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Sodium Trichloroacetate As a Denaturation Reagent for Proteins*

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Sodium Trichloroacetate As a Denaturation Reagent for Proteins* J. Biochem., 70, 63-73 (1971) Sodium Trichloroacetate as a Denaturation Reagent for Proteins* Akiko TAKENAKA, Osamu TAKENAKA, Teruhiko MIZOTA, Kazuo SHIBATA and Yuji INADA Tokyo Institute of Technology, nleguro-ku, Tokyo, and The Institute of Physical and Chemical Research, Wako, Saitama Received for publication, December 21, 1970 Trichloroacetic acid (TCA) is known as a precipitant for proteins at acidic pH was applied at neutral pH to proteins such as ƒ¿-chymotrypsin [EC 3. 4. 4. 5], chymotryp sinogen, lysozyme [EC 3. 2.1.17], ribonuclease A [EC 2. 7. 7. 16] and insulin in order to examine its effects on the structures and activities of these enzymes and proteins at neutral pH and to compare them with the effects of urea, guanidine-HCl and halogen derivatives of acetate. The proteolytic activity of ƒ¿-chymotrypsin was inhibited completely by addition of 1.2 M Na-TCA at pH 7.8, while the activity was fully retained in the presence of 1.5 M sodium monochloroacetate, sodium dichloroacetate, sodium trifluoroacetate, urea or guanidine-HCl at the same pH value. The activities of ribonuclease A and lysozyme near neutral pH were also lost completely with Na- TCA at 1.2 M. The four tyrosine residues in the ƒ¿-chymotrypsin molecule ionized with 0.6 M Na-TCA at pH 12.0 while, in the presence of other reagents at 1.5 M, only two or three of them ionized at the same pH value. Some tyrosine residues in chymotrypsinogen, lysozyme and ribonuclease A, which do not ionize at pH 12.0 without reagent, ionized on addition of 1.5 M Na- A at the alkaline pH. These proteins when treated with Na-TCA at pH 7.0 underwent spectral shifts due to ex posure of tyrosine and/or tryptophan residues from the interior of protein molecules. The helical contents of ribonuclease A and lysozyme increased on addition of Na- TCA, whereas the helical content of insulin and ƒ¿-chymotrypsin decreased on the same treatment. Na-TCA was thus found to act as a denaturation reagent for proteins which is more effective at neutral pH than the other notable denaturation reagents. * Abbreviations : Na-TCA , sodium trichloroacetate; Na-Acet, sodium acetate, Na-AMA, sodium mono chloroacetate; Na-DCA, sodium dichloroacetate; Na-TFA, sodium triHaoroacetate; Gua-HCl, guanidine hydrochloride; Ac-Tyr, N-acetyltyrosine ; Ac-Trp, Na-acetyl tryptophan. Vol. 70, No. 1, 1971 63 64 A. TAKENAKA , O. TAKENAKA, T. MIZOTA, K. SHIBATA and Y. INADA Trichloroacetic acid (TCA) is known as a and recrystallized twice from methanol. powerful precipitation reagent for proteins Assay of Enzymatic Activity-The prote giving rise to aggregation of proteins at acidic olytic activity of ƒ¿-chymotrypsin was meas pH-I. Recently, Yang and Doty (1) reported ured by the method of Kunitz (3) with 1°Jo that the helical contents of bovine serum casein as substrate. The digestion was carried albumin, insulin and silk fibroin decreased out for 30 min at 25•Ž in 0.2 M Tris-HCl buffer when dissolved in liquid dichloroacetic or tri of pH 7.8. The assay of ribonuclease A was fluoroacetic acid. This suggests that trichloro performed by the method of Anfinsen et al. acetic acid might denature proteins before (f ), using yeast ribonucleic acid as substrate . forming aggregates of proteins. The present The hydrolysis was carried out for 25 min at study was undertaken to examine the effect 25•Ž in 0.1 M acetate buffer of pH 5 .0. The of sodium trichloroacetate (Na-TCA) and other lytic activity of lysozyme was measured by the halogenated acetate on proteins at neutral pH method of Smolelis and Hartsell (5) with at which proteins do not aggregate with these Micrococcus lvsodeikticus cells as substrate , reagents. Na-TCA as a denaturation reagent The lysis was conducted at room temperature was compared with urea and guanidine-HCl , in 0.1 M phosphate buffer of pH 6.2. The a-Chymotrypsin , chymotrypsinogen, ribo activities of ƒ¿-chymotrypsin, ribonuclease A nuclease A, lysozyme and insulin were chosen and lysozyme at varing concentrations of Na- as representative samples of proteins , and the TCA and other denaturants except urea were enzymatic activity, the helical content and the measured at a constant ionic strength . The states of tryptophan and tyrosine resiues as ionic strength was adjusted with NaCl , LiCl observed by spectral shift or transformation and KCl in the experiments with sodium, were studied in the presence of these re lithium and potassium salts of halogenated agents. acetates, respectively. The adjustment was not needed for solutions of urea carrying no EXPERIMENTAL net charge. Urea was dissolved without use of NaCl in 0.2 M Tris-HCl buffer of pH 7 .8. Materials-Crystalline ribonuclease A [EC An enzyme and its substrate were preincubated 2. 7. 7. 16], insulin and lysozyme [EC 12 . 1. 17] separately with reagents at 25•Ž for 2 hr before were purchased from Sigma Chemical Co ., mixing. Simizu Pharmaceutical Co . and Seikagaku Ionization of Tyrosine Residues-Ionization Kogyo Co., respectively . ƒ¿-Chymotrypsin [EC of tyrosine residues in proteins was measured 3. 4. 4. 51 and chymotrypsinogen were obtained spectrophotometrically in the following manner in a crystalline state from bovine pancreas by with a Shimadzu recording spectrophotometer the method of Kunitz and Northrop ( 2) , and model SV-50, using 1.0-cm cells . A difference recrystallized five times from ammonium sulfate in absorbance at 295 mƒÊ between an alkaline solution. N-acetyltryptophan (Ac-Trp) and N - solution (pH 12.0) of ribonuclease A , insulin or acetyltyrosine (Ac-Tyr) were purchased from lysozyme in 0 .1 M glycine buffer with a Tokyo Kasei Co . and the Institute of Protein denaturant and the same protein solution with- Research, Osaka University , respectively, out the denaturant in 0 .1 M phosphate buffer Casein, dried Micrococcus lvsodeikticus cells and (pH 7.0) was measured as a function of reagent yeast ribonucleic acid were purchased from E , M concentration. Similar difference spectrophoto erck Ag., Germany , Seikagaku Kogyo Co. metry was made for chymotrypsinogen and ƒ¿- and Sigma Chemical Co ., respectively. Tri chymotrypsin at 305 mƒÊ in order to eliminate chloroacetic acid obtained commercially was the effect of a small shift by denaturation of purified by distillation at 98•Ž%20 mmHg . The the tryptophan band near 280 mp . The differ solubility of Na-TCA at room temperature in ence in absorbance thus obtained was con water at pH 7.0 was 5 .6 M. Urea was re- verted to the molar concentration of ionized crystallized twice from methanol . Guanidine- tyrosine residues with aid of the difference, ƒ¢ƒÃ HCl was prepared from guanidine carbonate , in molar extinction coefficient obtained for . Biochem. J SODIUM TRICHLOROACETATE AS A DENATURATION REAGENT FOR PROTEINS 65 Ac-Tyr as a model substance. The J.- values persion of proteins was measured with a Jasco were 2,300 M-1 cm-1 at 295 nip ( 6) and 1,300 optical rotatory dispersion recorder model M-1 cm-1 at 305 mu in the absence of Na-TCA, ORD/UV-50, using a 10.0-cm cell. The param and increased linearly with increasing Na-TCA eter, b0, was determined by use of the Moffitt- concentration ; 2,400 M-1 cm-1 at 1 M Na-TCA Yang equation. Refractive indices of 1.0 M and 2,470 M-1 cm -1 at 2 M Na-TCA for 295 m ƒÊ, and 3.0 M Na-TCA solutions were 1.353 and and 1,400 M-1 cm-1 at 1 M Na-TCA and 1,550 1.399, respectively, and the value of 2.0 was M-1 cm -1 at 2 M Na-TCA for 305 mƒÊ. In the assumed to be 212 mƒÊ (12, 13). The mean case of lysozyme, the value of Jr at 295 trip residue weight was assumed to be 110 for was corrected for the overlapping effect of the ribonuclease A (11) 113 for insulin ( 15), 112 tryptophan band. The concentrations of amino for lysozyme (16) and 105 for ƒ¿-chymotrypsin acids and proteins were determined spectro (11). The helical content was estimated from photometrically, assuming their molar extinc the calculated value of bo, assuming the bo tion coefficients to be 5.50 x 103 M-1 cm-1 at value for the complete helical structure to be 280 m,a (Ref. /-)for Ac-Trp, 1.47 x 103 M-1 cm-1 -630 (Ref . 17). at 276 m,a (Ref. 7) for Ac-Tyr, 3.88•~10 M-1 cm -1 at 280 mƒÊ (Ref. 8) for lysozyme, 9.80 RESULTS •~ 101 M-1cm1 at 278 mƒÊ (Ref. 9) for ribonuclease A, 5.62•~10 M-1 cm-1 at 276 mƒÊ (Ref. 10) for Enzymatic Activity-The proteolytic ac insulin and 5.00 x 101 M-1 cm-1 at 282 mƒÊ (Ref. tivity of ƒ¿-chymotrypsin measured with heat 11) for a-chymotrypsin and chymotrypsino denatured casein as substrate was measured gen. at pH 7.8 in the presence of various reagents Spectral Shift of Proteins by Na-TCA- including Na-TCA, and the results are listed The shift of the tyrosine and/or tryptophan in Table I, where the activity without reagent band of Ac-Tyr, Ac-Trp, ribonuclease A, lyso and NaCl was taken as 100% to express the zyme, insulin, ƒ¿-chymotrypsin and chymo relative activities with urea (expressed with a trypsinogen caused by addition of Na-TCA was measured as follows ; a Na-TCA solution TABLE I. The effects of Na-TCA and other reagents (3 ml) buffered with 0.13M phosphate of pH on the enzymatic activity of ƒ¿-chymotrvpsi=n at pH 7.0 was added to a protein solution (I ml), and 7.8.
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