Biotransformation enzyme inhibition

Calu-3 cells have shown the ability to perform fatty acid esterification of budes-onide [132], In pre-clinical studies, this esterification results in a prolonged local tissue binding and efficacy, which is not found when the esterification is inhibited [133]. The precise mechanism remains undefined in that the identity of specific enzyme(s) responsible for this metabolic reaction is unclear [134], Assessment of the potential toxicity and metabolism of pharmaceuticals and other xenobiotics using in vitro respiratory models is still at its infancy. The development of robust in vitro human models (i.e., cell lines from human pulmonary origin) has the potential to contribute significantly to better understanding the role of biotransformation enzymes in the bioactivation/detoxication processes in the lung. 

In order to use PBTK modeling in the assessment of mixtures, Cassee et al. (1998) suggest that one of the components is first modeled and regarded as the prime toxicant being modified by the other components. Based on in vitro data on the other components, effects of, e.g., inhibition or induction of specific biotransformation isoenzymes can be incorporated in the model. Effects of competition between chemicals in a mixture for the same biotransformation enzymes may also be incorporated by translating the effects into effects on the Michaelis-Menten parameters that are then incorporated into the model. 

Similar or identical biotransformation pathways, including ability to induce or inhibit biotransformation enzymes... 

Enzyme inhibition. The enzymes of biotransformation may be inhibited by a single exposure to chemicals. This occurs by several mechanisms formation of a complex, competition between substrates, destruction of the enzyme, reduced synthesis of the enzyme, allosteric effects, and lack of cofactors. The consequences will depend on the role of metabolism in toxicity in the same way as induction (see above). 

The rate of biotransformation of a chemical depends on the amount and efficiency of the pertinent biotransformation enzymes. Enzyme activity is partly genetically determined but may also vary between and within people because of enzyme induction caused by previous exposure to the same or related chemicals. Variation in enzyme activity may also be caused by enzyme inhibition due to concurrent exposures. 

Alternatively we could measure the 1C50 or the Ki (inhibitory constant) for the perpetrator. The A) of a perpetrator that is capable of inhibiting an enzyme (or transporter) is the dissociation constant for the enzyme-inhibitor complex. Accurate estimation of the A) requires, among other things, the appropriate definition or specification of the type of enzyme inhibition (e.g., competitive, noncompetitive, or uncompetitive). The appropriate in vitro experiments require that multiple concentrations of the inhibitor must be used as well as a range of substrate concentrations that embrace the substrate Km, and from these experiments both the type of inhibition elicited by the perpetrator can be deduced and the A) value for the perpetrator can be estimated. The Ki will have units of concentration. Alternatively, K values can be computed from /C50 values for an inhibitor. The /C50 is defined simply as the inhibitor concentration that decreases the biotransformation of a substrate at a single, specified concentration by 50%. This parameter obviously also has units of concentration (e.g., pM), and can be related to the Ki as follows. 

The enzyme responsible for the biotransformation of capecitabine to 5 -deoxy-5-fluorocytidine (a precursor to 5-fluorouracil) was evaluated using purified enzyme, cytosol, and microsomes. The purified CES cytosolic enzyme, inhibited by the carboxylesterase inhibitors bis-nitrophenyphosphate and diisopropylfluorophosphate, was identified as belonging to the subgroup CES lAl based on the result of the N-terminal amino acid sequence. 

Metabolism study of new fluorine-containing taxoids. Enzyme inhibition studies have shown that the cytochrome P-450 (CYP) family of enzymes is playing a key role in the biotransformation of paclitaxel and docetaxel. It has been found that the CYP 2C enzyme subfamily is responsible for the 6a-hydroxylation of paclitaxel, while the CYP 3 A subfamily plays a key role in the hydroxylation of the Boc group of docetaxel as well as that of the 3 -phenyl and 2-benzoyl groups of paclitaxel (Figure 1) (11,25)... 

Pyruvate decarboxylase demonstrates high resistance to denaturation by ethanol (up to a concentration of 3 M). Additions of 2.0-3.0 M of ethanol to the reaction mixture increased the biotransformation reaction rate by 30-40%. Enzyme inhibition by benzaldehyde was observed at concentrations greater than 180 mM. 

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