Mechanisms of Biotransformation

The NADPH-cytochrome P-450 system, commonly known as the mixed-function oxygenase (MFO) system, is the most important enzyme system involved in the Phase I oxidation reactions. Cytochrome P-450 system, localized in the smooth endoplasmic reticulum of cells of most mammalian tissues, is particularly abundant in the liver. This system contains a number of isozymes which are versatile in that they catalyze many types of reactions including aliphatic and aromatic hydroxylations and epoxidations, 

N-oxidations, sulfoxidations, dealkylations, deaminations, dehalogenations, and others (Wislocki et al. 1980). These isozymes are responsible for the oxidation of different substrates or for different types of oxidation of the same substrate. Carbon monoxide binds with the reduced form of the cytochrome, forming a complex with an absorption spectrum peak at 450 nm. This is the origin of the name of the enzyme. As a result of the complex, inhibition of the oxidation process occurs. 

The cytochrome P-450 monooxygenase system. P-4503+ Cytochrome P-450 with heme iron in oxidized state (Fe3+) P-45021 cytochrome P-450 with iron in reduced state S substrate e electron. (Adapted from J. A. Trimbell, 1982. Principles of Biochemical Toxicology. Taylor Francis, London.) 

Contrary to the cytochrome P-450 system, most hepatic Phase II enzymes are located in the cytoplasmic matrix. In order for these reactions to occur efficiently, adequate activity of the enzymes involved is essential. In addition, it is clear that adequate intracellular contents of cofactors such as NADPH, 

glucose 1-phosphate, glucuronate, ATP, cysteine, and GSH are required for one or more reactions. 

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Figure 18.7. Mechanism of biotransformation of bromobenzene in rat liver. [Adapted from...

In this chapter the mechanisms involved in metabolism of warfare nerve agents are discussed. Mechanisms of biotransformation of OP pesticides are beyond the scope of this chapter and interested readers are referred to other publications (Jokanovic, 2001 Chambers et al, 2001 Tang et al, 2006). 

Nitrosamines are not carcinogenic at the point of application. They require bioactivation. One possible mechanism of biotransformation is by enzymatic transformation to a carbonium ion. Activation is known to proceed first by hydroxylation of an a-carbon. The resulting hydroxyalkyl moiety is eliminated as an aldehyde, and an unstable primary nitrosamine is formed. The unstable nitrosamine ultimately tautomerizes to a carbonium ion. The highly reactive carbonium ion readily alkylates with the nearby cellular macromolecules. Cancer and mutagenicity develop when reactive nitrosamine metabolites alkylate to genetic macromolecules. 

Stojiljkovic and Jokanovic review the security and medical aspects of terrorist poisoning of civilian population with nerve gases sarin and VX performed by AUM Shinrikyo fanatic religious cult in 1994 and 1995 in Japan. In the second study they reviews current understanding of biochemical mechanisms of biotransformation of organophosphorus compounds (OPC) describing the role of the enzymes involved in this process... 

Fenner, H. (1974) EPR studies on the mechanism of biotransformation of tricyclic neuroleptics and antidepressants, in Forrest, I.S., Carr, C.J., and Usdin, E. (Eds.), The Phenothi-azine and Structurally Related Drugs, New York, Raven Press, pp. 5-13. 

The problems posed by species differences for the development and screening of new drugs are considerable. Knowledge of the mechanism of biotransformation of a new chemical agent in animals is, however, fundamental to its safety evaluation. 

A more complicated yet extremely enlightening example of the consequence of redox conditions on the ultimate expression of toxic outcomes is the mechanism of biotransformation through the oxidative family of enzymes know as the cytochrome P450 (GYP) superfamily. This enzyme system is responsible for an enormous array of biotransformations for substances that represent an extremely diverse range of molecular classes. 

The mechanism of biotransformation of HD has been elucidated. Once HD enters the target organ and blood, it forms an intermediate sulfonimnion whichis transformed... 

MetaboHsm is concerned with a deterrnination of the biotransformation of the parent material, the sites at which this occurs, and the mechanism of the biotransformation. 

PAHs can be bioconcentrated or bioaccumulated by certain aquatic invertebrates low in the food chain that lack the capacity for effective biotransformation (Walker and Livingstone 1992). Mollusks and Daphnia spp. are examples of organisms that readily bioconcentrate PAH. On the other hand, fish and other aquatic vertebrates readily biotransform PAH so, biomagnification does not extend up the food chain as it does in the case of persistent polychlorinated compounds. As noted earlier, P450-based monooxygenases are not well represented in mollusks and many other aquatic invertebrates (see Chapter 4, Section 4.2) so, this observation is not surprising. Oxidation catalyzed by P450 is the principal (perhaps the only) effective mechanism of primary metabolism of PAH. 

John DM, GF White (1998) Mechanism for biotransformation of nonylphenol polyethoxylates to xenoestro-gens in Peudomonas putida. J Bacterial 180 4332-4338. 

Even though all OP insecticides have a common mechanism of action, differences occur among individual compounds. OP insecticides can be grouped into direct and indirect ACHE inhibitors. Direct inhibitors are effective without any metabolic modification, while indirect inhibitors require biotransformation to be effective. Moreover, some OP pesticides inhibit ACHE more than PCHE, while others do the opposite. For example, malathion, diazinon, and dichlorvos are earlier inhibitors of PCHE than of ACHE. In these cases, PCHE is a more sensitive indicator of exposure, even though it is not correlated with symptoms or signs of toxicity. 

Atrophy of the thymus is a consistent finding in mammals poisoned by 2,3,7,8-TCDD, and suppression of thymus-dependent cellular immunity, particularly in young animals, may contribute to their death. Although the mechanisms of 2,3,7,8-TCDD toxicity are unclear, research areas include the role of thyroid hormones (Rozman et al. 1984) interference with plasma membrane functions (Matsumura 1983) alterations in ligand receptors (Vickers et al. 1985) the causes of hypophagia (reduced desire for food) and subsequent attempts to alter or reverse the pattern of weight loss (Courtney et al. 1978 Seefeld et al. 1984 Seefeld and Peterson 1984) and excretion kinetics of biotransformed metabolites (Koshakji et al. 1984). 

The other factor affecting the use of organic nitrates is nitrate tolerance, the mechanism of which is unclear. An early explanation of tolerance was thiol depletion [68] but that now seems unlikely as their is an abundance of thiol in most tissue [69]. A more likely explanation is down regulation of the enzymes involved in the biotransformation but few details are available. An interesting suggestion is that GTN induces increased production of superoxide from the vascular wall and tolerance is caused by reaction of NO, produced enzymatically from GTN, with superoxide to give peroxynitrite and then nitrate [70] (Eq. (16)). 

Extensive biotransformation studies have been conducted with the As-pidosperma alkaloid vindoline, but much less work has been done with monomeric Iboga and dimeric alkaloids from this plant. The long-standing interest in this group of compounds stems from the clinical importance of the dimeric alkaloids vincristine and vinblastine, both of which have been used for more than 2 decades in the treatment of cancer. Few mammalian metabolites of dimeric Catharanthus alkaloids have been characterized. Thus the potential role of alkaloid metabolism in mechanism of action or dose-limiting toxicities remains unknown. The fact that little information existed about the metabolic fate of representative Aspidosperma and Iboga alkaloids and Vinca dimers prompted detailed microbial, mammalian enzymatic, and chemical studies with such compounds as vindoline, cleavamine, catharanthine, and their derivatives. Patterns of metabolism observed with the monomeric alkaloids would be expected to occur with the dimeric compounds. 

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