Monooxygenase nonspecific

Since endosulfan is a cytochrome P450-dependent monooxygenase inducer, the quantification of specific enzyme activities (e.g., aminopyrine-A -demethylase, aniline hydroxylase) may indicate that exposure to endosulfan has occurred (Agarwal et al. 1978). Because numerous chemicals and drugs found at hazardous waste sites and elsewhere also induce hepatic enzymes, these measurements are nonspecific and are not necessarily an indicator solely of endosulfan exposure. However, these enzyme levels can be useful indicators of exposure, together with the detection of endosulfan isomers or the sulfate metabolite in the tissues or excreta. 

One important finding from purification studies as well as cloning and expressing of individual isoforms is that the lack of substrate specificity of microsomes for monooxygenase activity is not an artifact caused by the presence of several specific cytochromes. Rather, it appears that many of the cytochromes isolated are still relatively nonspecific. The relative activity toward different substrates does nevertheless vary greatly from one CYP isoform to another even when both are relatively nonspecific. This lack of specificity is illustrated in Table 7.2, using human isoforms as examples. 

In the early 1960s, during investigations on the N-demethylation of aminoazo dyes, it was observed that pretreatment of mammals with the substrate or, more remarkably, with other xenobiotics, caused an increase in the ability of the animal to metabolize these dyes. It was subsequently shown that this effect was due to an increase in the microsomal enzymes involved. A symposium in 1965 and a landmark review by Conney in 1967 established the importance of induction in xenobiotic interactions. Since then, it has become clear that this phenomenon is widespread and nonspecific. Several hundred compounds of diverse chemical structure have been shown to induce monooxygenases and other enzymes. These compounds include drugs, insecticides, polycyclic hydrocarbons, and many others the only obvious common denominator... 

Oxidation is by far the most important Phase I metabolic reaction. One of the main enzyme systems involved in the oxidation of xenobiotics appears to be the so called mixed function oxidases or monooxygenases, which are found mainly in the smooth endoplasmic reticulum of the liver but also occur, to a lesser extent, in other tissues. These enzymes tend to be nonspecific, catalysing the metabolism of a wide variety of compounds (Table 9.2). Two common mixed function oxidase systems are the cytochrome P-450 (CYP-450) and the flavin monoxygenase (FMO) systems (Appendix 12). The overall oxidations of these systems take place in a series of oxidative and reductive steps, each step being catalysed by a specific enzyme. Many of these steps require the presence of molecular oxygen and either NADH or NADPH as co-enzymes. 

Monooxygenations are those oxidations in which one atom of molecular oxygen is reduced to water while the other is incorporated into the substrate. Microsomal monooxygenation reactions are catalyzed by nonspecific enzymes such as the flavin-containing monooxygenases (FMOs) or the multienzyme system that has cytochrome P450s (CYPs) as the terminal oxidases. 

The location, structure, classification, regulation, and mechanism of action of CYPs are discussed in Chapter 9. This chapter summarizes the reactions involved in the oxidation of xenobiotics by CYP and other enzymes. Although microsomal monooxygenase reactions are basically similar with respect to the role played by molecular oxygen and in the supply of electrons, the enzymes are markedly nonspecific, with both substrates and products falling into many different chemical classes. It is convenient, therefore, to classify these activities on the basis of chemical reactions, bearing in mind that not only do the classes often overlap, but the same substrate may undergo more than one oxidative reaction. 

Resistance to insecticides can be due to enhanced oxidative metabolism caused by cytochrome P450 monooxygenases. This type of resistance usually results in producing less toxic metabolites. Even when the metabolites are more toxic, often resistance prevails, perhaps because the toxic metabolites are less stable, cannot reach the site of action due to change in polarity, or are neutralized by other factors. As we have already seen (Chapter 8), the cytochrome P450 enzyme system is rather nonspecific in its attack on organic compounds. Hence, this resistance factor is nonspecific, explaining much of the cross-resistance observed. 

Higher cholesterol levels (correlated with exposure levels), higher blood creatinine levels, marked disturbances of the hepatic cytochrome P-450 content and of the associated microsomal monooxygenase system, as well as the inhibition of succinic-oxidase enzyme activity, may also be considered as nonspecific biomarkers of carbon disulfide exposure. More research, however, needs to be done in order to determine whether a direct correlation exists between these parameters and carbon disulfide exposure. 

In the case of the nonspecific drug monooxygenase system in Uver and other tissues of many organisms one would expect a rather loose binding of the various drug substrates if a common binding site at only one enzyme were to exist. This,... 

Some relatively nonspecific enzymatic formation of caffeic (12), ferulic (13), and synapic (14) acids has been noted (Davin et al., 1992). Monooxygenases of microsomal fractions appear to be involved. For example, a specific p-coum-arate-3-hydroxylase has been isolated from mung beans. However, other work suggests that the carboxyl group of p-coumaric acid must be esterified as a quinic acid ester before... 

Cometabolism observed at methanotrophs is a result of nonspecific methane monooxygenase activity towards organic compounds that do not serve as carbon or energy sources (Brigmon, 2001). A soluble form of monooxygenase (sMMO), which is present in the type II methanotrophs, has low substrate specificity. It is able to oxidize several alkanes and alkenes, cychc hydrocarbons, aromatics, and halogenic aromatics (Grosse et al. 1999) for instance, trans-dichloroethylene, vinyl chloride (Yoon Semrau, 2008), dichloromethane (DCM) (Chiemchaisri et al. 2001), and 1,1,1-trichloroethane (TCA) (Kjeldsen et al. 1997). 

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