Xenobiotics, enzymatic activation

The decrease in enzymatic activity due to the effect of xenobiotic challenge. 

Enzyme levels and activities within the human population can vary considerably and many of the enzymes involved in the metabolism of xenobiotics are polymorphicaUy distributed in the human population. Genetic polymorphism (from Greek poly many , morph form ) is defined as the occurrence of at least two different alleles, with allele frequencies exceeding 1% at a particular locus. The allelic variants include point mutations as well as deletions and insertions and genetic polymorphism may cause an increase, a decrease, or no change in enzymatic activity. 

Reactive metabolites of xenobiotics may differ in reactivity, and therefore have varying impact on enzymatic activities in terms of proximity to their origin. For example, some intermediates are highly reactive and directly inhibit the enzyme that leads to their formation. These substances are commonly referred to as suicide inhibitors, for obvious reasons. Some suicide inhibitors, such as piperonyl butoxide (PBO), a pesticide synergist) are common inhibitors of certain CYP isozymes. PBO amplifies the toxicity of certain insecticides by inhibiting the insect s CYP enzymes that are involved in its degradation. It is metabolized to a highly reactive carbene, which forms an inhibitor complex with the heme iron of CYP, as shown in Scheme 3.6. 

Other enzymatic activities which are present in the cells, e.g. transferases, hydrolases, etc. which may also serve to metabolize the xenobiotic. In addition, the level of endogenous OR should be adequate for P450 catalytic activity. OR appears to be expressed in virtually all mammalian cell lines, however the levels can vary substantially (Sawada et al., 1991). Alternatively, OR coexpression can be performed. 

Systems reconstituted from purified CYP, NADPH-cytochrome P450 reductase and phosphatidylchloline will, in the presence of NADPH and 02, oxidize xenobiotics such as benzphetamine, often at rates comparable to microsomes. Although systems reconstituted from this minimal number of components are enzymatically active, other microsomal components, such as cytochrome bs, may facilitate activity either in vivo or in vitro or may even be essential for the oxidation of certain substrates. 

As previously mentioned, many of the enzymes involved in xenobiotic metabolism are inducible. Inducibility allows for more enzymatic activity, thereby ensuring an adequate detoxication response however, it also provides a mechanism whereby an activation pathway may be increased. This occurs in the example given earlier of the combined effects of ethanol and acetaminophen. When CYP2E1 is induced by ethanol prior to administration of acetaminophen, subsequent activation of acetaminophen to NAPQI is prevalent however, without induction by ethanol, CYP2E1 is not the predominant enzyme for metabolizing acetaminophen, and detoxication is favored. Interestingly, simultaneous administration decreases the toxicity of acetaminophen because both are substrates for 2E1 ethanol acts as a competitive inhibitor, thereby blocking the activation of acetaminophen. 

Activation of drugs to give toxic products is common. Apart from non-enzymatic activation (e.g., via autoxidation), activation by enzymatic one-electron oxidation or reduction frequently occurs. Several non-specific oxidases and reductases are encountered in mammalian tissues. Enzyme systems that have been studied in detail are peroxidases and microsomal oxidases and reductases. Xanthine oxidase also has received some attention. In many insta .ces the end products of the reaction are critically dependent upon the presence of oxygen in the system. This is because oxygen is an excellent electron acceptor, i.e., it can oxidize donor radicals, forming superoxide in the process. In this way a redox cycle is set up in which the xenobiotic substrate is recovered. The toxic effects of the xenobiotic often can be attributed to the oxidative stress arising from such a cycle. However, it seems that for some substrates, oxidative stress of this kind can be less damaging than anaerobic reduction. Anaerobic reduction can lead to formation of further reduced products with additional toxicity. 

A major organ of drag metaboUsm is the Hver, with a variety of DMEs impH-cated in the chemical alteration of drags and xenobiotics. This phase I metaboHsm of compounds frequently results in alterations such that the compounds are more rapidly eliminated from the body, and/or become ineffective or toxic. Oxidations mediated by enzymes of the GYP (cytochrome P450) superfamily account for a major proportion of these alterations. Thus, there is an urgent need to identify which compounds may interact with cytochrome P450s (as substrates) or which compounds may be (adversely) affected through induction/inhibition of cytochrome enzymatic activity. 

Most xenobiotics are biotransformed by enzymes in the liver, although there is also significant xenobiotic enzyme activity elsewhere in the body. These hepatic enzymes have broad specificity and can metabolize a wide variety of xenobiotics. Within the fiver the biotransforming enzymes are located in the microsomes (smooth endoplasmic reticulum), due to the solubility of fipophific xenobiotics in lipid membranes. There is also additional enzymatic activity in the cytosol and to a smaller extent in other areas of the cell. 

Metabolic enzymatic activity in the liver is one of the body s main neutralization strategies for xenobiotics, including drugs. Drug molecules, the substrates of interest here, are catalyzed into products called metabolites. Drug metabolism can be divided into three phases (Schultz 2006) ... 

The enzymatic activation of xenobiotics is known to take place in those organs in which the endoplasmic reticulum is developed. In insects, these functions are believed to be executed in fat bodies, Malpighian tubules, and various parts of the digestive tract. The age-activity profile of the relevant enzyme(s) in the silkworm shows that the most potent activity is seen in the late larval stage hence, the detection can be most effectively carried out by administering a test compound at this stage. 

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