Phase II—Conjugation of Toxicants

Department of Environmental and Molecular Toxicology, North Carolina State University, Raleigh, North Carolina 27695 

Since both Phase I and II biotransformation processes can increase the polarity and, accordingly, the aqueous solubility of the toxicant, these biotransformations can essentially trap the toxicant in the cell by compromising its ability to passively diffuse across the surface membrane of the cell. The cellular elimination of toxicants is facilitated by membrane proteins that actively transport Phase I and II biotransformation products out of the cell and make them available for elimination from the body. The active cellular elimination processes are often referred to as Phase III detoxification/elimination processes. 

Conjugation reactions may be divided into two types of mechanistic reactions. The first involves the formation of a conjugate in which the xenobiotic reacts with a high-energy or reactive endogenous ligand. 

Type I Xenobiotic + reactive conjugating ligand = conjugated product 

Molecular and Biochemical Toxicology, Fourth Edition, edited by Robert C. Smart and Ernest Hodgson Copyright 2008 John Wiley Sons, Inc. 

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Endogenous substances other than metallothionein may be involved in minimizing the effects of heavy metals and excreting them from the body. Hepatic (liver) glutathione, discussed as a phase II conjugating agent in Section 7.4, plays a role in the excretion of several metals in bile. These include the essential metals copper and zinc toxic cadmium, mercury(II), and lead(II) ions and organometallic methyl mercury. 

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. 

Subcellular localization studies have identified P-450-dependent monooxygenase activity in adult hairless mice sebaceous glands. Phase II conjugation pathways have also been identified in skin. Extracellular enzymes including esterases are present in skin, which has been utilized to formulate lipid-soluble ester prodrugs which penetrate the stratum corneum and then are cleaved to release active drug into the systemic circulation. Finally, co-administration of enzyme inducers and inhibitors modulate cutaneous biotransformation and thus alter the systemic toxicity profile. These metabolic interactions that occur in skin have attracted a great deal of research attention and clearly illustrate that skin is more than a passive barrier to toxin absorption. 

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). 

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.

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). 

Figure 4-1. Metabolism of acetaminophen to harmless conjugates or to toxic metabolites. Acetaminophen glucuronide, acetaminophen sulfate, and the mercapturate conjugate of acetaminophen are all nontoxic phase II conjugates. Ac is the toxic, reactive phase I metabolite. Transformation to the reactive metabolite occurs if hepatic stores of sulfate, glucuronide, and glutathione are depleted or overwhelmed or if phase I enzymes have been induced.

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