Time course experiments, experimental

The very slow dissociation rates for tight binding inhibitors offer some potential clinical advantages for such compounds, as described in detail in Chapter 6. Experimental determination of the value of k, can be quite challenging for these inhibitors. We have detailed in Chapters 5 and 6 several kinetic methods for estimating the value of the dissociation rate constant. When the value of kofS is extremely low, however, alternative methods may be required to estimate this kinetic constant. For example, equilibrium dialysis over the course of hours, or even days, may be required to achieve sufficient inhibitor release from the El complex for measurement. A significant issue with approaches like this is that the enzyme may not remain stable over the extended time course of such experiments. In some cases of extremely slow inhibitor dissociation, the limits of enzyme stability will preclude accurate determination of koff the best that one can do in these cases is to provide an upper limit on the value of this rate constant. 

Recently, this method was adapted to label two commercially available liposomal formulations doxorubicin encapsulated in polyethylene glycol (PEG)-coated liposomes (Caelyx /Doxil ) (14) and daunorubicin encapsulated in small distearoyl-phosphatidyl-choline/cholesterol liposomes (Daunoxome ) (15). Although no DTPA was encapsulated in these liposomes, the labeling efficiency was typically between 70% and 80% and the radiolabeled preparations were stable in vivo during the time course of the experiment (four hours). Most likely, the lipophilic In-oxine avidly associates with the lipid bilayer and encapsulation of DTPA might not be necessary when the experimental observation period does not exceed four to six hours. 

Implantation of polymer matrices that contain angiogenic factors requires quantification of the extent of vessel ingrowth. This can either be analysed immunohistochemically or by haemoglobin/red blood cell count in the tissue. These models generally do not allow analysis of the time course of vascularization since this would require the sacrifice of animals. Application in a dorsal skin fold chamber circumvents this experimental problem, as it provides the opportunity to monitor vessel formation at various time points during the experiment. 

Of course, if we do the experiment only once, the particle will be either in or out of the box and p will be pretty much meaningless (unless p = 1 or p = 0). Quantum mechanics does not typically allow us to predict the outcome of any one experiment. The only way to find the probability p experimentally is to do the experiment many times. If we do the experiment N times and find the particle in the box i times, then the experimental value of p is i/N. Quantum mechanics provides predictions of this experimental value of p. 

The outcome of many biochemical experiments can be expressed as simple descriptive statements, such as the desired band or peak was observed . In others, such as studies of the rates of processes or the affinity of a ligand for its target, the results need to be given in quantitative form as numerical values of one or more parameters. These quantitative results are derived by mathematical analysis of the raw experimental data. As an example, Figure 8-4 illustrates the time course of an enzymatic reaction. The raw data are the dependence of product concentration (the dependent variable, conventionally shown on the y-axis) on the time (the independent variable, conventionally on the x-axis). Another example is the result of the fluorescence titration of a protein with DNA shown in Figure 7-14. In this case, the independent variable is the concentration ratio of protein/DNA and the dependent variable is the observed fluorescence intensity. 

Measurements of motor activity have been used to evaluate the potential central nervous system (CNS) effects of a wide variety of drugs and toxicants. One of the advantages of such measures of motor function is that no training is required of the subject. In addition, measures of motor activity can be made repeatedly across time so that the time course, including the onset and reversibility of toxicant effects, can be determined. In these types of repeated measurement experiments, moreover, an animal can serve as its own control, meaning that the experimenter looks for a change in the animal s normal pattern of motor activity after receiving the toxicant compared to the pattern observed before the treatment. 

Fig. 4. Time-resolved assembly experiment, showing (from top to bottom) the time course of the intensities in the scattering regions C (top), R (middle), and M (bottom, compare Fig. 2) the solid trace below the experimental curves is the temperature T. Note the pronounced undershoot in the top curve accompanying the temperature jump, due to the disappearance of ring oligomers. The subsequent rise in the first and third curve is due to the nucleation and growth of microtubules. From [11]...

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