Biphasic kinetic behavior

As mentioned above, V(0 Pr)3-modiricd silica treated with BuOOH shows no activity towards cyclohexene at room temperature. However, the material obtained by grafting Ti(0 Pr)4 directly on unmodified silica heated at 500°C, eq 4, does convert olefins to their epoxides upon treatment with BuOOH. The catalyst 4 also shows biphasic kinetic behavior towards cyclohexene, Figure lc. 

Chiba et al. [163] studied the oxidative metabolism of omeprazole in 14 human liver microsomes in relation to the 4 -hydroxylation capacity of S-mephenytoin. The formation of 5-hydroxyomeprazole and omeprazole sulfone from omeprazole exhibited a biphasic kinetic behavior, indicating that at least two distinct enzymes are involved in either of the metabolic pathways of omeprazole. These findings suggest that S-mephenytoin 4 -hydroxylase is an enzyme primarily responsible for the 5-hydroxylation of omeprazole and further metabolism of omeprazole sulfone, but not for the sulfoxidation of omeprazole in human liver microsomes. 

By means of stopped-flow spectrophotometry, the reaction of flavopapain IV withN-benzyl-l,4-dihydronicotinamide (NBzNH) has been studied anaerobically. Using conditions of excess substrate and following the decrease in absorbance of Eox (at 427 nm) with time, we observed biphasic kinetic behavior. The experimental data could be fitted to a scheme using two consecutive first-order processes, and rate constants could be determined for the two phases. A reasonable interpretation of our results is that a labile intermediate is formed in the reaction, as illustrated in Equation 4, where ES represents the intermediate. In this interpretation, the second, substrate-independent, slower phase of the reaction corresponds to the breakdown of the intermediate ES (fc3 step in Equation 4). The initial, faster phase of the reaction corresponds to the formation of the intermediate from Eox and N-benzyl-l,4-dihydronicotinamide. Using Equation 5, the calculated rate constants for this phase, fcf, can be related to Ks and k2- Values of kCat and Km were calculated from the relationships of Equations 6 and 7, using the measured values of the kinetic parameters k2, 3, and Ks, and the numbers obtained were in reasonable agreement with the values obtained aerobically. 

C. The Kinetic Behavior of a Two-Enzyme System in Biphasic Media ... 

Kinetic behavior of the two-enzyme system (lipase-lipoxygenase) in biphasic media (curve c in Fig. 5) is compared with kinetics of lipoxygenase in the same biphasic medium (b) and in an aqueous medium (a). These curves demonstrated that the configuration of the media influences the production rate of HP. As previously stated, lipoxygenase in biphasic media has an apparent kinetic behavior different from that in aqueous media (see difference between curves a and b in Fig. 5). 

In the meantime temperature-dependent stopped-flow measurements were conducted on the latter complex in order to determine the activation parameters of the N-N cleavage reaction (24). Plots of the absorption intensity at 418 nm vs. time at T — —35 to +15°C indicate biphasic kinetics with two rate constants 0bs(p and obs(2)> in analogy to our measurements of the tungsten complex. This time, however, both rates depended upon the acid concentration. Interestingly much smaller rate constants 0bs(i) and 0bs(2)> were found for all acid concentrations than given by Henderson et al. for his (single) rate constant kobs (up to 1 order of magnitude). Furthermore plots of 0bs(i) and kohs(2) vs. the acid concentration showed no saturation behavior but linear dependencies with slopes k and k and intercepts k und k, respectively (s — acid dependent and i — acid independent), Eq. (2) ... 

Preliminary time-resolved fluorescence measurements give a somewhat better indication of what the cage lifetime might be. ° Both the rise in the fluorescence of the anion and the decay of neutral emission of 8-hydroxy-1,3,6-pyrene were measured. Careful examination of both the rise of the anion and the decay of the neutral indicate the presence of biphasic behavior. The biphasic kinetics can more readily be seen in DjO. It is quite possible that the faster rate constant is due to protons that immediately escape the cage, while the slower decay is due to the destruction of the cage. 

The sorption of mono-CB on marine sediment required 3 hours to reach a sorp-tion/desorption equiUbration (Zhao et al, 2001). Compared with sediment organic carbon content, surface and microporosity of the sediment might have a more important effect on the sorption of mono-CB (Zhao et al, 2001). Temperature does not have great influence on the adsorption behavior of mono-CB in seawater, but the saturate sorption capacity decreases as the temperature increases. The Tenax-mediated desorption of HCB from four freshly spiked, artificial sediments exhibited biphasic kinetics the first-order rate constants of the fast desorbing phase and slow desorbing phase were 9.6 x 10 and 7.2 x lO h respectively (Chai et al, 2007). The fast desorption fractions for the four sediments varied from 41.2 to 68.8% (Chai et al, 2007). 

More recently, the use of fast reaction techniques such as stopped flow and temperature jump with better resolution, has allowed the observation of more than one phase. Scott and Scheraga (1963) reported biphasic kinetics for thermal unfolding of RNase studied under certain conditions by stopped-flow techniques and followed by the change in tyrosyl absorbance at 287 nm (pH 1.3). Complex kinetics were observed in the lower one-third of the thermal transition curve, and a practically two-state behavior was described in the upper two-third of the transition curve. 

Reversible unfolding of chymotrypsinogen, at low pH was found to follow biphasic kinetics, a fast phase was detected in the millisecond time range in addition to the slow phase (in seconds) previously observed by Pohl. The amplitude of the fast process was small and increased at temperature near the upper end of the transition curve the same behavior was observed for RNase. The kinetics of unfolding were followed by temperature jump and by pH jump identical results were obtained at the same final pH and temperature (Tsong and Baldwin, 1972a,b). 

Analysis of the scheme in Fig. 6.61 demonstrates that the fluctuating enzyme with only two conformational channels can display complex kinetic behavior including substrate inhibition (Fig. 6.62A), sigmoidal kinetics (Fig. 6.62B), convex biphasic (Fig. 6.63A and B), and concave biphasic behavior (Fig. 6.63C). The first three phases exhibit positive cooperativity, whereas the last one shows negative cooperativity, observed experimentally. 

The lipases demonstrated very high stability in media partially or totally composed of organic solvent. In such media, the lipases catalyze esterification, transesterification, and resolution of enantiomers [19,45,75,97-100]. Nevertheless, several biphasic systems (organic-aqueous) are used for hydrolysis of lipid and fats [7,34,101]. Kinetic studies in biphase media or in inverted micelles demonstrate that the lipase behavior is different... 

Mean clearance (CL) values for cetuximab are displayed as a function of dose in Fig. 14.3. Mean CL values decreased from 0.079 to 0.018 L/h/m2 after single cetuximab doses of 20 to 500 mg/m2, respectively. In the dose range 20 to 200 mg/m2, CL values decreased with dose. At doses of 200 mg/m2 and greater, CL values leveled off at a value of approximately 0.02 L/h/m2. This biphasic behavior suggests the existence of two elimination pathways. The elimination of cetuximab apparently involves a specific, capacity-limited elimination process that is saturable at therapeutic concentrations, in parallel with a nonspecific first-order elimination process that is non-saturable at therapeutic concentrations. Increasing doses of cetuximab will therefore ultimately lead to the saturation of the elimination process that is capacity-limited and that follows Michaelis-Menten kinetics, whereas the first-order process will become the dominant mechanism of elimination beyond a particular dose range. 

Figure 8.4 shows the influence of e on the x (r) shape. For fixed (k, A), we simulated the time courses for e = 0.5, 1, 2, 5. It is noted that the shape of the substrate profiles varies remarkably with the values of e thus profiles of biphasic, power-law, and nonlinear type are observed. So, the sensitivity of the kinetic profile regarding the available substrate and enzyme amounts is studied by using several e values for low substrate or high enzyme amounts the process behaves according to two decaying convex phases, in the reverse situation the kinetic profile is concave, revealing nonlinear behavior. 

The transition from spherical to rod-like micelles changes the dimensionality of the diffusion space and affects the reaction dynamics [84]. The decay kinetics were predicted to follow approximate biphasic first-order kinetics, with the separate components corresponding to the two characteristic micelle dimensions. This behavior has been verified experimenfally [85[. 

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