isolation and properties of crystallineisolation and properties of crystalline papain* by a. k....

ISOLATION AND PROPERTIES OF CRYSTALLINE pAPAIN*

BY A. K. BALLS AND HANS LINEWEAVER

(From the Food Research Division, Bureau of Agricultural Chemistry and Engineering, United States Department of Agriculture,

Washington)

(Received for publication, July 1, 1939)

Proteolytic enzymes from plant tissues have in general resisted purification more successfully than those of animal origin. Pro- teinases have been extracted from many plants, among them yeast (9), pineapple (27), pumpkin (27), beans,’ and wheat (4), but much concentration and purification are usually required before a preparation of even moderate activity is obtained. This presents difficulties similar to those encountered in obtaining tissue enzymes from animal material. On the other hand, the investigation of latex is a comparatively simple matter, for ‘an initial high degree of purity is often found. For unpurified papaya latex the proteolytic activity on hemoglobin per unit of protein nitrogen is already one-fourth to one-half that of the crystalline pancreatic enzymes. In the papaya the latex appears to be a specialized enzyme-bearing material comparable in this sense to pancreatic or gastric juice.

There is no apparent distinction between latex-borne and other plant proteinases as such, possibly because the source of the latter may also have been a system of latex vessels. The proteinases of beans, wheat, and pineapple, as well as ficin and papain, may be activated by HZ’S, HCN, and similar reducing agents, and inactivated by iodoacetic acid, hydrogen peroxide, and other oxidants. These enzymes are therefore all to be classed as papainases, as suggested by Waldschmidt-Leitz (24).2 On the

* Food Research Division Contribution No. 443. 1 Davis, W. B., private communication. 2 It should be noted that catheptic proteinase does not appear to meet

this characterization (2).

669

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670 Crystalline Papain

other hand, the proteinase of the squash family may be of a different type, for it is inhibited by HCN and H&S (27).

Two plant proteinases have recently been crystallized, ficin from fig latex by Walti (25), and papain in this laboratory from the latex of the green papaya (6). In the latter case, the crystalline enzyme has shown proteolytic activities on hemoglobin (per unit of protein N) of the same order of magnitude as those of the crystalline pancreas enzymes. There is thus no marked distinction between the potency of proteinases elaborated by animals and by plants.

Willstiitter and Grassmann (26) achieved a 2- to 3-fold increase in the specific activity of papain by alcohol precipitation from a solution previously incubated for 3 days with cyanide at 40”. Kraut and Bauer (14) combined this method with various adsorption experiments, using aluminum hydroxides, lead phos- phate, etc., to obtain a total increase in activity in one case as high as IO-fold. In spite of the fact that direct comparisons are difficult to make because these workers used commercial material and reported the activities in terms of gelatin hydrolysis per unit of dry weight in contrast with the methods used in this investigation, it is probable that the purest preparations obtained by them represented very fine material. The same is probably true of the recent work of Okumura (Maeda) (20) in which a lo- to 20-fold increase in specific activity computed on a dry weight basis was obtained with commercial papain.

The crystals of papain have been prepared from papaya latex, as reported previously (6), by the use of salting-out methods similar to those successful in the past with other proteolytic enzymes. This paper relates a method of crystallization improved in many details over that originally described, and also presents such observations on the properties of the crystalline enzyme as the authors have had opportunity to make with the material so far obtained.

Isolation and Crystallization

The protein extracted from undried papaya latex by water or dilute cyanide solution can be fractionated by 0.4 saturation with ammonium sulfate or almost complete saturation with sodium chloride. The protein remaining in solution requires about 0.7


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A. K. Balls and H. Lineweaver

TABLE I

671

Preparation of Cr&alline Papain -

Material and manipulation

Papain milk- dotting units

Frac- ,lUrnl tion of NO. tctioI Br mg.

rotein N

ml.

1 kilo wet papaya latex 17 1 1. 0.04 M NaCN* and 100 gm. diatomaceous 1 .540 17

earth added to 1 kilo wet latex (180 gm. dry weight) and stirred for 1 hr.; pH should be 6.5-7t (green to brom-thymol blue). After filtration residual liquid squeezed out of solid by supporting filter paper with cheese-cloth. Liquid fractions combined. Filtrate

Filtrate 1 brought to pH 9 (slightly greenish to 2 15.5 thymolblue) with2 M NaCN (25 ml.) and centrifuged. Supernatant liquid made 0.4 saturated with solid ammonium sulfate (250 gm. per l.), cooled, and filtered on a weighed layer of Hyflo filter aid. 73 gm. precipitate

Precipitate 2 dissolved in 600 ml. 0.02 M NaCN 3 420 16.5 (resulting pH, 8-8.5) and filtered to remove Hyflo. 60 gm. NaCl added to filtrate. Sus- pension cooled and centrifuged while cold. 0.02 M cyanide solutjon, pH 6.4, added to centrifuged precipitate to make 400 ml. and pH of suspension adjusted to 6.5 (yellow-green to brom-thymol blue) with 0.1 N HCl. Sus- pension

r0tai : 10-a

100

-70

Suspension 3 cooled after standing about 30 4 310 22.5 min. at room temperature. The precipitate which formed was centrifuged at 5’ after standing 18 hrs. in cold. This crystalline precipitate dissolved in 300 ml. neutralized 0.02 M cyanide at room temperature; then 10 ml. saturated NaCl solution added slowly to solution. Suspension of crystals

Recrystallizations made by repeating treat- 5 155 26 ment of Suspension 3. Suspension of second crystals

39

9.7

4.5

4.0

* If the preparation is to be made in the absence of added activator, 1 N NaOH is used to make the alkaline pH adjustments; otherwise the procedure is as given.

t Wherever pH is used in this table it refers to the apparent pH as given with Clark’s indicators on a spot-plate.


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saturation with ammonium sulfate before it is completely precipitated. After this the solution still gives a strong nitroprusside test for sulfhydryl and contains a natural activator of papain that is not precipitated by complete saturation with ammonium sulfate. Crystals of papain have been prepared only from the protein fraction obtained in the first precipitate. This protein was found to differ from the original mixture in that it now precipitated from very dilute salt solutions at low temperatures. Such precipitates are usually partly crystalline. At 5” and pH 6 the crystals are almost completely salted-out by 2 per cent NaCl. The crystalline protein resembles the prolamins to the extent that it is soluble in 70 per cent ethyl or methyl alcohol. The protein can be salted-out of such solutions with lithium sulfate and subsequently recrystallized from water without appreciable loss of activity,

The details of a satisfactory procedure for obtaining crystals of papain are given in Table I.

Properties of Crystals

Activity of Papain Preparations-Table II shows the specific activity observed with preparations made according to the schedule in Table I, and also with preparations made by deviating from this method. The differences are discussed in later para- graphs. The enzymic activity was estimated by the milk-clotting technique and by the rate of hemoglobin digestion and hippurylamide splitting. The methods and units are described at the end of the paper. The technical assistance of Mr. S. Schwimmer is acknowledged with thanks.

E$ect of Oxidation during Preparation-The inactivating effect of oxidation was at first avoided by working in the cold room. At room temperature, however, crystals were obtained indistinguish- able in form from those prepared in the cold, but with varying and lower activity. On repeated crystallization the activity became lower still. This inactivation was not the usual reversible type, for the assays were uniformly made in the presence of an activator, either cyanide or cysteine. Crystals of high activity were obtained if activators (cyanide, sulfide, or cysteine) were present throughout the preparation, or if the air was excluded by nitrogen.


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A. K. Balls and H. Lineweaver 673

After several crystallizations under these conditions the enzyme was found to be relatively stable toward oxygen; its original instability appears therefore to be due to an impurity. Table III illustrates the lability of papain to oxygen and shows that a

TABLE II

Activity of Papain Preparations

1. Final cyanide method

2,a. Method 1 (oxidation occurs due to small size of preparations)

2,b. Same as (2,a), but recrystallizations were in presence of cysteine

3. Hexagonal crystals formed from (2,a) above 4. Same as (2,a) but NI gas present 5. Alkali method (see foot-note, Table I) 6. Same as (2,a) but a large preparation re-

duced extent of oxidation Commercial preparation Freshly prepared water extract of latex High salt fraction

1 21 1 19 2 25 3 27 4 27 8 25 1 15 5 12 5 14

12 22 22 22

Specifio activity*

0.17 120

0.21 0.2 0.21 0.11 0.09 0.12

120 135 120 135 135 115

0.1 120

9 0.03 275 16 0.06 250 14 0.06 230

* One sample of four times crystallized papain gave 3.2 X 10-d Pa. u.l&$ p. N compared to 1.7 X lo-’ for a commercial sample and 6.13

[Pa. u.1::;; N The form01 titration method with 5 per cent Hammarsten

casein solution, pH 7.3, was used to obtain the latter figure.

carrier such as cysteine accentuates the lability. It is quite possible that this loss of specific activity without loss of crystal form is due in part to contaminating heavy metals. This is sup- ported by the fact that the rate of loss is smaller for the purer


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preparations and by the literature reports concerning the cata- lytic effect of metals with regard to activation (21) and inactivation (15).

It is apparent from Table II that the crystals varied in activity with the methods of preparation. The highest activity attained, however, appears to represent a maximum value per unit of protein nitrogen which is constant for preparations made in several ways.

There are grounds for assuming that the maximum activity represents that of the original enzyme. It is difficult to see how oxidation could occur in crude wet latex which contains a large

TABLE III

Loss of Activity of Crystalline Papain at 30’ As Influenced by Oxygen Gas and Suljhydryl

Materid added

Trace of cysteine ‘I ‘I “

Cyanide “ + cysteine

.-

-

PH

6.0 6.0 6.6 6.6

Air Nz Air NZ Air ‘,

-Y--

1

_-

-

Nitroprusside teat (18 hrs.)

- I

0 0

: 0 +

+CN _-

f _-

[Pa. U.]M,i’ . Lossin I activity

3 hrs.

1.20 1.20 1.15 1.15 1.14 1.07

I

-

18 hm. --

pet cent 1.08 10 1.15 4 0.92 20 1.08 6 1.14 0 1.10 -3

*The reproducibility is ~0.05. The activities were determined by the cyanide method of activation, so that the loss in activity measured is irreversible.

quantity of reducing sulfhydryl compounds. Furthermore, many samples of latex collected from trees of different varieties and at different seasons of the year all gave crystals with the same activity when these were prepared by the cyanide method. The same was found to be true of latex that was stirred into a paste with solid ammonium sulfate at the moment of collection.

Crystal Forms of Papain-The crystals obtained by the method described here are thin needles (Fig. 1). In undiluted suspensions they have a mat-like appearance similar to that of pepsinogen (12). They are too thin to permit a satisfactory test for birefringence or other crystallographic studies with our apparatus. The suspen-


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A. K. Balls and II. Lineweaver 675

sions appear to be quite homogeneous under the microscope and careful inspection has failed to show any amorphous material.

The variation in the specific activity of the crystals has been mentioned. No differences in crystal form could be observed in these cases. When prepared by the cyanide method and freed from cyanide by recrystallization, the maximum activity (or nearly this)3 was obtained without the addition of any activator. Other preparations could be brought to the maximum activity only by the use of cyanide or cysteine; still others were inactive and could not be activated to the full extent. Crystals showing less than

FIG. 1 FIG. 2

FIG. 1. Crystalline papain, needle form. X 400. FIG. 2. Crystalline papain, plate form. X 100.

half the maximum activity in the presence of cyanide have been obtained frequently; these have been recrystallized several times from cyanide without further loss or gain in activity. It appears as though oxidation produces first an inactive but activatable form of the protein and later an unact-ivatable form, without material change in the appearance of the crystals. No apparent differences in the chemical behavior of the protein have been observed, and no

3 It is difficult to be sure of the activity of the crystals in the absence of added activator, because some inactivation of the enzyme is almost certain to occur in the assay process. The crystals without added activator are at least partially active in the milk-clotting test. See Table IX.


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significant change of molecular weight has been found between two crystalline preparations having different degrees of activity (see Table V).

Table IV shows that crystals of inferior milk-clotting ability are at least not easily separated by fractional crystallization into por- tions of higher and lower activity.

Needle-like crystals prepared without protection from oxygen were seen to change on several occasions to another crystal form of about the same proteolytic activity (Table II, hexagonal crystals). The new crystals appeared in suspensions of needles after several months standing at 8’. No deliberate attempt to induce their rapid formation by seeding or otherwise has yet been successful. They are large plates with an elongated hexagonal face, as shown in Fig. 2. The rate of solution of these large crystals

Fractional Recrystallization OJ Low Activity Papain Crystallized Six Times

Description

Unfractionated ..................... Fraction l..........................

“ 2 .........................

Remainder .........................

Per cent tots1 protein N

100 11.8

23 11 -12 66 11.5 11 11 -13

in water was much slower than that of the needles, so t,hey were readily washed free from contamination.

The ratio of milk clotting to hemoglobin activity compares with t,hat of all needle cryst,als but is in contrast with that of commercial papain and fresh latex. Needle crystals have been obtained from the hexagonal plates by dissolving them in water and reprecipitating with salt in the usual manner. Since each crystal form has been obtained from the other, there seems to be no reason at the moment to question the identity of the substance.

Ratio of Milk-Clotting Activity to Hemoglobin-Digesting Activity- Examination of Table II shows that in the presence of added activator, usually cyanide, the ratio of milk-clotting activity to that of hemoglobin digestion is reasonably constant for the crystalline preparations. The irreversible oxidative inactivation de- creased bot.h properties to the same extent, which makes it prob-


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able that these propert,ies are related. On the other hand, the ability to clot milk compared with the ability to digest hemoglobin for both fresh latex extracts and commercial papain was more than twice as great as for crystalline papain. The fraction of protein not precipitated from the original extract by 0.4 or 0.5 saturation with ammonium sulfate (but precipitated by 0.7 saturation) was likewise found to show a larger ratio of milk clotting to hemoglobin digestion. From these observations it appears probable that papaya latex contains another enzyme with n higher ratio of milk-clotting to hemoglobin-digesting activity. Compare the recent work of Frankel et al. (8).

Molecular Weight and Di$usion Coejicient---- The molecular weight was found to be 27,000 f 2000, as determined by the osmotic pressure method of Northrop and Kunitz (18), with cellophane tubing and a water manometer. The small solubility of papain in even dilute salt solutions below 12” prevents the accurate measurement of the osmotic pressure in salt concen- trations greater than 0.1 M. However, Table V shows that the Donnan effect is probably negligible, since otherwise the molecular weight should vary with the concentration of both salt and protein.

The diffusion coefficient was determined by the membrane method of Northrop and Anson (17, 3). Although Table VI shows that the diffusion coefficient falls with time, the decrease is practically the same whether determined by protein nitrogen or by activity. The enzyme activity, therefore, appears to be identical with the diffusing protein.

The data of Table VI can hardly be used to calculate the molecular weight. The most reasonable value of the diffusion coefficient, 0.060 cm.?’ per day at lo”, leads to a value higher than that obtained by the osmotic pressure measurements. However, it is to be noted that the limits of the molecular weight as calculated from the coefficients reported are 24,200 and 55,700.

The sedimentation in a quantity type ultracentrifuge was compared with that of crystalline lactoglobulin. Estimation of the molecular weight of crystalline papain from comparative sedimentation factors obtained by the method of Hughes, Pickels, and Horsfall (13) gave a result of 33,000, as shown by Table VII. This is slightly higher than the most probable value derived from


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Crystalline Papain

osmotic pressure measurements, but is nevertheless in substantial agreement therewith. The authors thank Dr. L. F. Martin for his help with the centrifuge.

TABLE V

Molecular Weight of Papain (pH 6.4) by Osmotic Pressure (10’)

Sample No. [Pa. u.]&. p. N “~~~,$[4”‘” Ionic strength

~__

C 13.5 0.306 0.09

“ 13.5 0.328 0.09 3 25 0.74 0.04

4 28 0.59 0.04

At 18” 24 0.177 0.154.2

Average............................................

Mol. wt.

27,000

27,500 25,500 27,400

31,000 Ca.

27,000 f 2000

TABLE VI

Diffusion Experiment

Solvent, 0.02 M NaCN, 0.02 M N&l, pH 6.4. The original solution

contained 0.79 mg. of protein N and 0.185 hemoglobin papain units per ml. Viscosity of solvent at So, 0.014 erg sec. cm.-3

The molecular radius corresponding to D = 0.060 is 2.26 X 10 cm.-’ The molecular weight The molecular volume, cm. per mole, is 29,400.

is 38,200.

Time interval

hrs.

0

21 44 24

Final inside

Amount diffused

-

Protein N

mg. per 1.

7.3 12.9

7*

[Pa. ..]Hb

unit3 per 1.

1.65

2.9 1.45

- I

0.23 0.22 0.22

0.21* 0.23

Diffusion coefficient by

Protein N Activity

cm.-2 per day

0.070 0.068

0.060 0.058

0.060* 0.053

* This figure is estimated from the color given by the diffusate with

the phenol reagent.

Furthermore, a less accurate but entirely different method of estimating the molecular weight also gave comparable results.


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When amounts of iodoacetate too small to inactivate the enzyme completely were added to the active protein at pH 7.0, it followed on the assumption of an equimolecular combination between iodoacetate and papain that the molecular weight of the latter was about 36,000. Such an assumption makes no allowance for a possible equilibrium between active papain and iodoacetate, or for the attack of iodoacetate upon groups in the protein unessential

TABLE VII

Sedimentation oj Papain and Lactoglobulin Centrifuged Simultaneously

The centrifuge was run for 2.75 hours at an average field of 64,000 times gravity. The phenol color values were determined by the technique of Anson (1) with a standard curve to convert the readings to protein.

Level No.

Total

Papain Laotoglobulin

Protein N Phen$~lor Activity Protein N Pho;;mouolor

Sedi- Sdi- Sedi- Sodi- Sodi- Par men- PC%- men- Par men- Per men- Per men-

cent 22t

cent tation cent ja;a cent tation cent tation factor faot‘x factor

-- __- --~~--

13.00 3.67 13.36 3.31 13.21 3.46 13.24 3.43 13.58 3.09 15.70 0.93 15.90 0.77 15.79 0.88 15.02 1.65 15.09 1.58 16.09 0.58 16.15 0.52 16.63 0.04 15.79 0.88 15.62 1.05 16.09 0.58 16.42 0.25 16.30 0.37 16.54 0.13 16.60 0.07 17.07 0.40 16.82 0.15 17.84 1.17 17.13 0.46 16.68 0.01 22.08 5.41 21.40 4.73 19.90 3.23 22.37 5.70 22.35 5.68 ___~ -__--- ~--

11.57 9.73 9.15 12.25 11.48

Aver- age.... 10.15 11.87

With 39,000 for the molecular weight of lactoglobulin, the molecular weight of crystalline papain from these data is 33,000.

to its activity, and to this extent is probably in error. Neverthe- less, the results appear to permit the conclusion that no gross errors are involved in the other molecular weight determinations.

Isoelectric Point of Papain-Ringer (22) in a’ cataphoresis experiment with commercial papain found that the active constituent migrated toward the cathode, and indicated an isoelectric point greater than pH 8. The observation was confirmed in this laboratory on crude papain; the experiment was also extended to


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Crystalline Papain

show that the protein nitrogen and the active constituent, as determined by the casein form01 method, by the hippurylamide method, and by milk clotting, migrated to about the same extent and indicated an isoelectric point above 8.5. The same material gave a maximum turbidity in the region of pH 9 to 9.5. The observation of Willstatter and Grassmann (26) that papain differs from most other enzymes in being only slightly adsorbed by alumina in acid solution, better in neutral, and best in alkaline solution also points to the basic nature of papain. The solubility-pH function of the crystalline material is quite similar to that just described, a fact that has been taken advantage of in the method of purification.

Analytical Results-The nitrogen values reported in Table VIII were determined on a sample prepared by dialysis for 24 hours against running distilled water at l”, thereafter precipitated and washed with acetone, and later with ether, and finally dried in vacua at 50”. The protein nitrogen content of the various preparations has been multiplied by the conversion factor 6.5 to obtain the protein content. Amino nitrogen determinable by the method of Van Slyke (with a Warburg apparatus) amounted to 2.4 per cent of the total nitrogen of the undried crystals, ir- respective of whether these were active, reversibly inactive, or irreversibly inactive. Assuming a molecular weight of 30,000 for papain, there are apparently eight such NH2 groups per molecule, and the reversible inactivation of the enzyme is not con- nected with their disappearance. This conclusion also follows from the behavior of the active protein with small amounts of iodoacetate, which at pH 7 inactivate equivalent amounts of enzyme without altering the amino nitrogen so determined.

There is no phosphorus in the crystalline material. It is seen that the cystine and nitrogen contents of the crystalline

and non-crystalline protein fractions of the latex are nearly alike. The difference between the cystine content of the crystals of high and of low activity (Samples 1 and 2) is not significant, but the cystine content of the high salt fraction appears to be definitely lower. The authors feel that no correlation between the activity and the cystine content is justifiable on the basis of these figures on account of the large probable error of the analyses.

The nature of the essential, reversibly activatable group in


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papain has been the subject of much research. It is commonly thought to be a sulfhydryl group, as suggested by Bersin (7) and Hellermann (11). Okumura (Maeda), however, considers it to be an aldehyde group (20). We have not as yet reached any definite conclusions on this point with respect to the crystalline material.

The active enzyme protein gave no demonstrable test for -SH with nitroprusside, even in the presence of weakly alkaline

TABLE VIII

Analytical Data on Crystalline Papain

Sample No.

1. Four times crystallized

2. Four times crystallized

3. High salt fraction

Per cent N’

15.5 ZL 0.3

15.5 f 0.1

15.5 f 0.1

Per cent

P

0.0

-

1

_-

I 1.2 f 0.

Per cent s, Per cent cystine cystinet

0.94 f 0.04 3.7 zlz 0.1:

0.86 f 0.04 3.4 f O.l!

0.76 f 0.03 3.0 f 0.1

* The same value for the per cent N was obtained whether prepared by precipitation with acetone or trichloroacetic acid. This sample for cystine determination was dried by precipitating with trichloroacetic acid and washing with water.

t We thankMr. Sam R. Hoover of this laboratory for making the cystine determinations for us. He used the method of Dr. Sullivan (23).

cyanide (pH 9). A similar test with an amount of cysteine representing one -SH group in the quantity of protein used gave a distinctly positive test, alone or mixed with the protein. On the other hand, denaturation of the enzyme with strong alkali or heat permitted the development of a definitely positive nitroprusside test.

An attempt was also made to determine the -SH content of crystalline papain by the newly described method of titration with porphyrindin (10). The data are shown in Table IX.


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The behavior of this reagent toward papain did not correspond to that described in the literature for other proteins. At room temperature no end-point was obtained, even after the addition of

TABLE IX M.

Effect of Porphyrindin on Papain ([Pa. u.lmg p N = 63) . .

16 mg. of papain were used for each determination.

1. Untreated enzyme 2. 30 min. after addition of dye 3. 30 “ ‘I ‘I ‘I ‘I

4. 2 hrs. “ “ “ “ 5. 18 ‘( (I ‘I ‘I ‘I

6. Sample 5, 2 hrs. after further addition of dye

7. Sample 6 after 15 days at 8” 8. 30 min. after addition of dye 9. Titrated at pH 4.6

10. 30 min. after addition of dye to enzyme inactivated by iodoacetic acid

-

Total volume

of PorPhY- rindint added

-i ‘8. U.]$ after titrstibn

-

NO ctiva- 1 tar

ml.

0.00 23 0.09 y 24 0.80 r 19 0.80 “ 15 0.80 “ 15 1.60“ 10

1.60“ 5.1 br 1.2 r 0.05 y 5

-

i&N

28 28 25 21

17

8 9

7

Pa- min re- .ain- IW :tive ftm rent- want

Pm :ent

82 86 76 71 71 58

0 11 25

39

701 68

75

squiv* lent8 If dye used

Per lllde

of nzym e

0.5 4.4 4.4 4.4 8.8

8.8 28

0.27

* The pH of the protein solution was 7.2 f 0.1 unless otherwise stated. t 1 ml. of porphyrindin solution was equivalent to 0.5 mg. of cysteine

hydrochloride. We thank Dr. L. F. Martin of this laboratory for the porphyrindin. y signifies the solution was blue for about 2 minutes and faded to a yellow which was stable for more than 30 minutes. r and br signify respectively that the solution was red and blue-red after standing for 30 minutes or for the times indicated.

1 The loss measured by the hemoglobin digestion technique was 75 per cent.

more porphyrindin than was equivalent to the total protein sulfur. At 2-3” no discharge of color occurred for several minutes when a very small amount of porphyrindin was added, thus giving practically a zero titration. In all cases, the addition of a large quantity


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of dye finally produced a red color (even at a pH as low as 4.6). There is, however, no reason to assume that the change in color from blue to red indicates a sulfhydryl group. Tyrosine and other phenols in alkaline solutions may be oxidized by porphyrindin to yield a similar red color. A group that reduces porphyrindin does appear to exist in crystalline papain, and to judge solely from the color produced, it might be a phenol group. Whatever this group may be, however, it is not directly responsible for the enzymic activity. As shown by experiments with iodoacetate, the reac- tion of one group in the papain molecule is capable of inactivating the enzyme; yet the addition of much more.porphyrindin than this failed to inactivate it. Furthermore, pretreatment with enough iodoacetate to inactivate over two-thirds of the papain had no influence on the development or intensity of the red color formed with porphyrindin.

The inactivation of papain with iodoacetate and cystine indicates an essential sulfhydryl group in the active enzyme, in agreement with the results of many other workers. The results with nitroprusside and porphyrindin disagree with these observations. Either there is no sulfhydryl in active papain or the latter methods of testing for it are not consistently valid. It is therefore planned to investigate these conflicting data further.*

Methods

Proteolytic Activity (Hemoglobin)-The method of Anson (1) was used, except that the precipitated hemoglobin was allowed to stand for 30 minutes before filtering and the digestion temperature was 30”. (About 0.2 mg. of commercial papain or 0.01 mg. of latex protein nitrogen was used per assay tube.)

The initial rate of digestion at 30” by 1 activity unit [Pa. u.lHb is such that there is produced per minute in 6 ml. of digestion mixture an amount of color-producing substance not precipitable with trichloroacetic acid that gives Ohe same color with the phenol reagent as 1 milliequivalent of tyrosine. Subscripts such as in [Pa. u.],,. p. N or [Pa. u.],,., etc., refer to the number of units per mg. of protein nitrogen or per ml. of enzyme solution.

Milk-Clotting Activity-This was determined by the method of

4 Further data are now in press and will appear shortly in Nature.


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Balls and Hoover (5), except that 30’ and 5 ml. of Klim were used. Each Klim preparation wag standardized with a dried enzyme preparation kept for reference. This is desirable because occa- sional preparations of Klim varied considerably from the general run in clotting time.

1 activity unit, [Pa. u.l”*, is the amount of papain that will clot 5 ml. of standard milk preparation at 30’ in 1 minute.

Hippurylamide-SpZitting Activity-2 ml. of enzyme solution plus 1 ml. of wateror activator were added to 87 mg. of hippurylamide dissolved in 14 ml. of 0.1 M acetate buffer, pH 4.9. At various times during digestion at 30” samples of 0.7 ml. were removed and titrated with 0.02 N NaOH by a form01 titration technique (16, 19). An increase of 1 ml. is equivalent to 100 per cent splitting.

1 activity unit [Pa. u.lHA is defined as that quantity of enzyme which, in the titration carried out as above, causes an increase in the titration representing splitting of hippurylamide, at the initial rate of 1 milliequivalent per minute.

Casein Digestion-The substrate WM a 5 per cent solution of Hammarsten casein in 0.05 N NaOH heated to 90-95’ for 30 minutes and subsequently adjusted to pH 7.3. This is not to be regarded as a standard preparation, however.

Protein Nitrogen Determination-10 ml. of 2.5 per cent trichloroacetic acid were added to 1 ml. or less of protein solution, heated for 10 to 15 minutes at 75’, and centrifuged. The supernatant liquid was decanted and the precipitate washed with 2.5 per cent trichloroacetic acid and recentrifuged. The protein was then dissolved in 1 ml. of concentrated H#JO~ and gently warmed until slightly charred. Transfer to a micro-Kjeldahl digestion flask was then possible without precipitation.

SUMMARY

A method is described for the isolation of a crystalline protein of high enzymic activity from papaya latex. The enzyme is activated by cyanide, sulfhydryl compounds, and the like, and will digest hemoglobin with a velocity comparable to the pancreatic proteinases. It also clots milk and hydrolyzes hippurylamide. It is quite stable in dilute alkali (up to pH 10.5) but is unstable in dilute acid (below pH 4.5). The isolated protein is but slightly soluble in dilute salt solutions, particularly at low temperatures,


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A. K. Balls and H. Lineweaver

and behaves like a prolamin to the extent that it is soluble in 70 per cent alcohol. It is isoelectric at about pH 9.0 and has a molecular weight of about 27,000 measured by osmotic pressure. The protein contains 15.5 per cent nitrogen, 1.2 per cent total sulfur, 1.0 per cent cystine sulfur, and 0.0 per cent phosphorus.

Doubt is thrown on the validity of porphyrindin and nitroprusside as consistent indicators of -SH in proteins, because they give negative results with papain, while other reactions may be interpreted as indicating the presence of sulfhydryl groups.

The cooperation of the Hawaiian Agricultural Experiment Station in furnishing papaya latex is greatly appreciated, and particular thanks are due to Dr. J. H. Beaumont, Director of the Station, and to Mr. R. R. Thompson, who is conducting the work on papain there.

BIBLIOGRAPHY

1. Anson, M. L., J. Gen. Physiol., 20, 561 (1937). 2. Anson, M. L., J. Gen. Physiol., 20, 565 (1937). 3. Anson, M. L., and Northrop, J. H., J. Gen. Physiol., 20, 575 (1937). 4. Balls, A. K., and Hale, W. S., Cereal Chem., 16, 622 (1938). 5. Balls, A. K., and Hoover, S. R., J. Biol. Chem., 121, 737 (1937). 6. Balls, A. K., Lineweaver, H., and Thompson, R. R., Science, 86, 379

(1937). 7. Bersin, T., in Nerd, F. F., and Weidenhagen, R., Ergebnisse der Enzym-

forschung, Leipzig, 4, 68 (1935). 8. Frankel, M., Maimin, R., and Shapiro, B., Biochem. J., 31, 1926 (1937). 9. Grassmann, W., and Dyckerhoff, H., 2. physiol. Chem., 179,41 (1928).

Lo. Greenstein, J. P., J. Biol. Chem., 126, 501 (1938). 11. Hellerman, L., Physiol. Rev., 17, 454 (1937). 12. Herriott, R. M., J. Gen. Physiol., 21, 501 (1938). 13. Hughes, T. P., Pickels, E. G., and Horsfall, F. L., J. Exp. Med., 67,

941 (1938). 14. Kraut, H., and Bauer, E., 2. physiol. Chem., 164, 10 (1926). 15. Maschmann, E., and Helmert, E., Biochem. Z., 290, 191 (1935). 16. Northrop, J. H., J. Gen. Physiol., 16, 53 (1932). 17. Northrop, J. H., and Anson, M. L., J. Gen. Physiol., 12,543 (1928-29). 18. Northrop, J. H., and Kunitz, M., J. Gen. Physiol., 9, 354 (1926). 19. Northrop, J. H., and Kunita, M., J. Gen. Physiol., 16, 319 (1932). 20. Okumura, S., Bull. Chem. Sot. Japan, 13,534 (1938). Maeda, S., Bull.

Chem. Sot. Japan, 12, 319 (1937). 21. Purr, A., Biochem. J., 29, 13 (1935). 22. Ringer, W. E., Arch. furmacol. sper., 48, 99 (1930).


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23. Sullivan, M. X., Pub. Health Rep., U. S. P. H. S., suppl. 89 (1931). Sullivan, M. X., and Hess, W. C., J. Biol. Chem., 116,221 (1936).

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25. Walti, A., J. Am. Chem. Sot., 60, 493 (1938). 26. Willstitter, R., and Grassmann, W., 2. physiol. Chem., 138, 184 (1924). 27. Willstitter, R., Grassmann, W., and Ambros, O., 2. physiol. Chem.,

161,286 (1926).


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A. K. Balls and Hans LineweaverCRYSTALLINE PAPAIN

ISOLATION AND PROPERTIES OF

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