Study of the structure of troponin-C by measuring the relative reactivities of lysines with acetic anhydride

The structure of troponin-C² has been studied by measuring the relative reactivity of lysines with acetic anhydride using a competitive labeling method. Troponin-C was acetylated free and complexed with troponin-I and -T in the native state with [³H]acetic anhydride and combined with [¹⁴C]troponin-C that had been acetylated in 6 m-guanidine · HCl. Peptides containing labeled lysines were isolated following chymotryptic and tryptic digestion and identified in the published sequence. The ${}^3\text{H}{}^{14}\text{C}$ ratio of these peptides was used as a measure of relative accessibility of the lysines. Troponin-C contains 9 lysine residues. In free troponin-C Lys20 was the least reactive and Lys153 was the most reactive; the remaining 7 had intermediate reactivities. Lys52 was more reactive in the presence of 10^?5m-Ca²⁺ than in 0.2 mm-EGTA (+2 mm-MgCl₂). When troponin-C was labeled in the native troponin complex, Lys20 and 153 were the least and most reactive, respectively. Peptides containing Lys52, (84, 88, 90) and (136, 140) were reduced in reactivity relative to Lys37 and 153, suggesting that these regions are involved in binding to the other troponin components. The reactivities of Lys37 and (136, 140) were influenced by the calcium ion concentration. A similar pattern of reactivities was seen when troponin-C was complexed with troponin-I and complex formation with troponin-T resulted in reduced reactivity of Lys52 and (84, 88, 90). The results are related to structural studies of troponin-C and to the predicted three-dimensional structure based on carp parvalbumin.     

Study of the structure of troponin-T by measuring the relative reactivities of lysines with acetic anhydride

The structure of troponin-T has been studied by measuring the relative reactivity of lysines with acetic anhydride using a competitive labeling method. Troponin-T was acetylated free and complexed with -I and -C in the native state with [³H]acetic anhydride, purified, and then combined with ¹⁴[C]troponin-T that had been acetylated in 6 m-guanidine · HCl. Peptides containing labeled lysines were isolated following chymotryptic and tryptic digestion and identified in the published sequence. The ${}^3\text{H}{}^{14}\text{C}$ ratio of these peptides was used as a measure of relative accessibility of the lysines. Troponin-T contains 39 lysines; we have identified 35 of these in 22 different peptides. The region of troponin-T influenced by binding to the other troponin components is extensive and includes the C-terminal half of the molecule as well as some residues in the N-terminal half. The lysines showing the greatest change in reactivity are concentrated between residues 114 to 223. The reactivities of the troponin-T lysines labeled in native troponin were not significantly influenced by the binding of calcium to the calcium-specific binding sites of troponin-C. A model for the structure of troponin-T is proposed based on the present and previous studies.     

Recombinant rat guanidinoacetate methyltransferase: Study of the structure by trace labeling lysine residues with acetic anhydride

• 1.1. The reactivities of lysine residues of recombinant rat guanidinoacetate methyltransferase were determined by trace labeling with acetic anhydride.
• 2.2. Lys-113 and -160 were weakly reactive and Lys-178 and -234 were unreactive toward the reagent. The six lysines (Lys-38, -83, -104, -108, -152 and -180) showed moderate reactivities. The N-terminal amino group was very reactive.
• 3.3. S-Adenosylmethionine did not alter the reactivities of lysines significantly, but the reactivity of Lys-38 was substantially reduced in the presence of S-adenosylmethionine and guanidinoacetate.

     

Effects of interaction with calcineurin on the reactivities of calmodulin lysines

Calmodulin was trace labeled by acetylation with [3H]acetic anhydride in the presence and absence of a 30% molar excess of the phosphatase calcineurin; phenylalanine was included in the reaction mixtures as an internal standard. The level of 3H acetylation of each of the 7 lysines was determined and corrected for differences arising from reaction conditions using the labeling of the internal standard, following procedures that are closely similar to those used in a previous study of the interaction of calmodulin with myosin light chain kinase (Jackson, A. E., Carraway, K. L., III, Puett, D., and Brew, K. (1986) J. Biol. Chem. 261, 12226-12232). The interaction with calcineurin was found to produce a 10-fold reduction in the acetylation of lysine 75, with lesser but significant effects on lysines 21 and 148. A small but reproducible perturbation of lysine 77 was also observed. The results are similar to those that are produced by the interaction with myosin light chain kinase. However, when they are compared with two recent reports between which there are major discrepancies (Manalan, A. S., and Klee, C. B. (1987) Biochemistry 26, 1382-1390; Winkler, M. A., Fried, V. A., Merat, D. L., and Cheung, W. Y. (1987) J. Biol. Chem. 262, 15466-15471), our results are in good agreement with those obtained in the former study. From the location of the perturbed groups in the three-dimensional structure of calmodulin, it appears that the interaction site on calmodulin for calcineurin, as well as for myosin light chain kinase, is very extended and may include hydrophobic pockets at homologous sites near the carboxyl-terminal ends of the two halves of the molecule.     

Differential reactivities of lysines in calmodulin complexed to phosphatase

Calmodulin and calmodulin complexed with calcineurin phosphatase were trace labeled with [3H]acetic anhydride and the incorporation of [3H]acetate into each epsilon-amino lysine of calmodulin was measured. The relative reactivities of calmodulin lysines were higher in the presence of Ca2+ than in the presence of EGTA, and the order was: Lys-75 greater than Lys-94 greater than Lys-148 greater than or equal to Lys-77 greater than Lys-13 greater than or equal to Lys-21 greater than Lys-30. The changes in relative reactivity implied a change in conformation. When calmodulin was complexed with the phosphatase, Lys-21, Lys-77, and Lys-148 were most protected, implying that these residues are at or near the interaction sites or are conformationally perturbed by the interaction. Lys-30 and Lys-75 were slightly protected, lysine 13 showed no change, while lysine 94 significantly increased in reactivity. Comparison with results obtained from myosin light chain kinase using a similar technique (Jackson, A. E., Carraway, K. L., III, Puett, D., and Brew, K. (1986) J. Biol. Chem. 261, 12226-12232) reveals that calmodulin may interact with each of the two enzymes similarly at or near Lys-21, Lys-75, and Lys-148; one difference with phosphatase is that complex formation also involved Lys-77. These findings suggest that calmodulin interacts differently with its target enzymes.     

Effect of glycerol on protein acetylation by acetic anhydride

We investigated the basis for the previously unexplained stabilization of proteins by glycerol during reaction with acetic anhydride [S. Siegel and W. M. Award, Jr. (1973) J. Biol. Chem. 248, 3233-3240]. Model studies showed that glycerol competes successfully for acetylation against protein hydroxyl groups. In contrast, amino groups are much more potent nucleophiles and their acetylation is not apparently affected. Since alpha-amino and phenolic pKa's did not change significantly in increasing glycerol concentrations, these findings are ascribed to glycerol's lower pKa value as compared to water, leading to the decreased acetylation of tyrosine, threonine, and serine hydroxyl groups in Pronase guanidine-stable chymoelastase. An additional mechanism is important and predominates in the protection against inactivation of bovine delta-chymotrypsin during acetylation and is explained by the recently described basis for protein stabilization in glycerol [K. Gekko and S. N. Timasheff (1981) Biochemistry 20, 4667-4676; 4677-4686]. Those studies demonstrated that glycerol increased the hydrophobicity of nonpolar residues, augmenting their tendency to be removed from protein surfaces. Therefore, the stabilization afforded by glycerol for chymotrypsin is attributed in part to a favoring of the native folded state which forces the side chains of isoleucine-16 and valine-17 to be buried, increasing the apparent pKa of the alpha-amino group of isoleucine-16 as it forms the charge pair with the beta-carboxyl group of aspartate-194. This conclusion was supported by stopped-flow analyses of the interaction of delta-chymotrypsin with proflavin in increasing concentrations of glycerol.     

Effects of the binding of myosin light chain kinase on the reactivities of calmodulin lysines

The effects of the binding of smooth muscle myosin light chain (MLC) kinase on the microenvironments of different regions of calmodulin (CaM) were investigated by comparing the acylation rate constants of the seven lysine amino groups of free CaM with those of CaM complexed with MLC kinase. Equimolar amounts of CaM and CaM-MLC kinase complex were trace labeled with [3H]acetic anhydride in the presence of phenylalanine as a standard nucleophile. After completion of the reaction, equal amounts of a trace 14C-acetylated CaM sample, together with [14C]acetylphenylalanine, were added to each reaction mixture. The 3H/14C-labeled CaM and acetylphenylalanine were then isolated from each solution. After complete reaction with nonradioactive acetylating reagent, 3H/14C ratios (r) were determined for each epsilon-N-acetyllysine in the two CaM samples. These values were obtained either from isolated peptide fragments containing one lysine or from epsilon-N-acetyl phenylthiohydantoin lysine obtained by Edman degradation of peptide fragments containing two lysines. From the ratios, protection factors (= rfree/rcomplex) were determined as a measure of the perturbation produced by MLC kinase binding. These protection factors were corrected, using the isotope ratios of the internal standard, for differences in the degree of competition for labeling reagent between the two mixtures. In two separate labeling experiments employing different levels of trace labeling, very little change was observed in the reactivities of four lysines on MLC kinase binding (lysines 13, 30, 77, and 94). Small but reproducible decreases (about 2-fold) were observed in the reactivities of lysines 21 and 148, while lysine 75 underwent a major (more then 7-fold) decrease in labeling. In conjunction with previously published data, these results are interpreted as suggesting that the major perturbation in lysine 75 is a direct effect of MLC kinase contact with CaM and that a region in the central helix containing this residue, but not lysine 77, represents or is near the CaM-binding site for MLC kinase. The smaller changes in reactivities at lysines 21 and 148 may reflect a conformational change that occurs in CaM as a result of binding to MLC kinase.     

Radiolabeling of proteins and viruses in vitro by acetylation with radioactive acetic anhydride.

We describe a convenient, rapid, and reproducible method for labeling proteins in vitro by acetylation with [3H] or [14-C]acetic anhydride dissolved in small amounts of anhydrous dioxane. The reaction is carried out at neutral pH and does not require the use of detergents, water-immiscible organic solvents, oxidizing, or reducing agents. Thus undesirable solvent-induced alterations in protein structure and biological activity are minimized. A method for calculating the specific activity of the protein and the efficiency of acetylation at known concentrations of protein and acetic anhydride is presented. Radioacetylated proteins were shown to be suitable for use as molecular weight calibration standards and as protein markers in polyacrylamide gel electrophoresis, gel filtration, and enzyme studies. Acetic anhydride was used to label intact oncornaviruses, which consist of a complex ribonucleo-protein core within a lipid envelope. Some of the viral lipid and all of the viral proteins, including the internal ones, were labeled without detectable alterations in viral morphology or buoyant density. This result suggests that acetic anhydride, evidently by virtue of its small size and neutral charge, penetrates freely throughout the viral membrane and core structures. The reactivity of RNA with acetic anhydride was less than 1% that of protein under similar reaction conditions.     

Dissociation of single-stranded DNA from nucleosomes following modification with acetic anhydride

Modification with acetic anhydride of nucleosomes from chicken erythrocytes at low ionic strength (less than 0.1 M NaCl) is accompanied by the formation of residual particles and the release of free DNA. This DNA has been identified as single-stranded by thermal denaturation, digestion with nuclease S1, and elution from hydroxyapatite. In contrast, if modification takes place at 0.6 M NaCl, the liberated DNA is mainly double-stranded. The release of the free energy stored in folded nucleosomal DNA, triggered by the weakening of lysine-DNA interactions which takes place upon modification, might be responsible for the observed denaturation of DNA at low ionic strength.     

C-terminal sequencing method for proteins in polyacrylamide gel by the reaction of acetic anhydride

A successive C-terminal amino acid truncation reaction with acetic anhydride was applied on proteins in polyacrylamide gel. Protein bands separated by conventional SDS-PAGE were excised, partially fixed in the gel with glutaraldehyde ethanol solution, dehydrated with ACN and subjected to the truncation reaction with acetic anhydride formamide solution. Pre-treatment of the gel with pyridine aqueous solution was found to enhance the truncation reaction yields. After the truncation reaction, the products were treated with an aqueous solution of dimethylaminoethanol to hydrolyze oxazolone rings at the C termini of the truncated products and O-acetylated products of serine, threonine and/or tyrosine. Several commercially available proteins of 10-40 kDa, as determined by SDS-PAGE, such as myoglobin, trypsin inhibitor, alpha-hemolysin, cytochrome c, chymotrypsin C chain, elastase, acylase and histone H4, were subjected to the C-terminal analysis. The truncated proteins were in-gel digested with trypsin and the extracted peptides were analyzed by MALDI-TOF MS, giving rise to a series of molecular mass ions of the C-terminal truncated fragments corresponding to the C-terminal amino acid sequence of the relevant protein.     

A modification of neurotoxin RTX-III from Radianthus macrodactylus actinin with acetic anhydride

Upon modification of neurotoxin RTX-III at amino groups with [3H]acetic anhydride, four monoacetyl and four diacetyl derivatives have been obtained. Acetylation of the N-terminal amino group led to 12-fold decrease of toxicity in mice. Monoderivatives of the toxin with either Lys or two out of three C-terminal Lys residues modified showed a 2-fold drop in toxicity as compared with the native RTX-III. Diacetylation caused a 30 to 35-fold decrease in toxicity, the N-terminal amino group being modified in all the derivatives. As assessed by circular dichroism method, the modification of amino groups, except for N-terminal one, affected secondary structure of the toxin. The data suggest the N-terminal amino group to be essential for toxicity of RTX-III.     

The solvolysis of topotecan in alcohols and acetic anhydride

Six derivatives of 10-hydroxycamptothecin were prepared via solvolysis of topotecan in corresponding alcohols and acetic anhydride. We attributed the specific reactivity of topotecan to the internal hydrogen-bonding between 10-hydroxy and the nitrogen atom in position 9. As a result the reaction underwent through an intermediate ortho-quionomethlide species to reach equilibrium. Bioactivity screening data showed all products could potentially inhibit the proliferation of several cancer cell lines in vitro and a bigger size group in 9-position would be favorable for the anti-tumor activities observably.     

Simultaneous quantification of lyso-neutral glycosphingolipids and neutral glycosphingolipids by N-acetylation with [3H]acetic anhydride

We describe a new method that permits quantification in the pmol to nmol range of three lyso-neutral glycosphingolipids (lyso-n-GSLs), glucosylsphingosine (GlcSph), galactosylsphingosine (GalSph), and lactosylsphingosine, in the same sample as neutral glycosphingolipids (n-GSLs). Lyso-n-GSLs and n-GSLs are initially obtained from a crude lipid extract using Sephadex G25 chromatography, followed by their isolation in one fraction, which is devoid of other contaminating lipids, by aminopropyl solid-phase chromatography. Lyso-n-GSLs and n-GSLs are subsequently separated from one another by weak cation exchange chromatography. N-GSLs are then deacylated by strong alkaline hydrolysis, and the N-deacylated-GSLs and lyso-n-GSLs are subsequently N-acetylated using [3H]acetic anhydride. An optimal concentration of 5 mM acetic anhydride was established, which gave >95% N-acetylation. We demonstrate the usefulness of this technique by showing an approximately 40-fold increase of both GlcSph and glucosylceramide in brain tissue from a glucocerebrosidase-deficient mouse, as well as significant lactosylceramide accumulation.The application and optimization of this technique for lyso-n-GSLs and lyso-GSLs will permit their quantification in small amounts of biological tissues, particularly in the GSL storage diseases, such as Gaucher and Krabbe's disease, in which GlcSph and GalSph, respectively, accumulate.     

Labeling of high density lipoproteins with [3H] acetic anhydride.

Rat serum HDL was labeled by reaction with [3H] acetic anhydride at pH 7.2 for 30 min at room temperature by a modification of the method of Montelaro and Rueckert (1975. J. Biol. Chem. 250: 1413). Protein specific activities of 60 dpm/ng were achieved. Seven percent of the label was in lipid, of which 92 percent was recovered in phospholipid. The labeled HDL migrated as a single band as seen by electrophoretic or column chromatographic analysis. When the labeled HDL was injected into rats without re-isolation, the biological half-life was not significantly different from HDL labeled in vitro with 125I or in vivo with amino acids. All of the apoproteins were labeled; their specific activities were closer to one another than those obtained with 125I. For some applications, acetylation may provide a useful alternative to the 125I labeling procedure.     

C-terminal sequencing method for peptides and proteins by the reaction with a vapor of perfluoric acid in acetic anhydride

A successive C-terminal amino acid truncation reaction of peptides and proteins with a vapor generated from a low-concentrated perfluoric acid in acetic anhydride is presented. The reaction products were analyzed with matrix-assisted laser desorption/ionization-time of flight mass-spectrometry giving molecular mass ions of the C-terminal truncated peptides or proteins from which the C-terminal sequence information can be deduced. Acetylation reaction preceded the truncation reaction in order to protect the amino groups and other reactive groups in peptides and proteins, and after the truncation reaction, hydration reaction was carried out to afford cleaner mass spectra.