Dixon-plot

Plotting v against /J will result in a Dixon plot, which is plotted at different fixed [5] with the slope ... 

The plotting of Dixon plot and its slope re-plot (see 5.9.5.9) is a commonly used graphical method for verification of kinetics mechanisms in a particular enzymatic reaction.9 The proposed kinetic mechanism for the system is valid if the experimental data fit the rate equation given by (5.9.4.4). In this attempt, different sets of experimental data for kinetic resolution of racemic ibuprofen ester by immobilised lipase in EMR were fitted into the rate equation of (5.7.5.6). The Dixon plot is presented in Figure 5.22. 

Fig. 5.22. Dixon plot 1/v versus [P] at different initial substrate concentrations.

Fig. 5.23. Slope re-plot the slope of the Dixon plot was plotted against 1/[S].

Table 5.5. Slope re-plot (slope of Dixon plot versus 1/[S])...

In this scheme, EOH is the enzyme, IX is the inhibitor (either a carbamate or an organophosphate). EOH(IX) is analogous to the Michaelis Menton comploc seen with the substrate reaction. EOI is the acyl-enzyme intermediate for carbamates or a phosphoro-enzyme intermediate for the organophosphates. The equilibrium constant for this reaction (K ) is defined as k /k and the phosphorylation or carbamylation constant is defined as k2- In this study 42)y ANTX-A(S) was found to be more specific for AChE than BUChE. The double reciprocal and Dixon plot of the inhibition of electric eel AChE indicated that the toxin is a non-competitive inhibitor decreases, k remains unchanged) (Figure 2). 

Figure 9.6 Dixon plot representations of competitive (a), noncompetitive (b), uncompetitive (c) and mixed (d) inhibition.

In order to obtain further evidence of the involvement of CaM in the inhibition of CaM-PDEl, a kinetic analysis of the inhibition of the activity of PDEl was assessed using different amounts of CaM in the presence of different concentrations of 60 and in the absence of BSA. The BSA was eliminated in order to reduce, though not completely eliminate, the effect of the compound 60 on PDEl itself The results were analyzed by means of Dixon plots.In this analysis, the vertical axes are the reciprocal of the PDEl activity in the presence of each Ca -CaM and 60 concentrations, and the horizontal axes are the 60 concentrations. 

D. L. Purich H. J. Fromm (1972) Biochem. Biophys. Acta, 268, 1. Note Uncompetitive inhibitors display parallel lines in Dixon plots. 

See Double-Reciprocal Plot Hanes Plot Direct Linear Plot Dixon Plot Dixon-Webb Plot Eadie-Hofstee Plot Substrate Concentration Range Frieden Protocol Fromm Protocol Point-of-Convergence Method Dal-ziel Phi Relationships Scatchard Plots Hill Plots... 

EFFECTLVE MOLARLTY DITHIONITE DITHIOTHREITOL DIXON PLOT DIXON-WEBB PLOTS... 

Figure 2. Dixon plot of the inhibition of human urinary kalli-krein by the substrate analog KKI-6. Assay is done at pH 9.0, 37°C usingD-Val-Leu-Arg-pNA (S-2266, Kabi).

If the effect of an inhibitor on an enzyme is to be investigated, the Dixon plot is recommended. To obtain data for the Dixon plot, estimate the reaction rate at constant substrate concentration and vary the inhibitor concentration [I]. At competitive inhibition, all the obtained straight lines coincide at a point with the coordinates X = -Ki, y = 1/Vmax> and at non-competitive inhibition all the straight lines have the same intercept on abscissa at x = -Ki. At... 

Another useful method of data reduction is the Dixon plot, where l/v is plotted against [I], the inhibitor concentration, at a fixed [D]. This allows for the determination of without the need to determine the absolute concentration of [D]—a great advantage in cases in which the substrate is a polynucleotide or a protein, as is often the case in chemotherapy. 

To determine inhibitor constants (K ), repeat step 17 in the presence of one or two different concentrations of inhibitor, [I]. Alternatively, for a Dixon Plot, test a range of inhibitor concentrations at two different substrate concentrations. Then plot 1/v against [I] for each value of [S]. 

A, obtained by Dixon plots (Fig. 1) the value for NP-pdTp-NP is Km (approximately), obtained from Lineweaver-Burk plots under identical experimental conditions. AF° = li V In Ki. 

Rates measured using the [I4C] glucose incorporation assay. Except for bistris, Kj values calculated from Dixon plots, assuming competitive inhibition. 

Inhibition constants (K ) are estimated by Dixon plot analysis and linear regression using ordinary least squares. Apparent Km values are estimated by nonlinear regression (Vavricka et al. 2002). 

Based on the result from the IC50 determination, determination of additional kinetic parameters such as Ki and the inhibition mode are useful (variation of the substrate concentration e.g. Km/4 1 Km with time). Transformation of the Michaelis-Menten equation are used both for calculation the Ki value as well as for graphical depiction of the type of inhibition (e.g. direct plot ([rate]/[substrate], Dixon plot [l/rate]/[inhibitor], Linewaver-Burk plot [l/rate]/[l/substrate] or Eadie-Hofstee plot [rate]/[rate/substrate]). 

Burlingham BT, Widlanski TS. An intuitive look at the relationship of Kj and IC50 a more general use for the Dixon plot. J. Chem. Educ. 2003 80 214-218. 

Dixon Plots. Another linear method for plotting inhibition data, the Dixon plot, is shown in Fig. 17.11 (74). In this method the initial velocity is measured as a function of inhibitor concentration at two or more fixed substrate concentrations. By plotting Hv against [I] for each substrate concentration, the different types of inhibition can easily be distinguished. Further, in cases of competitive or noncompetitive inhibition, the value of K- may be determined from the x-axis value at which the lines intercept. Overall, the Dixon plot is probably the simplest and most rapid graphical method for obtaining a value. 

Figure 17.11. Dixon plots for (a) competitive, (b) uncompetitive, and (c) noncompetitive inhibitors. The solid lines represent enzymatic reactions in the presence of increasing concentrations of substrate. The dashed line represents the reaction at infinite substrate concentration.

Figure 4-24 (o) Dixon plot for a competitive inhibitor 1/u versus [1] in the... 

The Dixon plot of 1/v versus [1] provides another way of identifying the type of inhibition and of determining Ki. The equation for the plot is obtained by multiplying out the equation for the reciprocal plot and then regrouping terms ... 

Figure 4-27 Dixon plot for a noncompetitive inhibitor 1/f versus [I] in the presence of different fixed concentrations of substrate.

The reciprocal equation for noncompetitive inhibition can be rearranged to the equation for the Dixon plot. 

Figure 4 31 Dixon plot for an uncompetitive inhibitor l/v versus [IJ at different fixed concentrations of S.

Enzyme inhibition data are often presented as IC50, the concentration of the inhibitor to cause 50 percent inhibition at one chosen substrate concentration Kt, the inhibition constant (dissociation constant from the inhibitor-enzyme complex) determined by enzyme kinetic analysis (e.g., Dixon plot) and /Cin lcl, the time-dependent inhibition constant for mechanism-based inhibitors. IC50 values can be estimated from the study described earlier. A positive inhibition, defined as dose-dependent inhibition, with the inhibited activity lower than 50 percent of that of the negative control, will require further experimentation to define Ki for a better evaluation of in vivo inhibitory potential. Further, a study to determine Klwul may be performed to evaluate if the inhibitor acts via covalent binding to the active site of the enzyme, leading to time-dependent irreversible inhibition. 

Kj can be determined using Dixon plot with the reciprocal of the activity as the y-axis, and inhibitor concentration as the x-axis. Results with at least two substrate concentrations below are plotted, with Ki calculated as the negative of the x-intercept.29 can also be estimated with the aid of nonlinear regression analysis software such as SYSTAT (SPPS, Inc., Chicago, IL).30... 

Substrate concentration Minimum of two for the Dixon plot three is recommended. 

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