Phase II Conjugative Enzymes

Volume 400. Phase II Conjugation Enzymes and Transport Systems Edited by Helmut Sies and Lester Packer... 

Phase II conjugative enzymes metabolize drugs by attaching (conjugating) a more polar molecule to the original drug molecule to increase water solubility,... 

The passage from one step to the next along the continuum will often depend on a person s characteristics. A biomarker that allows the assessment of a person s susceptibility to alter the progression along the continuum is called a biomarker of susceptibility. Examples are enzymatic genotypes and phenotypes, such as those seen with glutathione-S-transferase M, a phase II conjugation enzyme that often contributes to the detoxification of some electrophilic compounds (Perbellini et al. 2002). 

There are a variety of phase II conjugating enzyme systems that react with functional groups such as -OH, -COOH, -NH2, and -SH, which are either present originally on the target xenobiotic or have been generated by phase I reactions. 

McCarver, D. G., and Hines, R. N. The ontogeny of human drug-metabolizing enzymes Phase II conjugation enzymes and regulatory mechanisms. J. Pharmacol. Exp. Ther. 300,361-366, 2002. 

If liver cells (hepatocytes) are isolated and grown in culture, drugs are exposed to a similar array of enzymes. Because CYP enzymes are bound to the internal membrane fraction of hepatocytes, the liver can be homogenized and a preparation of vesicles of the hepatocyte endoplasmic reticulum called microsomes can be incubated with drug molecules. This preparation suffers because many of the soluble Phase II conjugation enzymes that are found in the cellular cytoplasm are lost. An alternative method for measuring microsomal metabolism involves isolation of the so-called S9 fraction , which includes the cytosolic soluble conjugation enzymes. 

Figure 4.1 (a) Hormone nuclear receptors involved in the regulation of hepatic cytochrome P450 enzymes, phase II conjugation enzymes, and uptake/efflux transporters. (,b) Integrated regulation of phase I and II enzymes as well as uptake and efflux proteins by nuclear receptors. (Modified from [98].)... 

This polymorphism (NAT2) was discovered almost 50 years ago after differences were observed to isoniazid toxicity in tuberculosis patients (66). Subsequently, the differences in isoniazid toxicity were attributed to genetic variability in NAT2, a cytosolic phase II conjugation enzyme primarily responsible for deactivation of isoniazid (67). Indeed, the polymorphism was termed the "isoniazid acetylation polymorphism" for many years until the importance of the polymorphism in the metabolism and disposition of other drugs and chemical carcinogens was fully appreciated (65). 

NUCLEAR RECEPTOR-MEDIATED REGULATION OF PHASE II CONJUGATING ENZYMES... 

The establishment of NRs pregnane X receptor (PXR) and constitutive androstane receptor (CAR) as xenobiotic receptors was published in 1998 [1,2], PXR and CAR were initially found to regulate the phase ICYP3 A and CYP2B enzymes. Subsequent studies have shown that both receptors can also regulate the expression of phase II conjugating enzymes and phase III drug transporters and for this reason, PXR and CAR have been proposed to function as master xenobiotic receptors. 

Besides CYPs, other metabolic enzymes like hydrolases, phosphatases, peptidases, and even some Phase II conjugating enzymes are also found in Caco-2 cells, so careful LCMS analysis of Caco-2 wells in a detailed experiment can provide not only information about the permeability but also some information on the gut wall metabolism, raising early flags for metabolically unstable compounds. Also, unlike PAMPA, Caco-2 results can reflect a para-cellular transport component, at least to a limited extent. Caco-2 monolayers have an average pore size of about 4 A, compared to an estimated 8-13 A found in the intestinal epithelium, but withholding Ca from the medium or chelating it with EDTA can open these tight junctions somewhat to better reflect in vivo pore size." " ... 

Finally, in addition to mediating the process of CYP3A4 induction in humans, the PXR reeeptor is also responsible for the regulation of expression of Phase II conjugating enzymes such as UGTIAI, and it also upregulates... 

As a consequence, developmental variations in the metabolic fate of drugs can occur and are apparent for many phase I (primarily oxidation, cytochrome P450) and phase II (conjugation) enzymes. 

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