Phase II conjugate metabolites

Fig. 3. Phase I adds a small reactive portion to the drug molecule, and Phase II conjugates the Phase I metabolite to an...

The most significant instance of addition of a sulfur-containing group is the phase II conjugation to sulfate of a xenobiotic compound or its phase I metabolite (see Section 7.4.3) by the action of adenosine 3 -phosphate-5 -phosphosulfate, a sulfotransferase enzyme that acts as a sulfating agent ... 

In addition to the examples discussed above, a number of other xenobiotics are measured by their phase I reaction products. These compounds and their metabolites are listed in Table 20.1. These methods are for metabolites in urine. Normally, the urine sample is acidified to release the phase I metabolites from phase II conjugates that they might have formed, and except where direct sample injection is employed, the analyte is collected as vapor or extracted into an organic solvent. In some cases, the analyte is reacted with a reagent that produces a volatile derivative that is readily separated and detected by gas chromatography. 

Xenobiotics are biotransformed by phase I enzymes and phase II conjugation reactions to form a variety of metabolites that are generally more water-soluble and less toxic than the parent compound. Occasionally, the enzymic action of phase I or II systems leads to the formation of unstable intermediates or reactive metabolites that are toxic or carcinogenic. Many physiological factors influence the rate of xenobiotic metabolism and the relative importance of different pathways of metabolic activation or detoxication. 

The lactam derivative dibenz[h/]l 4-oxazepin-ll-(lOH)-one is a primary metabolic product of metabolism and a direct precursor of the urinary hydroxylated metabolites. In rats, the lactam, a dihydro-CR metabolite, an amino alcohol of CR, and an arene oxide are metabolites in CR degradation. In the rat, the major mechanism for elimination is sulfate conjugation and biliary excretion to a limited extent. Phase I metabolism by microsomal mixed fimction oxidases involves reduction of CR to the amino alcohol, oxidation to form the lactam ring, and hydroxylation to form the hydroxylactams. Phase II conjugation reactions sulfate the hydroxylactam intermediates for renal elimination. Amino alcohol intermediates are conjugated with glucuro-nide for biliary secretion. 

CYP450s include steroid hormones and lipid-soluble drugs (Brown, 2001). Oxidative reactions frequently lead to the formation of highly reactive epoxides. These toxic metabolites are usually detoxified rapidly by phase II conjugation or other mechanisms, such as microsomal epoxide hydrolases (Pineiro-Carrero and Pineiro, 2004 Watkins, 1999). 

Oxidation tends to decrease lipophilicity by the introduction of hydrophilic functional groups forming metabolites that may be more readily excreted. In addition, many of these functionalized metabolites are also substrates for Phase II conjugation reactions. 

The metabolites are generally identified as metabolites of Phase I oxidation or Phase II conjugation. If Phase I oxidation is concluded as the major pathway for the oxidative metabolism of the drug, a second study will be performed to evaluate which of the several oxidative pathways are involved. Phase II conjugation pathways can be generally recognized by the identity of the metabolite, and subsequent experiments to further identify the pathways may not be necessary. For instance, if the metabolite is a glucuronide, UGT can be identified as the enzyme involved. 

Figure 3.1 Metabolism of carbon tetrachloride with the production of electrophilic radicals and highly toxic metabolites. GSH denotes the important phase II conjugant glutathione. GSSG denotes oxidized glutathione.

Most of the phase II conjugation reactions (with microsome preparation) are nfissing, so the assumption is made that these reactions and the removal of the conjugates do not affect the kinetics of metaboUsm in the hver. In contrast, extensive metabolism can be obtained with in vitro systems, but trace amounts or different metabolites can be obtained in whole livers. This situation could be due to metabolism in whole liver by other cells or pathways factors such as blood or bile flow contribute in whole liver, but do not exist in in vitro systems. "Hie inability of compounds that can penetrate to the site of metabolism during perfusion is observed in vitro. 

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