Biotransformation rates

The volumetric ratio of the two liquid phases (j6 = Forg/ Faq) can affect the efficiency of substrate conversion in biphasic media. The biocatalyst stability and the reaction equilibrium shift are dependent on the volume ratio of the two phases [29]. In our previous work [37], we studied the importance of the nonpolar phase in a biphasic system (octane-buffer pH 9) by varying the volume of solvent. The ratio /I = 2/10 has been the most appropriate for an improvement of the yield of the two-enzyme (lipase-lipoxygenase) system. We found that a larger volume of organic phase decreases the total yield of conversion. Nevertheless, Antonini et al. [61] affirmed that changes in the ratios of phases in water-organic two-phase system have little effect upon biotransformation rate. 

Moreover, a-, p- and y-HBCD diastereoisomers are chiral. Thus HBCD have three pairs of enantiomers (+)a, (—)a, (+)p, (—)p, (+)y and (—)y. The enantiomers have identical physicochemical properties and abiotic degradation rates, but may have different biological and toxicological properties and therefore different biotransformation rates. These transformations may result in nonracemic mixtures of the enantiomers that were industrially synthesized as racemates [16, 19]. The rates of metabolisation process of the enantiomers of chiral environmental pollutants may be significantly different [20],... 

Table 3 AHTN and HHCB biotransformation rates in activated sludge measured by Federle et al. [12] and Artola-Garicano et al. [13]... 

It was found that three factors affect the biotransformation rates of A9PEO the origin of the bacterial culture, temperature, and the initial concentration of A9PE0n. Biotransformation kinetics of mixed bacterial cultures from the brackish water layer was faster than those from the saline water layer. Rate constants (based on first order kinetics) for... 

As it was shown before that the wastewater plume spreads mainly in the upper fresh/brackish layer of the estuary [6], the difference between biotransformation rates in the two water layers is explained by a better pre-adaptation of the brackish water bacteria to A9PEO in their natural habitat, due to higher pre-exposures. It seems that the bacterial populations in these two physically very close habitats are quite different. 

Figure 8. Response of first order biotransformation rate constants for naphthalene oxidation to naphthalene perturbation frequency in a continuous activated sludge biotreatinent reactor.

The majority of compounds that enter the organism require metabolism in order to be excreted. If the parent compound is responsible for the toxicity and its metabolites are less toxic, an increased biotransformation rate will reduce the toxicity, and conversely. However, if the chemical s toxicity is mainly due to its metabolite, stimulating the biotransformation will enhance the toxicity. 

Overall biotransformation rate in biphasic systems depends on both mass transfer and biochemical reaction. In this section, we focus on the parameters... 

Amot, J.A., Meylan, W., Tunkel, J., Howard, P.H., Mackay, D., Bonnell, M. and Boethling, R.S. (2009) A quantitative structure-activity relationship for predicting metabolic biotransformation rates for organic chemicals in fish. Environ Toxicol Chem, 28, 1168-1177. 

Following oral administration to animals, dichlorvos is rapidly absorbed from the digestive tract and extensively metabolized in the liver. The metabolism of dichlorvos has not been clearly elucidated because almost none of its potential metabolites has been yet unequivocally identified due mainly to its very rapid biotransformation rate (6). It appears, however, that the initial hydrolysis of dichlorvos, which occurs in all species, leads presumably to dichloroacetaldehyde (40), which is further metabolized by reduction to dichloroethanol or oxidation to dichloroacetic acid. In addition, dealkylation to desmethyldichlorvos appears to be another minor route of biotransformation, except in the mouse where desmethyldichlorvos constitutes at least 18% of the administered radioactivity. The metabolites of dichlorvos do not persist in tissues, whereas only trace levels occur in the milk of lactating mammals (41). There is no evidence tliat the metabolites of dichlorvos are toxic. 

Estimating Biotransformation Rates for Michaelis-Menten Cases... 

Illustrative Example 17.9 Estimating Biotransformation Rates of an Organic Pollutant in a Natural System... 

The major focus in the discussions below is on the chemical nature of the enzymatic catalysts and coenzymes used in the initial transformation step. We will also pay some attention to the details of these enzymatic mechanisms. This will provide a basis for understanding how mathematical expressions describing the associated transformation rates can be derived when enzyme-catalyzed reactions limit the overall biotransformation rate (i.e., steps 2, 3 or 4 shown in Fig. 17.1). 

The first process that can limit initial biotransformation rates involves delivery of the organic substrates from the environment to the microorganism s exterior. This... 

Biotransformation rate constant k = 0.096 d-1 for rainbow trout (Sijm et al. 1990). 

Biotransformation rate constant for bacterial transformation of 1 x 10 10 mLcelL lr1 in water (Mabey et al. 1982). 

Biotransformation rate constant of 1 x 10 7 mLcelL1 h 1 for bacterial transformation in water (Mabey etal. 1982). Bioconcentration, Uptake (k) and Elimination (k2) Rate Constants ... 

---