protein separation and purification in neat dimethyl sulfoxide

Vol. 176, No. 3, 1991 BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICATIONS

May 15, 1991 Pages 1462-1468

PROTEIN SEPARATION AND PURIFICATION IN NEAT DINETHYL SULFOXIDE

Nancy Chang, Stewart J. Hen, and Alexander N. Kli banov*

Department of Chemistry, Massachusetts Institute of Technology Cambridge, Massachusetts 02139

Received April 8, 1991

SUMMARY: Pure DMSO (instead of water) is used as the reaction medium for protein separations. It is shown that common extracellular proteins (i) have high solubility in DMSO (l-50 mg/ml), (ii) do not irreversibly inactivate in this solvent, and (iii) can adsorb onto carboxymethyl cellulose in DMSO and be subsequently fully desorbed in this solvent by inorganic salts. Ion-exchange chromatography on this resin in DMSO has been used to purify bovine pancreatic trypsin and to separate it from hen egg-white lysozyme in their mixture. Another approach to protein separation in DMSO, fractional precipitation with ethyl acetate (which does not dissolve proteins), has been verified with a mixture of bovine pancreatic chymotrypsinogen and chicken egg ovalbumin. 0 1991 Academic Press, Inc.

The advent of modern biotechnology has led to the ever-growing need for

efficient protein separation methodologies. Downstream processing is a major

bottleneck in recombinant-DNA-based protein manufacturing: the purification of

proteins can contribute up to 90% of the processing costs (1). Traditional

bench-scale methods of protein separation and purification (2-4) cannot always

be easily scaled up because of unattractive economics and insufficient

recoveries (5). Consequently, there has been a surge of recent research into

new approaches to downstream protein processing, e.g., extraction using

aqueous two-phase systems (6) and reversed micelles (7), separation by

immobilized metal affinity chromatography (8,9), and affinity cross-flow

filtration, partition and precipitation (10).

No single protein purification methodology is universally ideal, and

thus an optimal bioseparation scheme comprised of a combination of specific

techniques has to be selected for each case. The success of this strategy is

contingent upon the availability of a diverse range of protein separation

options. In this study, we explore a novel approach to protein separations

which is based on the use of the neat non-aqueous solvent dimethyl sulfoxide

(DMSO) , instead of water, as the process medium.

' To whom correspondence should be addressed.

Abbreviations: DMSO,dimethyl sulfoxide; CM-cellulose,carboxymethyl cellulose.

0006-291X/91 $1.50 Copyright 0 1991 by Academic Press, Inc. All rights of reproduction in any form reserved. 1462


EXPERIMENTAL PROCEDURES

Materials. Enzymes and other proteins were purchased from Sigma Chemical Co.: hen egg-white lysozyme (EC 3.2.1.17) and ovalbumin; and bovine pancreatic trypsin (EC 3.4.21.4), chymotrypsin (EC 3.4.21.1), ribonuclease (EC 3.1.27.5), and cr-chymotrypsinogen A. All chemicals and solvents were obtained commercially and were of analytical grade or purer. DMSO (Aldrich Chemical Co.) was 99.9% pure and used without further purification. Assays. Protein determinations were carried out according to the Lowry method (11) following at least a l&fold dilution of a DMSO solution with water (10 mM phosphate buffer, pH 7.0). Calibration curves were obtained beforehand for each protein under each set of conditions.

Lysozyme and trypsin activities were determined spectrophotometrically using dried cells of Micrococcus lvsodeikticus (12) and N-a-p-toluenesulfonyl- L-arginine methyl ester (13), respectively, as substrates. The concentration of the catalytically competent active centers in trypsin was measured in water by spectrophotometric titration with R-nitrophenyl R'-guanidinobenzoate (14).

The individual concentrations of chymotrypsinogen and ovalbumin in their mixtures were determined by HPLC. Protein solutions in DMSO were diluted lOO- fold with 8 M aqueous solutions of urea and then concentrated 5-fold by ultrafiltration using Amicon microconcentrators (Centricon 10). The resultant samples were injected onto a Waters Protein Pak (300 SW) HPLC size exclusion co1 umn. The mobile phase was 10 mM aqueous phosphate buffer, pH 7.5 (flow rate of 1 ml/min); the protein content was monitored by absorbance at 280 nm.

To determine their solubility in DMSO, proteins were first dissolved in distilled water at 10 mg/ml, the pH was adjusted to the desired value, and the solutions were lyophilized. Each solid protein sample was then added to DMSO and stirred at 30°C. Following complete dissolution, more protein was added, and the procedure was repeated until no more protein dissolved. The undissolved protein was subsequently removed by centrifugation, and the protein concentration in the supernatant was measured by the Lowry method.

RESULTS AND DISCUSSION

Although proteins are insoluble in almost all non-aqueous solvents,

there are a few solvents, such as DMSO and formamide, that dissolve

significant concentrations of common proteins (15). Consequently, we reasoned that it may be feasible to apply classical separation techniques to proteins

dissolved in such solvents instead of water; to our knowledge, this approach

has not been studied before. Since non-covalent interactions, and hence

protein conformations, should be different in organic media compared to water

(I5)Y separation behavior in them should be distinct as well, thereby

expanding the overall scope of bioseparation opportunities. In addition, by

working with such solvents as DMSO instead of water, a major practical

obstacle in bioseparations, protein degradation by endogenous proteases, will

be removed because enzymes are catalytically inactive in such media (15-17).

In this investigation, we chose DMSO to test this approach. DMSO (for a

review, see refs. 18 and 19) has a number of attractive properties as a

solvent, such as high stability, low toxicity, infinite miscibility with

water, lack of odor and color, and relatively low cost. This polar solvent

dissolves most organic and many inorganic compounds and can be easily

1463


separated from water by distillation and also lyophilized. Consequently, DMSO

has found a score of industrial and therapeutic applications.

First, we addressed the issue of protein solubility in neat DMSO. Two

model enzymes, hen egg-white lysozyme and bovine pancreatic a-chymotrypsin,

lyophilized from water at pH 6.0, both readily dissolved in DMSO at

concentrations as high as 50 mg/ml (3O'C); even more concentrated solutions

could be prepared, but upon standing they gradually solidified into

transparent gels. Several other proteins, including bovine pancreatic

trypsin, chicken egg ovalbumin, and horseradish peroxidase could be easily

dissolved in DMSO at least at 1 to 10 mg/ml under the same conditions.

Next, we examined enzyme stability in this solvent. All enzymes tested

thus far have been catalytically inactive in neat DMSO (15-17) because the

protein conformation in this solvent is grossly different from that in water

(15, 20, 21). However, note that from a bioseparations standpoint it does not

matter whether enzymes are active in DMSO as long as the enzymatic activity is

restored following the separation procedure, e.g., upon dilution with water or

lyophilization. Rees and Singer (16) reported that while trypsin was inactive

in DMSO, essentially 100% of its activity could be recovered by simple

dilution with water. We confirmed this observation and extended it to other

enzymes (lyophilized from pH 6.0) including lysozyme, chymotrypsin, and bovine

pancreatic ribonuclease. Nearly all original enzymatic activity was restored

from DMSO solutions not only upon immediate dilution with aqueous buffer but

even after a 24-hour incubation of enzymes in DMSO at 25°C. Thus at least in

the enzymes studied, DMSO causes only reversible inactivation.

Having established the feasibility of working with enzymes in neat DMSO,

we then explored their separation in this medium. The first technique

examined was ion-exchange chromatography. Since lysozyme's isoelectric point

is around 11 (22), the enzyme, lyophilized from aqueous solution at pH 6.0,

should have a net positive charge and, retaining this charge in a non-aqueous

medium (23), should adsorb to cation-exchangers. We tested the adsorption of

lysozyme to CM-cellulose in DMSO (both the enzyme and the resin lyophilized

from pH 6.0). With 25 mg of the ion-exchanger suspended in 10 ml of DMSO, up

to 5 mg of the enzyme (i.e., 200 mg of the protein per gram of the resin) was

found completely adsorbed on CM-cellulose at 30°C within an hour.

The next question was whether it is possible to desorb the enzyme from

CM-cellulose, e.g., by inorganic salts, as routinely done in ion-exchange

chromatography in water (2-4). Out of the many inorganic salts soluble in

DMSO (19), we initially selected one of the simplest, lithium chloride, whose

solubility in this solvent was found to be several moles/liter. When 0.5 M

LiCl was added to lysozyme adsorbed on CM-cellulose, complete desorption of

the enzyme occurred. The same result was obtained with 1.0 M NH,NO,. Thus it

1464


0.0

Volume, ml

Fiwre 1. CM-Cellulose column chromatography of hen egg-white lysozyme (A), bovine pancreatic trypsin (B), and their mixture (C) in neat DMSO. A glass column (13x2 cm) was packed with 9 g of granular CM-cellulose lyophilized from water (containing 20 mM citrate - 60 mM phosphate buffer) at pH 6.0. The packed column was equilibrated with DMSO at 3O'C and the flow rate of 1.2 ml/min. Then the column was loaded with 15 mg of either enzyme (or 7.5 mg each in the case of their mixture) dissolved in DMSO at 5 mg/ml (both enzymes had been lyophilized from water at pH 6.0). Following washing of the column with DMSO, the enzymes were eluted with LiCl solutions in DMSO at the salt concentrations indicated above the arrows in the figure. Throughout the chromatography, 3.5-ml fractions were collected and assayed for protein and enzymatic activity as described in Experimental Procedures. For lysozyme, the unit of enzymatic activity is a 0.1 decrease in absorbance at 450 nm per minute at 25'C; for trypsin the international units are given (13).

is possible to both adsorb the enzyme on an ion-exchanger in DMSO and desorb

it when desired, thereby allowing for chromatography in this solvent.

Figure 1A depicts the results of a column chromatography of lysozyme on

CM-cellulose in neat DMSO. One can see that the entire amount of enzyme

applied adsorbed to the column and that subsequent elution yielded the same

single protein and activity peak. In a typical experiment, the protein

recovery was 96% and that for the enzymatic activity 91%.

Similar experiments were carried out with another enzyme, bovine

pancreatic trypsin. Active site titration (14) of Sigma's commercial sample

1465


of the enzyme revealed that its purity was 57%. When this sample was

dissolved in DMSO and subjected to CM-cellulose column chromatography (Fig.

lB), only 64% of the protein adsorbed to the column. The non-adsorbed

fraction contained no enzymatic activity. The adsorbed fraction was eluted

with 0.05 M LiCl, and the competent active site titration showed its purity

be 80%. In agreement with this result, the specific activity of trypsin was

found to increase after the chromatography by approximately 40%. Thus ion-

exchange chromatography in DMSO significantly purifies the enzyme.

to

The data presented in Figs. 1A and 1B suggest that lysozyme and trypsin

can be separated by CM-cellulose chromatography in neat DMSO due to their

different affinities to this ion-exchanger. Such a separation was indeed

achieved, as illustrated by Fig. 1C: as expected, the inactive fraction of

trypsin went through the column, active trypsin adsorbed and was then eluted

by 0.05 M LiCl; lysozyme, however, remained adsorbed at that salt

concentration and required 0.25 M to be eluted.

While ion-exchange chromatography in water is profoundly affected by the

pH of the aqueous solution, the analogous parameter in DMSO seems the ptl of

the aqueous solution from which the resin was lyophilized. For example,

whereas trypsin (lyophilized from pH 6.0) was chromatographed into two peaks

on CM-cellulose lyophilized from pH 6.0, active and inactive (Fig. lB), the

resin lyophilized from pH 3.0 yielded a single enzyme peak (i.e., both trypsin

fractions bound to the column). Similar behavior was observed in the

following binding capacity experiments (25 mg of CM-cellulose was suspended in

10 ml of DMSO, then lyophilized lysozyme was added, and the amount of enzyme

in the supernatant was measured as a function of the total amount present; the

binding capacity is defined as the onset point of this dependence). The

binding capacity of CM-cellulose for lysozyme when both were lyophilized from

pH 6.0 was 0.24 mg/ml; however, when the pH of the aqueous solution from which

the resin was lyophilized was lowered to 2.0 or raised to 10.0, the binding

capacity dropped to 0.10 or increased to 0.32 mg/ml, respectively. These

differences can be rationalized in terms of the changes in the ionization

state of the resin which increases when the pH is raised. Thus the pH of the

aqueous solution from which the resin and, presumably, the enzyme (the two do

not have to be the same) were lyophilized, may be a useful operational

variable in bioseparations in non-aqueous media, as it is in enzymatic

catalysis in organic solvents (23).

We then investigated yet another, independent approach to protein

separations in DMSO. As pointed out earlier, the vast majority of organic

solvents do not dissolve proteins. Hence when such a solvent is gradually

added to a protein solution in DMSO, at some point the protein should

precipitate. If the dependence of protein solubility in DMSO on the

1466


Concentration of ethyl acetate, %(v/v)

0-l 0 20 40 60

Concentration of ethyl acetate, %(v/v)

Fioure 2. Precipitation of ovalbumin (curves a) and chymotrypsinogen (curves h) from their solutions in DMSO, induced by ethyl acetate. The proteins were lyophilized from pH 6.0 (A) or pH 9.1 (B). For other conditions, see text.

concentration of the non-dissolving co-solvent is sharp and distinct for

different proteins, then this phenomenon can be used for protein separation.

This rationale was verified using the common organic solvent ethyl

acetate and two proteins other than those separated by chromatography in DMSO

(Fig. l), chicken egg ovalbumin and bovine pancreatic chymotrypsinogen. Curve

a in Figure 2A represents the precipitation profile of ovalbumin (lyophilized

from pH 6.0) from DMSO by ethyl acetate. Up to approximately 30% (v/v) of

ethyl acetate the protein solution remained transparent, but at higher

concentrations ovalbumin began to fall out of solution; at 50% of ethyl

acetate, essentially all the protein precipitated. The same experiment was

conducted with chymotrypsinogen (also lyophilized from pH 6.0). As seen in

Fig. 2A (curve b), this protein also was precipitated from DMSO by ethyl

acetate, but significantly less ethyl acetate was needed than for ovalbumin.

An even more marked contrast in the fractional precipitation behavior of the

two proteins was observed when they were lyophilized from pH 9.1 (Fig. 28).

Comparison of curves a and b in Fig. 2 suggests that ethyl acetate

precipitation may be used to separate ovalbumin from chymotrypsinogen. Such a

bioseparation was carried out with 1 ml of DMSO containing 9 mg/ml each of

both proteins (lyophilized from pH 9.1). We added 0.8 ml of ethyl acetate to

it and stirred the resultant mixture for 30 min at 3O'C. The precipitate

formed was separated by centrifugation, the pellet was redissolved in neat

DMSO and, along with the supernatant, was analyzed by HPLC; the protein

content of both the precipitate and the supernatant was also determined by the

Lowry assay. It was found that the addition of ethyl acetate precipitated 69%

of chymotrypsin but only 32% of ovalbumin. Comparison of these data with

those in Fig. 2B indicates that when the proteins are present together they do

not precipitate independently; further investigation of this phenomenon is

1467


underway. At any rate, our results validate the idea of protein separation by

solvent fractional precipitation, in particular if it is used repeatedly.

In closing, we have demonstrated that it is possible to separate and

purify proteins in DMSO. Future work will be directed toward expanding this

strategy to other protein-dissolving organic solvents and separation

techniques as well as to understanding the mechanistic basis of protein

separation in non-aqueous solvents. The approach described herein may be

particularly suitable for the purification of membrane and other hydrophobic

proteins where it may obviate the use of detergents.

ACKNOWLEDGMENTS. This research was supported by NSF's Biotechnology Process Engineering Center at MIT. S.J.H. is an NSF predoctoral fellow.

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