[~7] REACTION OF PROTEIN SULFHYDRYL GROUPS 457

[37] Reaction of Protein Sulfhydryl Groups with Ellman's Reagent

By A. F. S. A. HJ~BE~a

The reagent 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB) was devel- oped by Ellman as a sulfhydryl reagent. 1 DTNB has been found to be a sensitive tool for the assay of thiol groups in tissues, body fluids, and proteins.

Ellman 1 described a method for its synthesis. However, the reagent is commercially available. DTNB is an aromatic disulfide, and, since it has a higher standard oxidation-reduction potential than aliphatic analogs, it will react with aliphatic thiols by an exchange reaction to form a mixed disulfide of the protein and 1 mole of 2-nitro-5-thiobenzoate per mole of protein sulfhydryl group (reaction 1).

P--SH +

P-- S--S ~ c o ~ O 2

+ (1)

Sulfhydryl-disulfide exchange reactions between disulfide compounds with sulfur directly attached to aromatic groups and simple alkyl mercap- tans should go to completion? Protein SH groups may behave similarly to simple alkyl mercaptans unless steric factors interfere with the course of reaction. Therefore, reaction (1) could be followed by reaction (2). Intramolecular and intermolecular disulfide formation may be considered

+ P--SH ~ -S--~ /~x---NO2 + P--S--S--P ~ _ _ _ _ ~

"CO0- (2)

1G. L. Ellman, Arch. Biochem. Biophys. 82, 70 (1959). 2 A. J. Parker and N. Kharasch, Chem. Rev. 59, 583 (1959).

458 MODIFICATION REACTIONS [37]

a possibility. Indeed, Kleppe and Damjanovich 3 observed a heavy mo- lecular weight component as a result of treatment of muscle phosphoryl- ase b with DTNB, suggesting that inter- and intramolecular disulfide bonds formed during the reaction.

However, whether reactions (1) or (1) and (2) occur, the same stoichiometry applies, namely, 1 mole of 2-nitro-5-thiobenzoate anion is formed per mole of protein sulfhydryl. The nitromercaptobenzoate anion has an intense yellow color with a molar absorptivity of 13,600 M-lcm -1 at 412 nm. With DTNB, a solution of 0.01 ~mole of sulfhydryl per milli- liter gives an absorbance of 0.136 (1-cm light path) at 412 nm. Simple thiols, e.g., cysteine, give complete color development within 2 minutes and the color is stable for 2 hours.

Determination of Total Protein Sulfhydryl

The sulfhydryl groups in proteins exhibit variable reactivity toward DTNB owing to steric factors. Therefore, determination of total sulf- hydryl content requires that the protein be denatured, preferably with sodium dodecyl sulfate. 4 About 0.01-0.04 ~mole of protein is dissolved in 6 ml of solution containing 2% sodium dodecyl sulfate, 5 0.08 M sodium phosphate buffer, pH 8, and 0.5 mg/ml EDTA. To 3 ml of the solution is added 0.1 ml DTNB solution (40 mg DTNB in 10 ml of 0.1 M sodium phosphate buffer, pH 8). The color is developed for 15 minutes and read at 410 nm against protein solution in SDS to give apparent ab- sorbance. A reagent blank is subtracted from the apparent absorbance to give the net absorbance. For calculation of sulfhydryl content, the net absorbance is employed with a molar absorptivity value of 13,600 M-lcm -1. Protein concentration is derived from the absorbance at 280 nm and the known value of ~1~ L~280 nm.

Glaser et al. 6 used an alcohol-buffer solution of DTNB as a sensitive spray for the visualization of thiols on chromatograms. The spray con- sists of 0.1% solution of DTNB in a 1:1 mixture of ethanol and 0 . 4 5 M

Tris buffer, pH 8.2.

Determination of Available Protein Sulfhydryl

The reaction is performed as described above, but in the absence of denaturing agents, and the absorbance at 412 nm is followed as a function of time. Thus it is possible to distinguish between the different classes of

3K. Kleppe and S. Damjanovich, Biochim. Biophys. Acta 185, 88 (1969). 4 M. J, Fernandez Diez, D. T. Osuga, and R. E. Feeney, Arch. Biochem. Biophys. 107, 449 (1964).

5A. F. S. A. ttabeeb, Biochim. Biophys. Acta 115, 440 (1966). e C. B. Glaser, H. M. Maeda, and J. Meienhofer, J. Chromatogr. 50, 151 (1970).

[37] REACTION OF PROTEIN SULFHYDRYL GROUPS 459

sulfhydryl groups that exist in a protein. For example, tryptophan- transfer ribonucleic acid synthetase ~ contains 8 sulfhydryl groups, of which 3 react very rapidly and the remaining 5 do so more slowly. The 8 sulfhydryls in the enzyme are revealed in native as well as in 8 M urea-denatured protein, indicating the absence of any buried sulfhydryls. In the presence of tryptophan and ATP, the 3 fast-reacting sulfhydryls are still available, but 4 of the 5 slow-reacting sulfhydryls are no longer detected.

Phosphorylase b 3 contains 3 classes of sulfhydryl groups distinguished by their reactivity with DTNB. In the presence of a low concentration of DTNB (0.33 mM) the first class, consisting of 2 SH groups, reacts. As the concentration of DTNB is increased to 6.6 mM, the second class, consisting of 4 SH groups, reacts, followed by the third class, consisting of 10 SH groups, after further increase of the concentration of DTNB.

The rate of reaction of sulfhydryl groups has been used to detect conformational differences between proteins. Guidotti s attributed the difference in the velocity of the reaction of the sulfhydryl groups in oxyhemoglobin and carbon monoxide-hemoglobin with DTNB to differ- ences in the conformation of the two proteins. Phillips et al2 used DTNB to determine the number of sulfhydryl groups in several fl- lactoglobulins (cow, sheep, and goat) and to obtain evidence for con- formational differences between them. To the protein solution in 9 ml of 0.01 M potassium phosphate, pH 7.6, containing 0.001 M EDTA, was added 0.2 or 0.5 ml of 0.01 M DTNB; after mixing, the absorbance at 412 nm was recorded at intervals. The results showed that native fl-lactoglobulins from cows, goats, and sheep react with DTNB at the same rate and each contains 2 sulfhydryl groups per mole of protein. The similarity in availability of the sulfhydryls suggested that the gross conformations of these fl-lactoglobulins are similar. When the COOH- terminal sequence His-Ile was removed, the modified fl-lactoglobulins exhibited increased reactivity toward DTNB, indicative of conforma- tional changes as a result of loss of tIis-Ile.

Determination of Sulfhydryl Groups in Animal T i s s u e s

Autoxidation of thiols in tissue homogenates is frequently prevented by the presence of 0.2 M EDTA 1°,11 or by the presence of EDTA and

M. I)eLuca and W. D. McElroy, Arch. Biochem. Biophys. 116~ 103 (1966). 8 G. Guidotti, J. Biol. Chem. 240, 3924 (1965). 9 N. I. Phillips, R. Jenness, and E. B. Kalan, Arch. Biochem. Biophys. 120, 192 (1967). lo H. Sakai and K. Dan, Exp. Cell Res. 16, 24 (1959). ~1 G. S. Tarnowiski, R. K. Barclay, I. M. Mountain, M. Nakamura, H. G. Satter-

white, and E. M. Solney, Arch. Biochem. Biophys. 110, 210 (1965).

460 MODIFICATION REACTIONS [37]

a,a'-dipyridyl. 1" For determination of acid-soluble thiols, 11 1.0-ml ali- quots of tissue homogenates prepared in 0.2 M EDTA solution, pH 4.5, are mixed with 1 ml of 0.2 M EDTA, pH 4.5. Then 1 ml of 15% per- chloric acid is added and the mixture is heated in a boiling water bath for 5 minutes and filtered through Whatman No. 54 paper. Precipitates are washed 3 times with 1-ml aliquots of 0.2 M EDTA, pH 4.5. The pH of the filtrates is adjusted to 7.5 and the samples are diluted to volume (10 ml) with 0 . 2 M EDTA, pH 7.5. Colorimetric assays are made on 2-ml aliquots by adding 1.8 ml of 0.1 M Tris buffer, pH 7.5, and 0.2 ml of 0.001 M DTNB solution in Tris buffer. To calculate the content of acid-soluble thiols in tissues, a molar absorptivity at 412 nm of 13,600 M-lcm -~ is used. With rdduced glutathione as an internal stand- ard the recovery is 83 to 89%.

Sedlak and Lindsay 13 devised a procedure for the determination of protein-bound, nonprotein, and total sulfhydryl groups (PB-SH, NP-SH, and T-SH, respectively) in various tissues, as follows. Tissues (200- 400 mg) were homogenized in 8 ml of 0.02 M EDTA, pH 4.7, in an ice-bath. Under these conditions T-SH and NP-SH did not change during 2-4 hours. For determination of T-SH, a 0.5-ml aliquot of tissue homogenate was mixed with 1.5 ml of 0.2 M Tris buffer, pH 8.2, and 0.1 ml of 0.01 M DTNB in methanol. The mixture was brought to 10 ml with absolute methanol. A reagent blank (without sample) and a sample blank (without DTNB) were prepared. Color was developed for 30 min- utes, after which the solution was filtered. The absorbance was read at 412 nm and a value for the molecular absorptivity of 13,600 M-lcm -1 was used for calculation. For NP-SH, a 5-ml aliquot of the homogenate was mixed with 4 ml of distilled water and 1 ml of 50% trichloroacetic acid. After shaking for 10-15 minutes, it was centrifuged. To 2 ml of supernatant was added 4 ml of 0.4M Tris buffer, pH 8.9 and 0.1 ml DTNB, and the sample was shaken. The absorbance was read at 412 nm. The PB-SH was calculated by subtracting the NP-SH from T-SH.

This procedure requires that the pH of the final reaction mixture be above pH 8.0, since the intensity of color produced below pH 8 varies with pH. Color intensity was constant from pH 8 to 9, and its develop- ment from reduced glutathione and cysteine was linear and constant for identical concentrations, and was not altered when water, 0.5% sodium dodecyl sulfate, or 8 M urea were substituted for methanol. The color developed to maximum intensity within 5 minutes and remained stable for 1 hour in the three media. Color production in dodecyl sulfate was

~2 G. Calcutt and D. Doxey, Brit. J. Cancer 16, 562 (1962). ~'J. Sedlak and R. H. Lindsay, Anal. Biochem. 25, 192 (1968).

[37] REACTION OF PROTEIN SULFHYDRYL GROUPS 461

unaffected by pH in the range of 7.5-9 and that in 8 M urea was un- affected from pH 7-9. Maximum absorbance from bovine serum albumin and reduced glutathione was identical with methanol, 0.5% dodeeyl sulfate, and 8 M urea; however, the values obtained from rat liver were considerably lower with urea than with dodeeyl sulfate or methanol. In addition, high tissue blanks with both dodeeyl sulfate and urea in rat liver indicate that methanol is the most suitable diluent for T-SH de- termination in tissue preparations. The recovery of sulfhydryl added as albumin, reduced glutathione or cysteine to T-SH and NP-SH ranged from 96 to 102%. Whereas both bovine serum albumin and reduced glutathione were measured in T-SH, bovine serum albumin was not detected in NP-SH, while reduced glutathione was quantitatively re- covered in the NP-SH fraction.

Determination of Disulfide Bonds with DTNB after Reduction

Disulfide bonds play an important role in stabilizing the three- dimensional structure of proteins, and therefore a knowledge of their number, reactivity, and availability to denaturing agents is important. Methods have been developed in which the protein is first reduced, then assayed for liberated sulfhydryl groups by DTNB.

Reduc t ion w i th Sodium Borohydride . Cavallini et al. 14 used the fol- lowing method: To a test tube containing 1.44 g urea was added 0.1 M sodium EDTA, 0.5-1 ml of protein sample, 1 ml of 2.5% NaBI-I4, water to 3 ml, and a drop of octyl alcohol as an antifoaming agent. The tubes were mixed at 38 ° , and reduction was allowed to proceed for 30 minutes at 38 °, after which 0.5 ml of 1 M KH2P04 containing 0.2 N HC1 was added. After 5 minutes, 2 ml of acetone was added, then nitrogen was bubbled through for 5 minutes. Then 0.5 ml of 0.01 M DTNB was added, and the volume was made to 6 ml with water. Nitrogen was bubbled for 2 minutes and the tube was stoppered, ensuring that the gas space was filled with nitrogen. After the preparation had stood for 15 minutes, the absorbance was determined at 412 nm. Blanks containing all reactants except the protein solution were included. A molar absorptivity of 12,000 M-lcm -~ was used for calculating the number of sulfhydryl groups formed after reduction. With the exception of chymotrypsinogen, which gave low values, trypsin, ribonuclease, lysozyme, insulin, and bovine serum albumin gave satisfactory results.

Maeda et al. TM developed a method for visualization of eystine- containing peptides in peptide maps. The chromatogram was sprayed

I~D. Cavallini, M. T. Graziani, and S. Dupre, Nature (London) 212, 294 (1966). 15 H. Maeda, C. B. Glaser, and J. Meienhofer, Biochem. Biophys. Res. Commun. 39,

1211 (1970).

462 MODIFICATION REACTIONS [37]

with a freshly prepared 0.4% NaBH4 solution in 95% ethanol. After 20 minutes or more, the paper was dipped in an acid reagent (acetic acid, 6N HC1, and acetone 8:2:90) to decompose excess NaBH4. The paper was air-dried for 1.5 hours, followed by exposure to ammonia vapors to neutralize the acid reagent. Excess ammonia was removed by air drying for 15 minutes, then the paper was sprayed with DTNB solution (0.1% DTNB in a 1:1 mixture of ethanol and 0.45M Tris buffer, pH 8.2). Yellow spots appeared immediately. These were eluted and analyzed to establish the amino acid composition of the various cystine-containing peptides in lysozyme.

Reduc t ion wi th Dithioerythri tol . Zahler and Cleland 16 developed a method for assay of disulfide groups based on reduction with dithio- erythritol and determination of the resulting monothiols with DTNB in the presence of arsenite. The arsenite forms a tight complex with dithiols but not with monothiols. The disulfide in 0.2 ml of solution is mixed with 0.1 ml of 0.05 M Tris pH 9 and 0.1 ml of 0.003 M dithio- erythritol. Reduction is allowed to proceed for 20 minutes or for a period of time sufficient for the disulfide to be reduced. After reduction, 0.2 ml of 1 M Tris, pH 8.1, 1.5 ml of 0.005 M sodium arsenite, and water to give 2.9 ml are added, and the solution is mixed and allowed to stand for 2 minutes. DTNB is then added (0.1 ml of 0.003M solution in 0.05M acetate, pH 5) and the absorbance at 412 nm is recorded for at least 3 minutes. The absorbance resulting from the monothiols is determined by extrapolation of the linear portion of the curve to the time of addition of DTNB and subtraction of a blank value for a sample containing no disulfide. Solutions of cystine, pantethine, oxidized glutathione, and hydroxyethyl disulfide were completely reduced in 20 minutes. On the other hand, oxidized coenzyme A required 90 minutes at 0.009M dithioerythritol. Proteins may give doubtful results, as Zahler and Cleland 16 found for bovine serum albumin tested with and without urea in the solution. The resulting polythiol peptide chains may form arsenite complexes that are too stable to permit assay by this method. Walsh et al. 17 adapted the method of Zahler and Cleland 16 for continuous detection of cystinyl peptides in the effluent of ion-ex- change chromatograms of peptide mixtures.

Reduc t ion wi th f l -Mercaptoethanol . Reduction can be done in the absence and presence of varying concentration of denaturing agent. The resulting sulfhydryl groups are determined by DTNB. This serves

~6 W. L. Zahler and W. W. Cleland, J. Biol. Chem. 243, 716 (1968). ~ K. A. Walsh, R. M. McDonald, and R. A. Bradshaw, Anal. Biochem. 35, 193 (1970).

[37] REACTION OF PROTEIN SULFHYDRYL GROUPS 463

as a measure of disulfide groups that are available to reduction in the native molecule and also the ease with which the disulfides become exposed as a function of concentration of the denaturing agent. The procedure is as follows18: To about 6 mg of protein contained in 4 ml of Tris.glycine buffer (0.043M Tris and 0.046M glycine) adjusted to pH 7 and containing 0.5 mg of EDTA is added 1 ml of 0.25 M fl- mercaptoethanol in the same Tris.glycine buffer. Several 6-mg samples of the protein are reduced in the presence of varying concentrations of a guanidine salt. At 1, 3, and 6-hour intervals, 1.5 ml is withdrawn and added to 10 ml of 5% trichloroacetic acid to precipitate the pro- tein. The precipitate is washed twice with 10 ml of 5% trichloroacetic acid to remove p-mercaptoethanol. Then the precipitate is dispersed with a glass rod and dissolved in 1 ml of 8 M urea in Tris.glycine, pH 8, con- taining 0.5 mg of EDTA per milliliter. The solution is diluted with 5 ml of 2% sodium dodecyl sulfate in Tris.glycine buffer, pH 8, containing 0.5 mg/ml EDTA. To 3 ml of the solution is added 0.1 ml of DTNB (4 mg per milliliter of Tris.glycine, pH 8) and the solution is read at 410 nm against a blank with no DTNB. A reagent blank is subtracted to give net absorbance at 410 nm. The concentration of protein is obtained from the absorbance at 280 nm of the remainder of the protein solution (to which no DTNB was added) and the known ~1% The sulfhydryl ~ 2 8 0 a m .

content is calculated from a molar absorptivity of 13,600 M-lcm -1 at 410 nm for 2-nitro-5-thiobenzoate anion. The use of Tris.glycine buffer TM

instead of the phosphate buffer previously used TM is found to be advan- tageous when guanidine salts are used as denaturants. There is a loss of sulfhydryl groups with time and with increased guanidine salt concen- tration in phosphate buffer. This is eliminated in Tris-glycine buffer. I t is likely that guanidine hydrochloride can contain impurities which react with sulfhydryl groups, but which are eliminated in the presence of Tris.glycine buffer.

The difference in susceptibility of disulfide bonds to reduction in a native protein and its modified counterpart form the basis of a method to detect conformational changes that result from chemical modi- ficationY 2° Such conformational changes were found to increase in bovine serum albumin (BSA) after modification, as follows: succinyl BSA > acetyl BSA > nitroguanyl BSA > guanyl BSA > BSA2 ,2° An increase in reducible disulfides accompanied the nitration of tyrosine-20

18 M. Z. Atassi, A. F. S. A Habeeb, and L. Rydstedt, Biochim. Biophys. Acta 200, 184 (1970).

19A. F. S. A. Habeeb, Arch. Biochem. Biophys. 121, 652 (1967). A. F. S. A. ttabeeb, Fed. Proc. Fed. Amer. Soc. Exp. Biol. Abstr. 24, 224 (1965).

464 MODIFICATION REACTIONS [38]

and tyrosine-23, 21 and the modification of 6 tryptophan residues 22 in lysozyme, behavior indicative of conformational changes. The sequence homology between lysozyme and a-lactalbumin was found not to be reflected in a conformational homology, is as the two proteins differ in the reducibility of their disulfide bonds and in the ease with which they unfold in guanidine hydrochloride solutions.

~1 M. Z. Atassi and A. F. S. A. Habeeb, Biochemistry 8, 1385 (1969). A. F. S. A. ttabeeb and M. Z. Atassi, Immunochemistry 6, 555 (1969).

[38] T h e R a p i d D e t e r m i n a t i o n of A m i n o G r o u p s

w i t h T N B S

By ROBERT FIELDS

TNBS 1 was originally proposed by Okuyama and Satake 2 for deter- mining amino acids and peptides and for protein modification studies, as the conditions required for its reaction were mild compared with those with ninhydrin, s,4 and the reaction was more specific than that of aryl halides: no reaction with tyrosine or with histidine side chains was detected.

Sulfite is displaced from TNBS by an attacking nucleophile.

NO 2 NO~

02N O s- ~ O,N NH--

NO~ NO~

+ SO,'- + H +

Its generation has presented a serious nuisance, since it associates reversibly with TNP-amino groups or TNP-thiol groups to form com- plexes whose absorption spectrum is altered, thus making quantitation of the reaction difficult. Reaction mixtures have been acidified to dis- sociate the sulfite complexes before reading, ~,8 empirical corrections have

1Abbreviations: TNBS, 2,4,6-trinitrobenzenesulfonic acid; TNP-, 2,4,6-trinitro- phenyl-.

z T. Okuyama and K. Satake, J. Biochem. (Tokyo) 47, 454 (1960). 3 S. Moore and W. H. Stein, J. Biol. Chem. 211, 907 (1954). 4 H. Rosen, Arch. Biochem. Biophys. 67, 10 (1957). ~K. Satake, T. Okuyama, M. Ohashi, and T. Shinoda, J. Bioehem. (Tokyo) 47,

654 (196o). ~A. F. S. A. ttabeeb, Anal. Biochem. 14, 328 (1966).