Biotransformation enzyme kinetics

Microbial Biotransformation. Microbial population growth and substrate utilization can be described via Monod s (35) analogy with Michaelis-Menten enzyme kinetics (36). The growth of a microbial population in an unlimiting environment is described by dN/dt = u N, where u is called the "specific growth rate and N is microbial biomass or population size. The Monod equation modifies this by recognizing that consumption of resources in a finite environment must at some point curtail the rate of increase (dN/dt) of the population ... 

Hence, sometimes phenomena associated with enzyme kinetics control the rate of biotransformations. If suitable enzymes are present in the microbial community, for example due to consumption of structurally related growth substrates, then we may see immediate degradation of compounds of interest like BQ when they are added to these metabolically competent microbial communities (Fig. 17.17). For such cases, if the abundance of the bacteria is varied, the rate of removal changes accordingly. Consequently, the removal of BQ could be described by a second-order rate law (Smith et al., 1978) ... 

Dixon et al. (2001) described a preliminary PB-PK model to predict JP-8 concentrations in Air Force fuel-cell maintenance workers. The model used data from PB-PK models of naphthalene inhalation in mice and rats and nonane inhalation in rats. In addition to inhalation, a pathway of dermal exposure and a skin compartment were included. For highly exposed people, the PB-PK model was generally in agreement with exhaled-air naphthalene concentrations however, predictions for the low-exposure scenarios were grossly underestimated, especially in female workers, by a factor of 10. The model did not predict blood and urinary concentrations. The major limitation of the Dixon et al. (2001) study was the lack of appropriate human data (e.g., metabolic measures, blood and tissue partition coefficients, and diffusion rates). The Dixon et al. (2001) model predicted a rapid decline in naphthalene concentrations in all compartments after exposure except liver, fat, and brain. The model predicted accumulation in liver, brain, and fat tissues for a 7-day period that included 4-hr exposures on 5 days. Competition for enzyme does not occur only from interactions of different inhaled compounds. Interactions can also occur between inhaled compounds and metabolites formed in the body that require similar enzymes for biotransformation. Detailed kinetic studies with both benzene and -hexane show inhibition of later metabolic steps, phenol to hydroquinone or methyl -butyl ketone to 2,5-hexane dione, by high concentrations of inhaled benzene or hexane, respectively (Medinsky et al. 1989 Andersen and Clewell 1984). 

The use of ILs as solvents for biotransformation reactions is, of course, not limited to kinetic resolutions and the use of hydrolytic enzymes. Numerous other... 

Both purified laccase as well as the crude enzyme from the WRF Cerrena unicolor were used to convert the dyes in aqueous solution. Biotransformation of the dyes was followed spectrophotometrically and confirmed by high performance liquid chromatography. The results indicate that the decolorization mechanism follows MichaeliseMenten kinetic and that the initial rate of decolorization depends both on the structure of the dye and on the concentration of the dye. Surprisingly, one recalcitrant azo dye (AR 27) was decolorized merely by purified laccase in the absence of any redox mediator [46],... 

In cases where the depuration of HOCs from BMOs involves enzyme-mediated biotransformations (Eq. 7.4) or active transport mechanisms, and environmental concentrations are high (e.g. near a point source), depuration rates have been shown to follow Michaelis-Menten kinetics (Spade and Hamelink, 1985). Michaelis-Menten kinetics is elicited when an enzyme or active transport system is saturated with a chemical. This type of kinetics is characterized by lower values of keS at sites with high HOC concentrations. If k s are unchanged at high concentration sites, Michaelis-Menten kinetics will result in elevated BAFs. However, if chemical concentrations become toxic, finfish likely avoid the area and sessile organisms such as mussels may close their valves for extended periods (Huckins et al., 2004). 

Biocatalytk decarboxylation is a imique reaction, in the sense that it can be considered to be a protonation reaction to a carbanion equivalent intermediate in aqueous medimn. Thus, if optically active compoimds can be prepared via this type of reaction, it would be a very characteristic biotransformation, as compared to ordinary organic reactions. An enzyme isolated from a specific strain of Alcaligenes bronchisepticus catalyzes the asymmetric decarboxylation of a-aryl-a-methyhnalonic acid to give optically active a-arylpropionic acids. The effect of additives revealed that this enzyme requires no biotin, no co-enzyme A, and no ATP, as ordinary decarboxylases and transcarboxylases do. Studies on inhibitors of this enzyme and spectroscopic analysis made it clear that the Cys residue plays an essential role in the present reaction. The imique reaction mechanism based on these results and kinetic data in its support are presented. 

Biphasic systems consisting of ionic liquids and supercritical CO2 showed dramatic enhancement in the operational stability of both free and immobilized Candida antarctica lipase B (CALB) in the catalyzed kinetic resolution of rac- -phenylethanol with vinyl propionate at 10 MPa and temperatures between 120 and 150°C (Scheme 30) 275). Hydrophobic ionic liquids, [EMIM]Tf2N or [BMIM]Tf2N, were shown to be essential for the stability of the enzyme in the biotransformation. Notwithstanding the extreme conditions, both the free and isolated enzymes were able specifically to catalyze the synthesis of (J )-l-phenylethyl propionate. The maximum enantiomeric excess needed for satisfactory product purity (ee >99.9%) was maintained. The (S)-l-phenylethanol reactant was not esterified. The authors suggested that the ionic liquids provide protection against enzyme denaturation by CO2 and heat. When the free enzyme was used, [EMIM]Tf2N appeared to be the best ionic liquid to protect the enzyme, which... 

When only a fixed amount of drug is eliminated in a given interval of time, because enzymes for biotransformation and elimination are saturated, the kinetics of drug elimination are zero order (16). Alcohol is the classic example, where blood levels rise exponentially with increased amounts ingested, because elimination mechanisms are saturated and only a certain fraction of the total dose taken can be eliminated before the next dose is ingested. 

In sum, biotransformations may be limited by (1) delivery of the chemical to the organisms metabolic apparatus capable of transforming the chemical, (2) the enzyme s ability to mediate the initial transformation of the chemical, or (3) the growth of a population of microorganisms in response to the presence of a new substrate. Depending on what limits the rate of biotransformation, different mathematical frameworks are required to describe the kinetics of the process both with respect to the nature of the equations and the parameters they require. 

Finally, assuming bioavailability and biouptake do not limit the rate of biodegradation, then we expect the biodegradation kinetics to reflect the rate of growth due to utilization of a substrate or the rate of enzyme processing of that compound (both discussed in more details below). In these cases, the rate of biotransformation, khm, has been studied as a function of the substrate s concentration in the aqueous media in which the microorganisms or enzymes occur. Hence, for an environmental system, we can write ... 

A description of transdermal drug delivery has been produced which is based on the physicochemical properties of the permeant. At this time transdermal delivery is limited to the administration of potent drugs. Higher doses may be accessible if penetration enhancers are incorporated into the formulation. The kinetic model shows what properties these should have and that they are a function of the physico-chemical properties of the drug. Various loss processes, e.g. microbial biotransformation, skin enzyme metabolism can be identified but cannot, as yet, be quantified. 

Many biotransformations are simple functional group conversions, with rates dependent on a range of properties of the molecule and the organism. The various possible biotransformations, including spontaneous reactions, can be viewed as competing reactions kinetically slow biotransformations are frequently only apparent in the absence of alternative rapid biotransformations. Noncovalent protein binding of chemicals may reduce the availability for enzymic metabolism and... 

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