Inhibition slope

FIGURE 14.16 Lineweaver-Burk plot of mixed noncompetitive inhibition. Note that both intercepts and the slope change in the presence of I. (a) When Ki is less than Ki (b) when Ki is greater than Ki. ... 

Participation in the electrode reactions The electrode reactions of corrosion involve the formation of adsorbed intermediate species with surface metal atoms, e.g. adsorbed hydrogen atoms in the hydrogen evolution reaction adsorbed (FeOH) in the anodic dissolution of iron . The presence of adsorbed inhibitors will interfere with the formation of these adsorbed intermediates, but the electrode processes may then proceed by alternative paths through intermediates containing the inhibitor. In these processes the inhibitor species act in a catalytic manner and remain unchanged. Such participation by the inhibitor is generally characterised by a change in the Tafel slope observed for the process. Studies of the anodic dissolution of iron in the presence of some inhibitors, e.g. halide ions , aniline and its derivatives , the benzoate ion and the furoate ion , have indicated that the adsorbed inhibitor I participates in the reaction, probably in the form of a complex of the type (Fe-/), or (Fe-OH-/), . The dissolution reaction proceeds less readily via the adsorbed inhibitor complexes than via (Fe-OH),js, and so anodic dissolution is inhibited and an increase in Tafel slope is observed for the reaction. 

Figure 3.11 illustrates the mass transfer coefficient for batch-grown R. rubrum and was computed with various acetate concentrations at 200 rpm agitation speed, 500 lux light intensity, and 30 °C. As the experiment progressed, there was an increase in the rate of carbon monoxide uptake in the gas phase and a gradual decrease in die partial pressure of carbon monoxide. Also, a decrease in the partial pressure of carbon monoxide was affected by acetate concentration in the culture media. The value of the slope of the straight line increased with the decrease in acetate concentrations, i.e. 2.5 to 1 g-l. The maximum mass transfer coefficient was obtained for 1 g-l 1 acetate concentration (KLa = 4.3-h 1). The decrease in mass transfer coefficient was observed with the increase in acetate concentration. This was due to acetate inhibition on the microbial cell population as acetate concentration increased in the culture media. The minimum KLa was 1.2h 1 at 3g l 1 acetate concentration. 

The values determined from Figure 5.23 agree well with the values calculated from the equations (Table 5.5), with an error of 3.81% for the slope and 4.65% for the intersect, respectively. The obtained experimental data were consistent with the proposed enzymatic reaction and the reaction mechanisms with uncompetitive substrate inhibition and the noncompetitive product inhibition model. 

There have been a number of observations which show increased excitation and/or reduced inhibition in slices prepared from human epileptic brain tissue. Thus burst discharges can be evoked with stimuli that would not do so in normal animal tissue and these can be blocked by NMD A receptor antagonists. The inhibitory postsynaptic currents (IPSCs) in hippocampal dentate granule cells in slices prepared from temporal lobe epileptic tissue are in fact reduced by stimulation that activates NMDA currents (Isokawa 1996), which are more prolonged than usual and show changes in slope conductance. 

It can be seen from Fig. 14.7 that the polarization curve for this reaction involving p-type germanium in 0.1 M HCl is the usual Tafel straight-line plot with a slope of about 0.12 V. For -type germanium, where the hole concentration is low, the curve looks the same at low current densities. However, at current densities of about 50 AJvcF we see a strong shift of potential in the positive direction, and a distinct limiting current is attained. Thus, here the first reaction step is inhibited by slow supply of holes to the reaction zone. 

Ordinary least squares regression of MV upon MX yields a slope of 17.7 and an intercept of 2.46. Using the previously derived values for and K, and setting Y equal to 40, we can derive the inhibition constant of the competitively inhibited Michaelis-Menten reaction (eq. (39.117))... 

Before trying to interpret this partial inhibition of S by PCP, we need to consider the influence of membrane potential on the 86Rb efflux. To obtain information about the relationship between component S and membrane potential, we measured S (increase in the slope of 86Rb efflux between 2 and 4 seconds) as a function of [K]0 in the efflux solution, in the absence of drug. These data are shown in figure 3 (solid circles) the calculated depolarization, due to increasing [K]0, is given in the upper abscissa seale. 

This is a linear equation, and we can thus expect kobs to track linearly with inhibitor concentration for an inhibitor conforming to the mechanism of scheme B. As illustrated in Figure 6.4, a replot of kobs as a function of [/] will yield a straight line with slope equal to k3 and y-intercept equal to k4. It should be noted that in such an experiment the measured value of k3 is an apparent value as this association rate constant may be affected by the concentration of substrate used in the experiment, depending on the inhibition modality of the compound (vide infra). Hence the apparent value of Ki (Kfw) for an inhibitor of this type can be calculated from the ratio of... 

Inhibition of ALAD and stimulation of ALAS result in increased levels of ALA in blood or plasma and in urine. For example, in a case report of a 53-year-old man with an 11-year exposure to lead from removing old lead-based paint from a bridge, a PbB level of 55 pg/dL was associated with elevated urinary ALA (Pollock and Ibels 1986). The results of the Meredith et al. (1978) study on lead workers and controls indicated an exponential relationship between PbB and blood ALA. Numerous studies reported direct correlations between PbB level and log urinary ALA in workers. Some of these studies indicated that correlations can be seen at PbB levels of <40 pg/dL (Lauwerys et al. 1974 Selander and Cramer 1970 Solliway et al. 1996), although the slope may be different (less steep) than at PbB levels of >40 pg/dL. In a study of 98 occupationally exposed subjects (51 pg/dL, mean PbB) and 85 matched controls (20.9 pg/dL. mean PbB) it was found that log ZPP and log ALA in urine correlated well with PbB levels (Gennart et al. 1992a). In the exposed group, the mean ZPP was 4 times higher than in the controls, whereas urinary ALA was increased 2-fold. 

These three classes of inhibition can be distinguished by virtue of the effect of variations in inhibitor concentration on the slopes and intercepts of reciprocal plots. For competitive inhibition only the slope varies. For uncompetitive inhibition only the intercept varies, while for noncompetitive inhibition both the slope and the intercept vary. 

Enzymes can be used not only for the determination of substrates but also for the analysis of enzyme inhibitors. In this type of sensors the response of the detectable species will decrease in the presence of the analyte. The inhibitor may affect the vmax or KM values. Competitive inhibitors, which bind to the same active site than the substrate, will increase the KM value, reflected by a change on the slope of the Lineweaver-Burke plot but will not change vmax. Non-competitive inhibitors, i.e. those that bind to another site of the protein, do not affect KM but produce a decrease in vmax. For instance, the acetylcholinesterase enzyme is inhibited by carbamate and organophosphate pesticides and has been widely used for the development of optical fiber sensors for these compounds based on different chemical transduction schemes (hydrolysis of a colored substrate, pH changes). 

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