1. CYSTEIC ACID AND CYSTEIC ACID PEPTIDES FROhf OXIDIZED PAPAIN*

Size: px

Start display at page:

Download "1. CYSTEIC ACID AND CYSTEIC ACID PEPTIDES FROhf OXIDIZED PAPAIN*"

Isabella Floyd
6 years ago
Views:

1 CRYSTALLINE PAPAIN 1. CYSTEIC ACID AND CYSTEIC ACID PEPTIDES FROhf OXIDIZED PAPAIN* BY J. R. KIhIMEL, E. 0. P. THOMPSON, AND EMIL L. SMITH (From the Laboratory for the Study of Hereditary and Metabolic Disorders, and the Departments of Biological Chemistry and Medicine, University of litah College of Medicine, Salt Lake City, Utah) (Received for publication, April 18, 1955) In an earlier study (1) of the amino acid composition of papain, it was assumed that all of the sulfur could be attributed to cysteine and cystine since methionine is absent from this protein. However, because of the sulfhydryl nature of this enzyme, the content of these amino acids is of some importance; we have used the method of Schram, Moore, and Bigwood (2) for this estimation. The results indicate that 6 of the 8 sulfur atoms of this protein are present as cysteine or cystine; 2 sulfur atoms are present in an unidentified form. The importance of the cysteine residues has also prompted a study of partial hydrolysates of oxidized papain. A number of peptides containing cysteic acid have been isolated and identified. EXPERIMENTAL Oxidation of Papain and Mercuripapain-For the determination of cysteic acid, twice recrystallized papain or mercuripapain was dried with acetone and ether (1) and oxidized in preformed performic acid essentially by the method of Schram, Moore, and Bigwood (2). The oxidation was carried out at room temperature, as well as at - 10 to - 15, and for varying periods of time. Residual performic acid was removed either by repeated concentration in vacw) at 40 or by drying twice from the frozen state. The oxidized papain is a light brown powder which is virtually insoluble except at extreme ph values. When it was treated with fluorodinitrobenzene (FDNB) by the method of Sanger (3) and hydrolyzed with 6 N HCl, dinitrophenyl (DNP)-isoleucine was recovered and was accompanied by less than 0.1 mole of other DNP amino acids per mole of oxidized papain, in agreement with earlier results on the unoxidized protein (4). It is evi- * This investigation was aided by research grants from the National Institutes of Health, United States Public Health Service, and from the American Cancer Society upon recommendation of the Committee on Growth of the National Research Council. 151

2 152 CRYSTALLINE PAPAIN. V dent that the oxidation procedure produces no significant hydrolysis of peptide bonds. The amino acid composition of a 70 hour hydrolysate of this protein was determined by the procedure of Moore and Stein (5) as described earlier from this laboratory (1). The results were identical with those obtained on a similar hydrolysate of unoxidized papain (1) except for a small loss of tyrosine. Such a loss is expected, since it is known that chlorination of tyrosine occurs to a certain extent during or after the oxidation procedure in the presence of HCl; the chlorinated derivatives of tyrosine do not emerge with tyrosine from Dowex 50 columns (6). TABLE Recovery of Cysteic Acid from Hydrolysates of Oxidized Bovine Albumin and Papain The results are computed as half cystine residue weight in per cent, although the amino acid separated was cysteic acid. No corrections have been applied to these data. In each case the protein was hydrolyzed at in 6 N HCl under anaerobic conditions for 20 hours. Preparation No. Time of oxidation Recovery of half cystine Ills. I per cm1 1. Albumin , ( 20* 4.57, 4.80, Papain...., (3.01) 4. Ls , * 2.61, * 2.43, 2.52 * The residual performic acid was removed by repeatedly drying from the frozen state in high vacuum. In the other experiments the acid was removed at 40 in vacua. Cyst&c Acid Content of Oxidized Papain-Oxidized papain or mercuripapain was hydrolyzed in 6 N HCl by procedures previously described (1). The cysteic acid was isolated by chromatography on a 0.9 X 15 cm. column of Dowex 50-X8. A 0.2 M citrate buffer at ph 3.42 was the eluting solvent (5). Cysteic acid emerged at the column volume and was estimated in the effluent fractions by the photometric ninhydrin technique of Moore and Stein (7). The recoveries of cysteic acid from 20 hour hydrolysates of oxidized papain are given in Table I. The results are expressed as half cystine residue weight in per cent. The average of these figures is 2.48 f 0.08 per cent. Schram, Moore, and Bigwood (2) obtained a yield of cysteic 1 We are indebted to Vina Buettner-Janusch and Anne Stockell for assistance with these determinations.

3 J. R. KIMMEL, E. 0. P. THOMPSON, AND E. L. SMITH 153 acid of 90 f 2 per cent of the theoretical yield upon oxidation of cystine and cysteine with performic acid. In similar experiments in this laboratory 86 per cent of the theoretical quantity of cysteic acid was obtained. However, hydrolysis of oxidized, crystalline bovine serum albumin gave a recovery of 4.56 per cent half cystine (Table I) which is 82 per cent of the value found by Brand (8). Although the differepce between 82 and 86 per cent is small, it appears that the value of 82 per cent is more applicable to the papain data, since a closer relationship exists between determinations with a protein than between a protein and the free amino acids. When the recovery factor of 82 per cent is applied to the average half cystine value of 2.48 per cent obtained with papain, the half cystine content of papain is calculated to be 3.01 per cent. For a molecular weight of 20,340 (l), this corresponds to 6.03 residues of half cystine per mole of papain. In the calculation of the average half cystine recovery from crystalline papain the value 3.01 per cent obtained with Preparation 3 has been omitted, since it differs from the mean value by more than 4 times the average deviation. The inclusion of this figure does not significantly alter the results. Other workers have also studied the cystine content of papain. Kassel and Brand (9) found 3.58 per cent residue weight of half cystine. Inasmuch as they also found 0.4 per cent methionine, it appears that these authors were not dealing with the pure protein. Balls and Lineweaver (10) found 2.86 per cent half cystine residue weight in their crystalline papain by the Sullivan method. This corresponds to 5.8 residues of half cystine for a molecular weight of 20,340. These results are in excellent accord with our data. Close, Moore, and Bigwood (11) found only 1.82 per cent residue weight of half cystine in their preparations of crystalline papain. This value is considerably lower than the ones reported above. The reason for the discrepancy is not apparent. Crystalline papain and mercuripapain contain 8 atoms of sulfur per mole of enzyme (12). This value has now been confirmed several times with different preparations of the enzyme. Since methionine is absent from papain (1, lo), it is apparent that not all the sulfur can be accounted for as half cystine. The extra sulfur does not appear to be present as sulfate.2 A similar discrepancy between the total sulfur content and the sum of half cystine plus methionine residues of ribonuclease has been observed by Hirs, Stein, and Moore (13). Amino Acid Sequences Containing Cysteine-The crystalline papain used in these experiments was prepared as previously described (12). The protein (500 mg.) was hydrolyzed for 7 days in 12 N HCl at 37. The hydrolysate was concentrated to dryness in vacua several times and finally treated * B. J. Finkle and E. L. Smith, unpublished observations.

4 154 CRYSTALLINE PAPAIN. V with solid AgO to remove traces of HCl. The precipitated AgC12 and AgO were removed by centrifugation and the precipitate was washed once with water. The combined supernatant solution and washings were concentrated and finally dried in vacua. The dried hydrolysate was oxidized by solution in 8 ml. of anhydrous formic acid followed by addition of 0.5 ml. of H202. After 15 minutes at room temperature, the reaction mixture was diluted with 10 ml. of water, concentrated under reduced pressure several times, and dissolved in a volume of about 2 ml. An alternative procedure that gave greater yields of peptides containing cysteic acid consisted of oxidizing the protein first and then hydrolyzing it partially. The procedures used were the same as those described above. Two methods were used to isolate cysteic acid and cysteic acid peptides from the partial hydrolysates. Ionophoresis in an apparatus similar to that described by Flynn and de Mayo (14) had the advantage of separating free cysteic acid from the cysteic acid peptides but required considerable time for the collection of sufficient material for further study. The ionophoresis was carried out on a full sheet of Whatman No. 3 filter paper which was folded at the center line. The oxidized partial hydrolysate (0.2 ml.) was applied to the paper along this fold. The paper was suspended on glass rods with the ends dipping in 1 N acetic acid, ph 2.3, and a potential difference of 400 volts, direct current, was applied for 4 hours at room temperature. After completion of the ionophoresis the sheet of filter paper was dried in air and strips were cut from the center and from both edges at right angles to the center fold. These strips were sprayed with a solution of ninhydrin (0.1 per cent) in 95 per cent ethanol. The color was allowed to develop at room temperature. Five distinct bands appeared on the paper after staining with ninhydrin. The results are illustrated in Fig. 1. The stained paper strips permitted the location of t he bands on the unstained portions of the filter paper. These bands were cut out and the adsorbed material was eluted with water by the method of Sanger and Tuppy (15). Several ionophoretic runs were made under identical conditions and the eluates from the individual bands were pooled. To identify the amino acids in each band, samples of the pooled eluates were hydrolyzed in sealed tubes in 6 N HCl for 24 hours at 37. These hydrolysates were freed of excess HCl by repeated concentration in vacua and chromatographed on Whatman No. 4 filter paper with water-saturated phenol-o.3 per cent ammonia (16). Band 1 was found to contain only free cysteic acid. Band 2 contained high concentrations of cysteic acid, aspartic acid, glutamic acid, serine, and glycine. Band 3 showed only small amounts of cysteic acid and many other amino acids. Bands 4 and 5

5 J. R. KIMMEL, E. 0. P. THOMPSON, AND E. L. SMITH 155 contained no cysteic acid. Further investigation was performed only on material from Band 2. The second method for isolating the cysteic acid peptides was a column chromatography procedure with a 10 X 2.5 cm. column of Dowex 50-X4 (50 to 100 mesh). The resin was cycled several times with 2 N NaOH and with 2 N HCl. Finally, the resin was left in the hydrogen cycle and washed with water until the washings were neutral. The partial hydrolysate was placed at the top of the column and run through at a flow rate of about 1 ml. per minute. Under these conditions cysteic acid and some peptides containing cysteic acid emerge unretarded; all of the ot.her amino acids and peptides require elution at more alkaline ph values. The eluate containing the cysteic acid peptides was concentrated to dryness in vacua. ORiGlN FIG. 1. Ionophoresis of oxidized partial hydrolysates of papain. The sample was applied along the center line (origin) of a full sheet of Whatman No. 3 filter paper and subjected to a potential difference of 400 volts for 4 hours at ph 2.3. The bands were located by spraying sample strips with 0.1 per cent ninhydrin in 95 per cent ethanol and allowing the color to develop at room temperature. This is a scale drawing showing the distance and the direction that each band moved. Band 2 from the ionophoresis and the cysteic acid fraction from the column chromatography were fractionated further by two-dimensional paper chromatography. Approximately 0.2 of each fraction (in 0.05 ml.) was spotted on a full sheet of Whatman No. 3 filter paper. The solvent for development in the first dimension was water-saturated phenol-o.3 per cent ammonia and that for the second dimension was n-butanol-acetic acidwater (4: 1:5 volume per volume) (16). After complete development of the chromatogram, the paper was thoroughly dried and sprayed with per cent ninhydrin in 95 per cent ethanol. The color was allowed to develop at room temperature. When the spots had appeared, they were cut out and washed with acetone to remove unreacted ninhydrin. The peptide was elut.ed from the paper by the technique of Sanger and Tuppy (15). A portion of each eluate was hydrolyzed in 6 N HCl in a sealed tube for 24 hours at 105 and the constituent amino acids were identified by paper chromatography with one of the systems previously described. The remainder of the eluate was treated with FDNB as described by Sanger and Thompson (17). Excess FDNB was removed from the alkaline solution by extraction with ether. The aqueous phase was concentrated to dryness

6 156 CRYSTALLINE PAPAIN. V several times, redissolved in three times redistilled 6 N HCl, and hydrolyzed for 4 to 8 hours at 105. The hydrolysates were extracted three times with ether to remove the DNP amino acids, and both the ether and aqueous phases were freed of excess HCl by evaporation to dryness. The amino acids in the aqueous phase, including DNP-cysteic acid, were identified by paper chromatography as described above. DNP amino acids were identified by paper chromatography on Whatman No. 4 filter paper buffered -PHENOL.: 0.3 % NH3 0 I a I FIN. 2. Two-dimensional paper chromatogram of cysteic acid peptides from partial hydrolysates of papain. The mixture of peptides was placed on a full sheet of Whatman No. 3 paper at the spot marked 0. The solvent for the first dimension was water-saturated phenol-o.3 per cent ammonia; the second dimension was developed with n-butanol-acetic acid-water (4:1:5 volume per volume). The fifteen spots were located with per cent ninhydrin in 95 per cent ethanol. at ph 6.0 with 0.1 M phthalate. The mobile phase was tert-amyl alcohol saturated with 0.1 M phthalate buffer, ph 6.0. Fig. 2 shows the approximate location of the fifteen distinct spots on the two-dimensional chromatogram. Spot 5 which was free cysteic acid was not present in the two-dimensional chromatogram of Band 2 from the ionophoresis. All of the spots except Spot 9 contained cysteic acid. Table II gives the amino acid composition of each spot and the DNP amino acid obtained from the DNP peptide. In the case of dipeptides it was possible to establish the amino acid sequence with this treatment. At least two tripeptides were identified and in order to establish the amino acid sequence in these cases it was necessary to hydrolyze partially and repeat the dinitrophenylation. The tripeptide of Spot 8 was hydro-

7 J. R. KIMMEL, E. 0. P. THOMPSON, AND E. L. SMITH 157 lyzed for 3 days at 37. This hydrolysate was chromatographed on paper in the butanol-acetic acid-water system. The components were eluted from the paper and studied by the DNP method. As a result, it was possible to identify the two dipeptide sequences cysteylglycine and glycylaspartic acid. This established the amino acid sequence of the tripeptide TABLE Cysteic Acid Peptides Isolated from Partial Hydrolysales of Oxidized Papain The dinitrophenyl peptides were prepared as described in the text and then hydrolyzed in 6 N HCl. Spot No. ~ t 4 5t Serine Aspartic Cyst.eic DNP amino acid acid acid Serine Cysteic acid I Valine Glycine - II Amino acids Probable sequence* Cysteic acid I Aspartic? I Cysteic Serine? Cysteic acid Glycine Aspartic acid Glycine Glutamic acid Cysteic acid Glycine Cysteic acid Cysteic acid, serine, glycine, glutamic acid Cysteic acid Proline Ser. CySOIH. CySOJH (?) Asp. CySOsH CySOzH. Asp Ser. CySOIH CySOsH. Gly CySOsH. Gly Asp Val. CySOaH Gly.(Pro,CySOaH) * The convention of Sanger and Tuppy (15) has been used to designate known and unknown sequences. Abbreviations are as follows: Asp, aspartic acid; CyS03H, cysteic acid; Gly, glycine; Pro, proline; Ser, serine; Val, valine. t Not present in large enough quantities to be studied by the DNP method. from Spot 8 as cysteylglycylaspartic acid. There was insufficient material in Spot 15 to carry out similar studies, but, from the behavior of this tripeptide and its hydrolysates, certain inferences can be made. Acid hydrolysates of material from Spot 15 invariably appeared to contain more cysteic acid than glycine or proline. The rapid movement of this tripeptide in the phenol-ammonia system practically excludes the existence of more than 1 equivalent of cysteic acid in the peptide. The low yield of glycine and proline could be explained if the sequence glycylproline is present because this dipeptide will form the diketopiperazine during acid hy-

8 158 CRYSTALLINE PAPAIN. V drolysis (18). The diketopiperazine will not stain with ninhydrin. Since the tripeptide has glycine as its N-terminal residue, it is possible that the tripeptide in Spot 15 has the sequence glycylprolylcysteic acid. Consden and Gordon (19) have also noted that it is difficult to isolate proline dipeptides from oxidized partial acid hydrolysates of wool, although they could identify proline in larger peptides. The study of cysteic acid peptides from papain indicates that at least four of the cysteine (or half cystine) residues are accounted for. These sequences are serylcysteic acid, aspartylcysteic acid, valylcysteic acid, and glycylprolylcysteic acid. Although the sequence of the tripeptide is not conclusively established, the N-terminal glycine residue eliminates the possibility that it contains sequences already identified. The peptide serylcysteic acid occurs in great abundance in the partial hydrolysates of papain and this sequence may occur more than once. Furthermore, it is possible that the sequence cysteylcysteic acid is present, as indicated by observations on Spot 1 of the two-dimensional chromatogram of cysteic acid peptides. This peptide contains only serine and cysteic acid but is unquestionably different from the peptide in Spot 6 which also contains only serine and cysteic acid. The slow rate of movement in both solvents suggests that Spot 1 may contain 2 moles of cysteic acid. If these suggestions are correct, t,hen 6 cysteine residues are accounted for in peptide sequences. It is noteworthy that cysteine (or cystine) in peptide form from partial hydrolysates of various proteins (15, 19-22) has now been observed in combination with almost every other amino acid. The methods used by us and by others for isolation of cysteic acid peptides are all based on the acidic nature of cysteic acid. Dipeptides containing cysteic acid with lysine, arginine, or histidine would behave as neutral peptides and would not be isolated by these techniques. However, Thompson (22) has isolated neutral dipeptides containing cysteic acid and a basic residue by elution chromatography of partial hydrolysates of oxidized lysozyme. SUMMARY The cysteic acid content of oxidized papain has been determined by chromatography of hydrolysates on Dowex 50. It is concluded that papain contains 6 residues of half cystine per mole of enzyme. The following peptides containing cysteic acid have been isolated and identified: Asp. CySOaH, CySOaH. Asp, Ser. CySOsH, CySOaH. Gly, CyS03H. Gly. Asp, Val. CyS03H, Gly. (Pro,CySOsH). BIBLIOGRAPHY 1. Smith, E. L., Stockell, A., and Kimmel, J. R., J. Biol. Chem., 207, 551 (1951) 2. Schram, E., Moore, S., and Bigwood, E. J., Biochem. J., 67, 33 (1954).

9 J. R. KIMMEL, E. 0. P. THOMPSON, AND E. L. SMITH Sanger, F., Biochem. J., 39, 207 (1945). 4. Thompson, E. 0. I., J. Biol. (Them., 20 7, 563 (1954). 5. Moore, S., and Stein, W. II., J. Biol. Chem., 192, 663 (1951). 6. Thompson, 1~. 0. P., Biochim. et biophys. a&, 16, 440 (1954). 7. Moore, S., and Stein, W. II.,.l. Biol. Chem., (1948). 8. Braud, E.:., Ann. Arew I ork ~Zcad. SC., 47, 187 (1946). 9. Kassel, B., and Brand, 15., J. Viol. Chem., 126, 435 (1938). 10. Balls, A. K., and Lineweaver, H., J. Biol. Chem., 130, 669 (1939). 11. Close, J., Moore, S., and Bigwood, E. J., Enzymologia, 16, 137 (1953). 12. Kimmel, J. R., and Smith, E. L., J. BioZ. Chem., 20 7, 515 (1954). 13. Hirs, C. H. W., Stein, W. H., and Moore, S., J. Biol. Chem., 211, 941 (1954). 14. Flynn, F. V., and de Mayo, P., Lancet, 2,235 (1951). 15. Sanger, F., and Tuppy, H., Biochem. J., 49,463 (1951). 16. Block, R. J., LeStrange, R., and Zweig, G., Paper chromatography, New York (1952). 17. Sanger, F., and Thompson, E. 0. P., Biochem. J., 63, 353 (1953). 18. Smith, E. L., in Wolstenholme, G. E. W., and Cameron, M. P., The chemical structure of proteins, Ciba Foundation symposium, London (1954). 19. Consden, R., and Gordon, A. H., Biochem. J., 48,s (1950). 20. Flavin, M., Nature, 173,214 (1954). 21. Flavin, M., and Anfinsen, C. B., J. Biol. Chem., 211,375 (1954). 22. Thompson, A. R., Biochim. et biophys. acta, 14, 581 (1954).

10 CRYSTALLINE PAPAIN: V. CYSTEIC ACID AND CYSTEIC ACID PEPTIDES FROM OXIDIZED PAPAIN J. R. Kimmel, E. O. P. Thompson and Emil L. Smith J. Biol. Chem. 1955, 217: Access the most updated version of this article at Alerts: When this article is cited When a correction for this article is posted Click here to choose from all of JBC's alerts This article cites 0 references, 0 of which can be accessed free at tml#ref-list-1

Exam I Answer Key: Summer 2006, Semester C

Exam I Answer Key: Summer 2006, Semester C 1. Which of the following tripeptides would migrate most rapidly towards the negative electrode if electrophoresis is carried out at ph 3.0? a. gly-gly-gly b. glu-glu-asp c. lys-glu-lys d. val-asn-lys