Linearization of the Bradford Protein Assay Increases Its Sensitivity ...

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A 590 Å e 590 DP 1 K 1 f b 1 n 1 [P] A 450 e 450 D t / e 590 D . [12] e 450 D LINEARIZATION OF BRADFORD PROTEIN ASSAY 305 Equation [12] predicts a linear relationship between the ratio A 590 /A 450 and total protein concentration, [P] t , when the binding is far from saturation (Expression [11]). As explained above, for simplicity the binding sites were assumed to be homogeneous and noninteracting. Under assay conditions, however, binding to a heterogeneous population of binding sites occurs (9). Yet, we found that the same form of linear equation (Eq. [12]) is obtained. The slope of this equation includes the parameters of all different binding sites: equilibrium constants (K) and number of binding sites on a protein molecule (n). FIG. 3. Plots for the free CBBG dye at 450 nm (l) and at 590 nm () under the usual assay conditions. Experimental Support of the Theory The following observations are consistent with the shows a straight line obtained upon plotting A 590 /A 450 linear Eq. [12]: as a function of BSA concentration. (a) While the regular Bradford calibration graph (Fig. 2A) markedly deviates from linearity, Fig. 2B (b) The theoretical equation predicts that the linear curve will intercept the Y axis at a value that equals the ratio of the dye’s molar absorption coefficient at 590 nm over that at 450 nm. These constants were determined and found to be 5200 and 11,300 M 01 cm 01 , respectively (Fig. 3). The ratio of the molar absorption coefficients is 0.46 { 0.01. This value is in excellent agreement with the experimental data of the calibration graph which is 0.48 { 0.02 (average of 15 independent determinations, cf. Figs. 2B, 4, 5, and 6) Calibration Graph The regular Bradford calibration graph (Fig. 2A) shows distinct curvature in the range of 0–20 mg/ml FIG. 2. Calibration graphs of 0–20 mg/ml BSA. (A) The conventional Bradford calibration graph. The linear regression line was calculated for the narrow range of 2–10 mg/ml BSA (). (B) Linearized Bradford calibration graph. The same set of samples was used for both plots. FIG. 4. Linearization of the Bradford calibration graph in the 0.25- ml microplate assay. The straight line was produced by the values derived of 7 points using 0–4 mg BSA. Only the range of 0–100 ng BSA is shown for the demonstration of sensitivity. The slope of the graph is smaller than that of Fig. 2B by approximately 250 due to the 4-fold smaller protein quantities and the 3 orders of magnitude difference in X-axis units.
306 ZOR AND SELINGER The procedure described can be applied to any standard protein, as shown in Fig. 5. Linear curves were obtained for both a-chymotrypsin and BSA, while the response of BSA is 2.8-fold higher than the response of a-chymotrypsin, a value that is in very good agreement with previous reports (11, 12). For both proteins the regular calibration graph was nonlinear (results not shown). It should be kept in mind that protein determination is always relative to the standard and not absolute. BSA is a commonly used standard in the Bradford assay because of its high color yield and availability while other standards may more closely resemble the proteins under determination (Fig. 5 and Ref. (13)). FIG. 5. Linearization of the Bradford calibration graph using a- Determination of Unknown Protein Samples chymotrypsin (l) and BSA (). The linear equations are Y Å 0.053X / 0.474 with R 2 Å 0.998 for a-chymotrypsin and Y Å 0.146X / 0.457 Although Fig. 4 shows that 0.2 mg/ml BSA can be with R 2 Å 0.999 for BSA. accurately determined, the limits of detection of an unknown protein sample must also be established. To this end, rabbit serum was diluted to give 3–110 nl per BSA. However, in a curved graph, a semilinear relationship can be seen over a narrow range of points. The ‘‘close to linear’’ range of the Bradford calibration graph is considered to be 2–10 mg/ml BSA since smaller protein concentrations are characterized by a small signal to noise ratio and therefore cannot be accurately determined from a nonlinear graph. It is important to note that the linear regression in Fig. 2A was calculated using only 5 points (shown as squares) within the ‘‘linear range.’’ Figure 2B shows that a linear correlation exists between A 590 /A 450 and BSA concentration in the range of 0–20 mg/ml, for the same set of samples used in plotting the nonlinear Fig. 2A. Statistical analysis gives standard deviations of 1.2 and 0.9% of error in the narrow range of 2–10 mg/ml BSA, for the regular and corrected calibration curves, respectively. The correlation parameter, R 2 , is increased from 0.976 in Fig. 2A to 0.999 in Fig. 2B. The difference is even more obvious in the full-scale graphs, where the standard deviation is decreased from 3.9% error for the regular calibration curve to 1.2% error for the corrected calibration curve. The statistical analysis proves the improvement in linearity over the standard concentrations range and demonstrates the linear relationship between A 590 /A 450 ratio and protein concentration over the entire range, 0–20 mg/ml. In order to determine the limits of sensitivity, an extended range of protein concentrations was tested. A quantity of 0.2 mg BSA can be accurately determined in the 1-ml assay while Fig. 4 shows that an amount as low as 50 ng BSA is accurately determined in the FIG. 6. Calibration graphs in the absence () or presence (l) of 0.25-ml microplate assay. The linear relationship exto 0.002% SDS. A volume of 0.8 ml dye reagent from Bio-Rad was added 0.2 ml protein sample without () or with (l) SDS. (A) Conven- ists up to 20 mg BSA in the 1-ml assay and up to 4 mg tional Bradford calibration graph. (B) Linearized Bradford calibra- BSA in the microplate assay. It can be concluded that the procedure presented here produces a full-scale linear graph. tion graph. The same set of samples was used for both plots. The linear equations are Y Å 0.205X / 0.457 with R 2 Å 1.000 () and Y Å 0.070X / 0.452 with R 2 Å 0.999 (l).
Page 1 and 2: ANALYTICAL BIOCHEMISTRY 236, 302-30
Page 3: 304 ZOR AND SELINGER K Å [DP] [nP]
Page 7: 308 ZOR AND SELINGER order to devel

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