes), the lysosomal extracts degraded collagen to a mixture of free amino acids and small peptides. Amino acid analysis established that approximately 30% of the amino acid residues of the collagen appeared in the lysosomal hydrolysate as free amino acids. Hydroxyproline and perhaps hydroxylysine were the only amino acids found in collagen which did not appear at least to some extent as the free amino acid in this hydrolysate.     

Heat capacity and conformation of proteins in the denatured state

Heat capacity, intrinsic viscosity and ellipticity of a number of globular proteins (pancreatic ribonuclease A, staphylococcal nuclease, hen egg-white lysozyme, myoglobin and cytochrome c) and a fibrillar protein (collagen) in various states (native, denatured, with and without disulfide crosslinks or a heme) have been studied experimentally over a broad range of temperatures. It is shown that the partial heat capacity of denatured protein significantly exceeds the heat capacity of native protein, especially in the case of globular proteins, and is close to the value calculated for an extended polypeptide chain from the known heat capacities of individual amino acid residues. The significant residual structure that appears at room temperature in the denatured states of some globular proteins (e.g. myoglobin and lysozyme) at neutral pH results in a slight decrease of the heat capacity, probably due to partial screening of the protein non-polar groups from water. The heat capacity of the unfolded state increases asymptotically, approaching a constant value at about 100 degrees C. The temperature dependence of the heat capacity of the native state, which can be determined over a much shorter range of temperature than that of the denatured state and, correspondingly, is less certain, appears to be linear up to 80 degrees C. Therefore, the denaturational heat capacity increment seems to be temperature-dependent and is likely to decrease to zero at about 140 degrees C.     

Assessing native and non-native conformational states of a protein by methylene carbene labeling: the case of Bacillus licheniformis beta-lactamase

Much knowledge of protein folding can be derived from the examination of the nature and size of solvent-exposed surfaces along conformational transitions. We exploit here a general photochemical modification with methylene carbene of the accessible surface area (ASA) of the polypeptide chain. Labeling of Bacillus licheniformis beta-lactamase (BL-betaL) with 1 mM 3H-diazirine yielded 8.3 x 10(-3) mol CH2/mol protein, in agreement with the prediction for an unspecific surface labeling phenomenon. The unfolded state U in 7 M urea was labeled 60% more than the native state N. This result lies well below the increment of ASA expected from theoretical estimates and points to the presence of residual organization in state U and/or of cavities or crevices favoring the partition of the reagent in state N. A partially folded state I was demonstrated from two sequential transitions occurring at 1.5-3.0 M and 3.5-6.5 M urea. This technique shows a close correlation with optical probes most sensitive to changes in tertiary structure, a statement supported by the fact that the largest change occurs along the N-I portion of the N-I-U transition and along the acid pH-induced N-A transition. In the latter case, state A is labeled 70% more than state N, an increment consistent with the loosening of tight interactions in the core of the protein. Fragmentation of labeled BL-betaL into peptides provides a sequential map of solvent accessibility. Thus, amino acid residues pertaining to the Omega-loop and to helices alpha5 and alpha6 line the major cavity of the protein, that is big enough to lodge the diazirine reagent. Methylene labeling, by introducing an original (and perhaps unique) experimental measurement of ASA, enlightens subtle aspects of complex transitions and makes possible a comparative structural characterization of the native as well as non-native states.     

Application of the pH-jump method to the titration of tyrosine residues in bovine alpha-lactalbumin.

A stopped-flow technique has been developed for the zero-time spectrophotometric titration of tyrosine residues in the purely native or in the purely alkaline denatured state of alpha-lactalbumin that undergoes an alkaline conformational transition in the pH region of tyrosine ionization. The progressive absorption change at 298 nm caused by a pH jump from neutral pH is shown to result from the change in ionization of the tyrosine residues brought about by a first-order process of the conformational transition. Extrapolation to zero time gives the titration curve for purely native alpha-lactalbumin. Similarly, the pH jump from highly alkaline pH gives the titration curve for the purely alkaline denatured protein. The method should be generally applicable to other proteins that contain tyrosines. Analysis of the titration curves suggests that the four tyrosines in native alpha-lactalbumin have pK values of 10.5, 11.8, 11.8, and 12.7, respectively. After the alkaline transconformation, all of them become titrated normally with a pK value of 10.3. A comparison of these results with the ionization behavior of tyrosines in hen egg white and human lysozymes is presented and discussed in terms of differences in the sequences of the proteins.     

Influence of the pH on the photodynamic effect in lysozyme A comparative kinetic study with the sensitized photooxidation of isolated amino acids

Summary The kinetics of the eosin-sensitized photooxidation ([O₂(¹_g)]-mediated) of the protein lysozyme (Lyso) was investigated under two different pH conditions (pH 7 and pH 11). Rates of oxygen consumption and the fade in the protein fluorescence spectrum upon sensitized irradiation were monitored. Parallel studies on both denatured Lyso (absence of the four-S-S- bridges in the protein) and different mixtures of the photooxidizable amino acids of Lyso were also carried out. The mixtures maintained the same molar ratio as in the native protein, and were selected just in order to throw into relief the preferential amino acids that were being photooxidized at both pH values.Under work conditions Lyso was only photooxidizable at pH 7, whereas the opposite accounted for the denatured protein: only measurable oxygen consumption was detected at pH 11. Nevertheless, Lyso at pH 11, evidenced an important physical quenching of O₂(¹_g) due to the Tyr and Trp residues.The results for the native protein were interpreted on the basis of a previously described dark complex Eosin-Lyso, which selectively favours the photooxidation of the bounded protein. The Trp residues were the main reactive entities in the native protein. The photodinamic effect in denatured Lyso was characterized by the prevalence of Tyr residues as photooxidizable targets.In the discussion of the results, a comparisson with the photooxidation kinetics of the mixtures of free amino acids was made.Abbreviations O₂(³_g ^–) ground state triplet oxygen - O₂(¹_g) singlet molecular oxygen - Lyso lysozyme - LysoD denatured lysozyme - Eos eosin - FFA furfuryl alcohol - Trp tryptophan - Tyr tyrosine - Cys cysteine - Cis cystine - Met methionine - His histidine - AA amino acid - a.u. arbitrary units     

Spectrophotometric titration of phenolic groups of pepsin.

The ionization of tyrosine residues in diazotized pepsin under various solvent conditions was studied. All tyrosyl residues of the protein titrated normally with a pK of 10.02 in 6 M guanidine hydrochloride solution. On the other hand, two stages in the phenolic group titration curve were observed for the inactivated protein in the absence of guanidine hydrochloride; only about 10 tyrosine residues ionized reversibly up to pH 11, above which titration was irreversible. The irreversible titration zone corresponds to the pH range 11--13 in which unfolding, leading to the random coil state, was shown to occur by circular dichroism and viscosity measurements. The number of tyrosine residues exposed in the native and alkali-denatured (pH 7.5) states of diazotized protein were also studied by solvent perturbation techniques; 10 and 12 groups are exposed in the native and denatured states, respectively.     

Reversible fluorescence labeling of amino groups of protein using dansylaminomethylmaleic anhydride

The reversible fluorescence labeling of insulin, catalase and lysozyme has been demonstrated. As a derivatizing reagent, dansylaminomethylmaleic acid (DAM) has been used after investigating the precolumn and precapillary derivatization conditions. This reagent (DAM) reacts with the amino groups of proteins via its anhydride in the presence of a suitable dehydrating reagent, which then could be liberated under mild acidic conditions and the native proteins are regenerated. After the derivatization of insulin, catalase and lysozyme with DAM, no peaks of these native proteins were observed while several peaks of the derivatized proteins due to the multiple labeling were observed. However, after the regeneration, increasing amounts of the native proteins were observed as the regeneration period increased. For the lysozyme, the bacteriolytic activity of the enzyme decreased after the derivatization, and only 0.9% of the activity remained. The activity increases by the regeneration, and 95.6% of the bacteriolytic activity of the native enzyme was observed after a 48-h regeneration at pH 2.5 and 40 degrees C.