C H A P T E R 17 Protein De termination Martin Guttenberger B. Bradford Assay I. INTRODUCTION From Bradford (1976): Coomassie brilliant blue G250 (Serva Blue G, Serva, Cat. No. 35050). The reagent for this assay is available commercially from Bio-Rad (Cat. No. 500-0006). The protein content of tissues or samples can serve a number of purposes: It can be a research topic of its own (e.g., in nutritional studies; Hoffmann et al., 2002), a loading control in gel electrophoresis (Unl/i et al., 1997), or a reference quantity in biochemical (e.g., yields in protein purification) or physiological (e.g., specific activities of enzyme preparations; Guttenberger et al., 1994) investigations. In addition, with the advent of proteomics, there is an increasing need for protein quantitation in complex sample buffers containing deterg.ents and urea as potentially interfering compounds (Unlii et al., 1997). In any case, care should be taken to obtain correct results. This article focuses on three techniques and outlines the specific pros and cons. C. Neuhoff Assay (Dot-Blot Assay) From Guttenberger et al. (1991) and Neuhoff et al. (1979): Ammonium sulfate for biochemical purposes (Merck, Cat. No. 1.01211), benzoxanthene yellow (Hoechst 2495, Merck Biosciences, Cat. No. 382057, available upon request), cellulose acetate membranes (Sartorius, Cat. No. SM 11200), glycine, and SDS (Serva, Cat. Nos. 23390 and 20763, respectively). Commercially available ammonium sulfate frequently contains substantial amounts of undefined UV-absorbing and fluorescing substances. These lead to more or less yellowish solutions. Use only colourless solutions to avoid possible interference in fluorometry. Solutions are prepared from bidistilled water. Bovine serum albumin (BSA, fraction V, Roche, Cat. No. 735086) is used as a standard protein. Ninety-six-well, flat-bottomed polystyrene microtiter plates (Greiner, Cat. No. 655101) are used for the photometric tests. II. MATERIALS AND INSTRUMENTATION The following reagents are from the indicated suppliers. All other reagents are of analytical grade (Merck): A. Lowry Assay III. PROCEDURES From Lowry et al. (1951): Folin-Ciocalteu phenol reagent (Merck, Cat. No. 1.09001). A detergentcompatible modification of the Lowry assay is available as a kit (Bio-Rad 500-0116). Cell Biology With respect to convenience and speed, microplate reader assays are described where appropriate. These assays can be read easily in conventional instruments 131 Copyright 2006, Elsevier Science (USA). All rights reserved. 13 2 PROTEINS by employing microcuvettes or by scaling up the volumes (fivefold). The composition of the sample (extraction) buffer requires thought with respect to the avoidance of artifactual alterations of the protein and to the compatibility with the intended experimental procedures. The former requires strict control of adverse enzyme activities (especially proteases and phenol oxidases) and, in the case of plant tissues, of interactions with secondary metabolites. A convenient, semiquantitative assay for proteolytic activities allowing for the screening of suitable inhibitors was described by Gallagher et al. (1986). There is some uncertainty as to which assay gives the most reliable results in combination with extracts from plant tissues rich in phenolic substances. The influence of such substances can never be predicted. It is therefore imperative to minimize interaction of these substances with protein in the course of sample preparation. For a more detailed discussion of this problem, see Guttenberger et al. (1994). A frequent source of ambiguity is the use of the term "soluble protein." Soluble as opposed to membranebound proteins stay in solution during centrifugation for I h at 105,000g (Hjelmeland and Chrambach, 1984). All assays described in this article quantitate protein relative to a standard protein. The choice of the standard protein can markedly influence the result. This requires special attention for proteins with a high content of certain amino acids (e.g., aromatic, acidic, or basic amino acids). For most accurate results, choose a standard protein with similar amino acid composition or, if not available, compare different assays and standard proteins. Alternatively, employ a modified Lowry procedure that allows for absolute quantitation of protein (Raghupathi and Diwan, 1994). The most efficient way to prepare an exact dilution series of the standard protein employs a handheld dispenser (e.g., Eppendorf multipette). Typically a sixpoint series is pipetted according to Table I. In any case, avoid a concentration gradient of the sample buffer. Usually samples and standards may be kept at -20~ for a couple of weeks. For longer storage intervals, keep at-80~ A. L o w r y Assay See Lowry et al. (1951). TABLE I P i p e t t i n g S c h e m e for Preparation of a Standard Dilution Series a Concentration Blank 0.2x 0.4x 0.6x 0.8x 1.0x Water Standard protein (2x) Buffer (2x) 5 0 5 4 1 5 3 2 5 2 3 5 1 4 5 0 5 5 a To prepare i ml of each concentration, 1 volume corresponds to 0.1 ml. determinations), dissolve 20 g Na2CO3 in 1 litre 0.10 M NaOH. Keep at room temperature in tightly closed screw-cap plastic bottles. 2. Reagent B: 0.5% CuSO4.5H20 in 1% sodium or potassium tartrate. To make 20 ml of reagent B, dissolve 0.1 g CuSO4-5H20in 20 ml 1% tartrate (0.2 g sodium or potassium tartrate dissolved in 20 ml water). Keep at room temperature. 3. Reagent C (alkaline copper solution): Mix 25 ml of reagent A and 0.5 ml of reagent B. Prepare fresh each day. 4. Reagent D (Folin-Ciocalteu phenol reagent): Dilute with an equal volume of water just prior to use Steps 1. Place 40 ~tl of sample (protein concentration 0.02I mg m1-1) or blank into cavities of a microplate or into appropriate test tubes. 2. Add 200 ~tl of reagent C and mix. Allow to stand for at least 10 min. 3. Add 20 ~tl of reagent D and mix immediately. Allow to stand for 30 min or longer. 4. Read the samples in a microplate reader or any other photometer at 750 nm. Modifications 1. The sample volume may be raised to 140 ~tl when samples are low in protein (0.02 mg ml q or less). In this case, employ double-strength reagent C. 2. If samples have been dissolved in 0.5 M NaOH (recommended for resolubilization of acid precipitates), omit N a O H from reagent A. B. B r a d f o r d A s s a y See Bradford (1976). Solutions Solutions Note: For samples low in protein (0.02 mg m1-1 or less), prepare reagents A and B at double strength. 1. Reagent A: 2% (w/v) sodium carbonate (Na2CO3) in 0.10 N NaOH. To make 1 litre of reagent A (5000 1. Protein reagent stock solution: 0.05% (w/v) Coomassie brilliant blue G-250, 23.8% (v/v) ethanol, 42.5% (w/v) phosphoric acid. To make 200 ml of stock solution (5000 determinations), dissolve 0.1 g Serva PROTEINDETERMINATION blue G in 50 m195% ethanol (denatured ethanol works as well), add 100 ml 85% phosphoric acid, and make up to 200 ml by adding water. The stock solution is available commercially (Bio-Rad). Keep at 4~ The reagent contains phosphoric acid and ethanol or methanol. Handle with due care (especially when employing a dispenser)! 2. Protein reagent: Prepare from the stock solution by diluting in water (1:5). Filter immediately prior to use. Steps 1. Place 4 ~tl of sample (protein concentration 0.11 mg m1-1) or blank into cavities of a microplate or into appropriate test tubes. 2. Add 200 ~tl of protein reagent and mix. Allow to stand for at least 5 min. 3. Read the samples within I h in a microplate reader or any other photometer at 595 nm. Modifications 1. For improved linearity and sensitivity, compute the ratio of the absorbances, 590 nm over 450 nm (Zor and Selinger, 1996). 2. Microassay: For diluted samples (less than 0.1 mg ml-1), proceed as follows: Employ 200 ~tl of sample and add 50 ~tl of protein reagent stock. C. Dot.Blot Assay See Guttenberger et al. (1991). Do not change the chemistry of the membranes. Nitrocellulose will dissolve in the staining solution; PVDF membranes develop a strong background. 13 3 4. SDS stock: To make 30 ml of 10% (w/v) SDS stock solution, dissolve 3 g SDS in approximately 20 ml of water, stir, and make up to 30 ml (allow some time for settling of foam). Keep at room temperature; it is stable for at least 1 year. 5. Elution buffer: 0.25 M glycine-sulfuric acid buffer (pH 3.6) and 0.02% (w/v) SDS. To prepare 1 litre, dissolve 18.8 g glycine in approximately 900 ml water and add 15 ml of 0.5 M sulfuric acid. Slight deviations from pH 3.6 are tolerable. Add 2 ml SDS stock and make up to 1 litre. Keep at room temperature; it is stable for months. The following solutions are not needed for the standard protocol. 6. Washing solution A: Saturated ammonium sulfate, adjust to pH 7.0 with Tris. To make 1 litre, stir ammonium sulfate in warm water (do not heat excessively). Let the solution cool to room temperature overnight and titrate to pH 7.0 with a concentrated (approximately 2 M) solution of Tris (usually approximately i ml is required). Keep at room temperature. As ammonium sulfate tends to produce lumps in the storage bottle it might be easier to weigh the entire bottle, add some water, remove the resulting slurry, and weigh the empty bottle again. To produce a saturated solution (53.1%, w / v ) , dissolve 760 g ammonium sulfate in 1 litre water. 7. Washing solution B: Methanol/acetic acid/water (50/10/40, v/v). To make i litre, mix 100 ml acetic acid and 500 ml methanol; make up to 1 litre. Keep at 4~ 8. Drying solution: 1-Butanol/methanol/acetic acid (60/30/10, v/v). To make 0.1 litre, mix 10 ml acetic acid, 30 ml methanol, and 60 ml butanol. Keep at 4~ use up to six times. Steps Solutions 1. Benzoxanthene stock: To prepare the stock solution add i ml of water to 0.5 g of the fluorescent dye (as supplied, weighing not necessary); keep at-20~ The toxicity of benzoxanthene is not thoroughly studied, it might be mutagenic! 2. Destaining solution: Methanol/acetic acid (90/10, v/v). To make 1 litre, mix 100ml acetic acid and 900 ml methanol. 3. Staining solution: To obtain 100 ml, dilute 80 ~tl benzoxanthene stock in 100 ml destaining solution. Be sure to pour the destaining solution onto the stock solution to prevent the latter from clotting. Keep staining and destaining solutions in tightly closed screw-cap bottles at 4~ in the dark. They are stable for months and can be used repeatedly. Take due care in handling the highly volatile solutions containing methanol! The dot-blot assay is a versatile tool; its different modifications enable one to cope with almost every potentially interfering substance. In the following description the steps for all modifications are included. 1. Preparation of filter sheets (cellulose acetate membrane). Handle the sheets with clean forceps and scissors, do not touch! Cut one corner to aid in orientation during processing of the sheet. Mark the points of sample application (see later). Mount the membrane in such a way that the points of sample application are not supported (otherwise a loss of protein due to absorption through the membrane may be encountered). There are two different ways to achieve these requirements. a. For routine assays it is recommended to mount the sheets in a special dot-blot apparatus (Fig. 1). Mark dot areas by piercing the sheets through small holes in the upper part of the device. 134 A PROTEINS I" I B FIGURE 1 Dot-blot apparatus. (A) Top view. (B) Section along the diagonal. The apparatus has not been drawn to scale. Dashed lines indicate the position of the cellulose acetate membrane. Large circles correspond to the application points, small ones to the holes that are used for piercing the membrane (arrows in B), and solid small ones to the position of the pins that hold together the apparatus. b. For occasional assays, mark the application points by impressing a grid (approximately 1-cm edge length) onto the filter surface (use a blunt blade and a clean support, preferably a glass plate covering a sheet of graph paper). Mount the sheets on a wire grating (preferably made from stainless steel, fixation by means of adhesive tape is recommended; cut off the taped areas prior to staining). 2. Apply samples (0.01-10 mg m1-1) to the membrane sheets in aliquots of 2 ~tl (piston pipettes are highly recommended; well-rinsed capillary pipettes may be used instead). Leave to dry for a couple of minutes. Dilute samples may be assayed by applying samples repeatedly (let the sample dry prior to the next application). 3. Perform heat fixation. Note: This step is imperative for samples containing SDS whereas it might prove deleterious to samples lacking SDS! Bake the dot-blot membranes on a clean glass plate for 10 min at 120~ (oven or heating plate). 4. Remove interfering substances. Note: This step is optional! Its use depends on the presence of potentially interfering substances (mainly carrier ampholytes, but also peptides and the buffer PIPES). Remove interfering substances prior to protein staining by vigorous shaking in washing solution A (3 x 5 min), followed by gentle agitation in washing solution B (3 x 2 min). 5. Stain and destain. Perform staining (10 min) and destaining (5, 5, and 15 min) in closed trays (polyethylene food boxes work very well) on a laboratory shaker at ambient temperature. For the last destaining bath, employ fresh destaining solution; discard the first destaining bath. The incubation times given here represent the minimal time intervals needed. As long as the vessels are closed tightly, each of these steps may be delayed according to convenience (in case of the last destaining bath, rinse in fresh destaining solution before proceeding). 6. Dry the stained membrane sheets. To facilitate cutting dot areas from the sheets, the following drying step is recommended. Shake the membranes in drying solution for exactly 2 min, mount them between two clamps 1 (Fig. 2), and leave them to dry in a fume hood. The dried sheets may be stored in the dark for later analysis. 7. Elute. Prior to elution, cut the dots from the membrane sheet. Perform elution (45 min in 2 ml of elution buffer) in glass scintillation vials on a laboratory shaker at ambient temperature (bright illumination should be avoided). Dried sheets have to be rewetted in destaining solution prior to immersion in elution buffer. It is recommended to dispense the destaining solution (25 ~tl) and the elution buffer with appropriate repetitive devices (e.g., Eppendorf multipette and Brand dispensette, respectively). 8. Take readings in a fluorometer (e.g., Luminescence Spectrometer LS 50B; Perkin-Elmer; Beaconsfield, UK) at 425 (excitation) and 475 (emission) nm. Modification Skip elution and take readings directly from the wet membrane sheets (step 6) with a video documentation system (e.g., DIANA, Raytest GmbH, Straubenhardt, Germany; Hoffmann et al., 2002). Depending on the choice of filters, there might be considerable deviation from linearity. IV. COMMENTS With the exception of protein solutions, most stock solutions have a long shelf life. Discard any stock solu1 Test for chemical resistance prior to first use: The edges of the clamp can be protected by a piece of silicon tubing cut open along one side. PROTEIN DETERMINATION Cons" High sensitivity to potentially interfering substances; least shelf life of the reagents employed. Recommendation: E m p l o y where absolute protein contents are of interest. O" . . . . . . . . . . . . . . . . . . . . . . . . . . . . .... 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 o o o o o o o 0 0 0 0 0 0 0 / O 13 5 7- FIGURE 2 Membrane mounted for drying. Be sure to mount the drying membranes between two clamps of sufficient size to prevent distortion by uneven shrinkage. The weight of the lower clamp should keep the membrane spread evenly. tion that changed its original appearance (e.g., got cloudy or discoloured). Calculate standard curves according to the m e t h o d of least squares. Appropriate algorithms are provided with scientific calculators and most spreadsheet programs for personal computers. It is better to compute standard curves employing single readings instead of means. Be aware of the basic assumptions m a d e in regression analysis. For additional reading on the statistics of standard curves, compare Sokal and Rohlf (1995). A. Lowry Assay Pros: The L o w r y assay exhibits the best accuracy with regard to absolute protein concentrations due to the chemical reaction with polypeptides. It is also useful for the quantitation of oligopeptides. This contrasts with the other two methods, which, as dyebinding assays, exhibit more variation d e p e n d i n g on the different reactivity of the given proteins (standards as well as samples). B. Bradford Assay Pros: The assay is widespread because of its ease of performance (only one stable reagent is needed, low sensitivity to potentially interfering substances, u n s u r p a s s e d rapidity), its sensitivity, and its low cost. Cons: High blank values, requires d u a l - w a v e l e n g t h readings for linearity, and possibly rather high deviations from absolute protein values (depending on the choice of standard protein). Recommendation: E m p l o y where relative protein contents are sufficient (in most cases such as electrophoresis) and where the assay shows no interference by sample constituents (compare Bradford, 1976). C. Dot.Blot Assay Pros: The dot-blot assay combines high sensitivity, an extended range of linearity (20 ng to 20 btg), and high tolerance to potentially interfering substances. The sample is not used up during assay. Hence, it m a y be reprobed 2 (Fig. 3) for immunological tests or detection of glycoproteins (Neuhoff et al., 1981). Cons: More d e m a n d i n g and time-consuming than the other assays and rather expensive (chemicals and instrumentation). Recommendation: E m p l o y where (1) the other assays show interference, especially with complex sample buffers used in one-dimensional 3 and twodimensional 4 electrophoresis; (2) the a m o u n t of sample is limited a n d / o r reprobing of the dotted samples is desirable; or (3) the mere detection of protein in aliquots, e.g., from column chromatography, is n e e d e d (spot 0.2-2 btl onto m e m b r a n e , process according to 2 Sheets containing single dot areas can be marked conveniently by cutting the edges (Fig. 3, Neuhoff et al., 1979). 3 Sample buffer according to Laemmli (1970): 62.5 mM TrisHC1 (pH 6.8), 2% (w/v) SDS, 10% (v/v) glycerol, 5% (v/v) 2mercaptoethanol, and 0.001% (w/v) bromphenol blue. Range of the assay: 0.04 to 10 mg m1-1, i.e., 80 ng to 20 btg in the test. 4 Sample (lysis) buffer according to O'Farrell (1975): 9.5 M urea, 2% (w/v) Nonidet P-40, 5% (v/v) 2-mercaptoethanol, and 2% (w/v) carrier ampholytes. Standards are prepared by a stepwise dilution of the BSA stock solution in a modified sample buffer lacking carrier ampholytes. These are added from a doubly concentrated stock solution (4%, w/v) in sample buffer. Range of the assay: 0.02 to 8 mg m1-1, i.e., 40 ng to 16 btg in the test. 13 6 PROTEINS 1 2 3 4 5 5. In microplates it is important to achieve uniform menisci: Prick air bubbles with a thin wire and mix the plates on a gyratory shaker. 6. Analysis of dilute samples by application of larger sample volumes also increases the amount of potentially interfering substances. Include appropriate controls. A. Lowry Assay FIGURE 3 Usefulincision patterns employed for marking membrane sheets prior to reprobing. Additional patterns may be generated by combination. standard protocol, prevent evaporation by covering the destained membrane with a thin glass plate, view under UV light). V. PITFALLS 1. Solutions containing protein exhibit an altered surface tension. Avoid foaming and pipette slowly and steadily. 2. Extraction or precipitation steps to eliminate interfering substances should be carefully controlled for complete recovery of protein (Lowry et al., 1951). The more demanding dot-blot assay frequently is a good alternative because of a considerable gain of convenience and accuracy with respect to a simplified sample preparation. 3. Omission of known interfering buffer components from just those samples that are intended for protein determination is strongly discouraged as the solubility of proteins might be influenced (carrier ampholytes, e.g., enhance solubilization of membrane proteins in two-dimensional electrophoresis sample buffer; for references, see Guttenberger et al., 1991). 4. In the case of photometers/fluorometers operating with filters (usually microplate readers), the correct wavelength may not be available. Instead, a similar wavelength may be employed [Lowry assay: 530800 nm, Bradford assay: 540-620 nm, dot-blot assay: 366-450 nm (excitation), 450-520 nm (emission)]. In the case of fluorometry, allow for a sufficient wavelength interval between excitation and emission (consult the operating instructions of your instrument). Be aware that considerable deviations from the standard wavelengths will be at the expense of linearity and sensitivity. 1. Many reagents used commonly in protein extraction interfere with this assay. The main groups of interfering substances are reductants (e.g., sulfhydryl compounds such as mercaptoethanol, reducing sugars such as glucose), chelating agents (e.g., EDTA), amine derivatives (many common buffering substances such as Tris), and detergents (e.g., Triton, SDS). A detailed list of interfering substances, along with remedies and tolerable limits, is provided by Petersen (1979). 2. Reagent D is not stable at a basic pH. Immediate mixing after the addition of reagent D is imperative. In microplates the use of a small plastic spatula is convenient for this purpose (change or rinse between samples). 3. The colour reaction takes about 80 min to come to completion. Prior to this, reading of samples over an extended period of time will give rise to experimental error (more than 20%; Kirazov et al., 1993). Keep the reading interval to a minimum. Alternatively, both incubation steps can be cut to 3 min by raising the incubation temperature to 37~ (Shakir et al., 1994). As the time to reach thermal equilibration will depend on the experimental setup, a test run in comparison to the original method is recommended. B. Bradford Assay 1. The commonly used standard protein BSA is highly reactive in this dye-binding assay. As a consequence the protein content of the samples is underestimated. This systematic error does not matter in comparative analyses but brings about wrong absolute values. Bovine y-globulin is a preferable standard. 2. The standard curves are not strictly linear in the original version of the assay. If the necessary equipment for the recommended dual-wavelength ratio is not available, do not extend the range of standard concentrations beyond one order of magnitude or do not calculate standard curves by means of linear regression. 3. Samples containing detergents (1% will interfere) must be diluted (if possible) or precipitated (compare Section V.2) prior to analysis. PROTEINDETERMINATION 4. The p r o t e i n - d y e complex is insoluble a n d will precipitate w i t h time (Marshall a n d Williams, 1992). For highest accuracy, take readings w i t h i n an interval b e t w e e n 5 a n d 20 m i n after a d d i t i o n of the reagent. With crude extracts (e.g., from mycelia of certain fungi), this interval m a y be considerably s h o r t e r m t o o short to take m e a n i n g f u l readings. In this case, alter the w a y of sample p r e p a r a t i o n or use another assay. 5. Plastic a n d glassware (especially quartz glass) tend to b i n d dye. R e m o v e the resulting blue colour b y one of the following procedures: (1) Rinse w i t h glassw a r e detergent (avoid strongly alkaline detergents w i t h cuvettes; rinse t h o r o u g h l y to r e m o v e detergent again), (2) rinse w i t h ethanol or methanol, or (3) soak in 0.1 M HC1 (takes several hours). C. Dot.Blot Assay 1. Generally, it is imperative to p r e v e n t the m e m brane sheets from d r y i n g d u r i n g one of the transfer steps (residual acetic acid will destroy the filter matrix). 2. In case of highly variable results, inspection of the stained filters (last destaining b a t h or dried) u n d e r UV illumination m a y be helpful: B a c k g r o u n d staining resulting from i m p r o p e r h a n d l i n g of the m e m b r a n e s will be visible (do not use UV-irradiated m e m b r a n e s for quantitative analyses). 3. After the w a s h i n g procedure, t h o r o u g h rinsing in w a s h i n g solution B is imperative. A m m o n i u m sulfate a c c u m u l a t i n g in the staining solution will interfere w i t h the assay. 4. A l t h o u g h the dot-blot assay is extremely insensitive to potentially interfering substances, it is advisable to include a p p r o p r i a t e controls (at least b l a n k buffer a n d buffer plus standard). 5. In the case of buffers containing detergent plus carrier a m p h o l y t e s , the storage conditions a n d the n u m b e r of f r e e z e - t h a w cycles m a y p r o v e i m p o r t a n t . Use fresh solutions or r u n a p p r o p r i a t e controls. 6. If m e m b r a n e sheets t u r n t r a n s p a r e n t u p o n drying, they have not been equilibrated p r o p e r l y in the d r y i n g solution (keep in time: 2 min) or the d r y i n g solution has been diluted by a c c u m u l a t i o n of destaining solution (do not reuse the d r y i n g solution too often). References Bradford, M. M. (1976). A rapid and sensitive method for the quantitation of microgram quantities of protein utilizing the principle of protein-dye binding. Anal. Biochem. 72, 248-254. 13 7 Gallagher, S. R., Carroll, E. J., Jr., and Leonard, R. T. (1986). A sensitive diffusion plate assay for screening inhibitors of protease activity in plant cell fractions. Plant Physiol. 81, 869-874. Guttenberger, M., Neuhoff, V., and Hampp, R. (1991). A dot-blot assay for quantitation of nanogram amounts of protein in the presence of carrier ampholytes and other possibly interfering substances. Anal. Biochem. 196, 99-103. Guttenberger, M., Schaeffer, C., and Hampp, R. (1994). Kinetic and electrophoretic characterization of NADP dependent dehydrogenases from root tissues of Norway spruce (Picea abies [L.] Karst.) employing a rapid one-step extraction procedure. Trees 8, 191-197. Hjelmeland, L. M., and Chrambach, A. (1984). Solubilization of functional membrane proteins. Methods Enzymol. 104, 305-318. Hoffmann, E. M., Muetzel, S., and Becker, K. (2002).A modified dotblot method of protein determination applied in the tanninprotein precipitation assay to facilitate the evaluation of tannin activity in animal feeds. Br. J. Nutr. 87, 421-426. Kirazov, L. P., Venkov, L. G., and Kirazov, E. P. (1993). Comparison of the Lowry and the Bradford protein assays as applied for protein estimation of membrane-containing fractions. Anal. Biochem. 208, 44--48. Laemmli, U. K. (1970). Cleavage of structural proteins during the assembly of the head of bacteriophage T4. Nature 227, 680-685. Lowry, O. H., Rosebrough, N. J., Farr, A. L., and Randall, R. J. (1951). Protein measurement with the Folin phenol reagent. J. Biol. Chem. 193, 265-275. Marshall, T., and Williams, K. M. (1992). Coomassie blue protein dye-binding assays measure formation of an insoluble proteindye complex. Anal. Biochem. 204, 107-109. Neuhoff, V., Ewers, E., and Huether, G. (1981). Spot analysis for glycoprotein determination in the nanogram range. Hoppe-Seyler's Z. Physiol. Chem. 362, 1427-1434. Neuhoff, V., Philipp, K., Zimmer, H.-G., and Mesecke, S. (1979). A simple, versatile, sensitive and volume-independent method for quantitative protein determination which is independent of other external influences. Hoppe-Seyler's Z. Physiol. Chem. 360, 1657-1670. O'Farrell, P. H. (1975). High resolution two-dimensional electrophoresis of proteins. J. Biol. Chem. 250, 4007-4021. Peterson, G. L. (1979). Review of the Folin phenol protein quantitation method of Lowry, Rosebrough, Farr and Randall. Anal. Biochem. 100, 201-220. Raghupathi, R. N., and Diwan, A. M. (1994). A protocol for protein estimation that gives a nearly constant color yield with simple proteins and nullifies the effects of four known interfering agents: Microestimation of peptide groups. Anal. Biochem. 219, 356-359. Shakir, F. K., Audilet, D., Drake, A. J., III, and Shakir, K. M. M. (1994). A rapid protein determination by modification ot the Lowry procedure. Anal. Biochem. 216, 232-233. Sokal, R. R., and Rohlf, F. J. (1995). "Biometry." Freeman, New York. Unl/i, M., Morgan, M. E., and Minden, J. S. (1997). Difference gel electrophoresis: A single gel method for detecting changes in protein extracts. Electrophoresis 18, 2071-2077. Zor, T., and Selinger, Z. (1996). Linearization of the Bradford protein assay increases its sensitivity: Theoretical and experimental studies. Anal. Biochem. 236, 302-308.