Phase II metabolism

The reactivity of both endogenous compounds and xenobiotics with CYP is fairly broad, and is governed by a complex combination of physiochemical and structural properties [5]. A comprehensive review of this enzyme system and its critical role in the mechanisms of toxicity for many important chemicals is beyond the scope of this chapter, and the reader is directed to reviews on the topic [6-10]. 

The Phase II enzymes play a major role in the conjugation and detoxification of nucleophiles (by sulfation or glucuronidation) and electrophiles (by reaction with 

Although in many cases the formation of the GSH conjugate protects cellular proteins from electrophilic attack, not all GSH conjugations with xenobiotics lead to detoxification. For example, in the case of haloalkanes, GST-catalyzed conjugation with GSH can lead to the formation of the highly electrophilic episulfonium ion [35], which can covalently bind DNA and cause mutations. 

Although GSH is found in many tissues, it is most abundant in the liver, where GSH levels may reach levels of 5mM or more [42]. GSH is maintained in the millimolar range by de novo synthesis and regenerative reactions however, levels may be severely depleted in times of oxidative stress, for example., as mentioned above in the case of acetaminophen (APAP) overdose and bioactivation, which leaves cellular proteins vulnerable to attack by electrophiles and free radicals. 

The other primary Phase II metabolic pathway is glucuronidation, which involves the formation ofglucuronic acid conjugates of xenobiotics. This is catalyzed by a family of UDP-glucuronosyl transferases (UGTs) in the presence of uridine diphosphate 

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Phase II metabolism The reaction of a phase I metabolite with an endogenous compound, e.g. glucuronic acid, to form a polar compound that is eliminated from the body. 

As inhibitors of certain enzyme reactions and apoptosis related to the development of cancer (Naasani et al, 1998 Yang et al, 2001), specifically by selective induction or modification of phase I and phase II metabolic enzymes so as to increase the formation and excretion of detoxified metabolites of carcinogens. 

The concept of microbial models of mammalian metabolism was elaborated by Smith and Rosazza for just such a purpose (27-32). In principle, this concept recognizes the fact that microorganisms catalyze the same types of metabolic reactions as do mammals (32), and they accomplish these by using essentially the same type of enzymes (29). Useful biotransformation reactions common to microbial and mammalian systems include all of the known Phase I and Phase II metabolic reactions implied, including aromatic hydroxylation (accompanied by the NIH shift), N- and O-dealkylations, and glucuronide and sulfate conjugations of phenol to name but a few (27-34). All of these reactions have value in studies with the alkaloids. 

The predicted metabolites are also the starting point for the phase II metabolic prediction, to find where glucuronidation could occur. All the probable metabolites obtained from CYP metabolism reactions are submitted to a possible phase II reaction catalyzed by UGTs, using the UGT structure(s) as a template. The accessibility component is computed in the UGTcavity to prioritize glucuronic acid transfer. The final metabolite structures are then reported in graphical output or saved to a file. 

A methodology is described to predict the site of metabolism and the potential MBI by CYPs for compounds as well as subsequent possible phase II metabolism by UGTs. On average, for about 85% of the cases, the method predicted the right site of metabolism within the first two atoms in the ranking list and for more than 80% of the MBI inhibitors. The same methodology can also be applied to predict phase II, UGT-mediated metabolism. 

The CYPs as Antitarget Enzymes 278 The UGTs as Antitarget Enzymes 279 The MetaSite Technology 282 Mechanism-Based Inhibitors 285 Phase II Metabolism by UGTs 287 The Flowchart of the Overall Method 287 Conclusions 289 Software Package 290 References 290... 

Table 10.2 Expression profiles of phase II metabolic enzymes in human bronchial mucosa and in vitro cell culture models.

Table 11.3 Summary of the Expression Pattern and Activities of Phase I and Phase II Metabolic Enzymes and Various Peptidases/Proteases in Human Peripheral Lung Tissues, Primary Rat AEC Culture and Human Alveolar Epithelial Cell Line, A549.

Most of the enzymes involved in mammalian phase II metabolism are, like P450s, poor candidates for in vitro biocatalysis, being membrane associated and requiring activated... 

Phase II) metabolism of drugs and the privileged use of a small number of bacterial or fungal species. 

For exogenous compounds such as drugs, various enzymes involved in both phase I and phase II metabolic routes are present, e.g. various isoforms of cytochrome p450, cytochrome b5, glucuronyl transferase and sulfotransferase [15]. 

The precise mechanism of 1,4-dichlorobenzene oxidation to 2,5-dichlorophenol has not thoroughly been investigated. 1,4-Dichlorobenzene is known to be metabolized by cytochrome P-450 (Azouz et al. 1955 Hawkins et al. 1980) in order to be presented to phase II metabolic pathways to increase its water solubility for excretion. A proposed metabolic pathway involving cytochrome P-450 with intermediate formations of metabolites has been outlined for 1,4-dichlorobenzene (Den Besten et al. 1992). No... 

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