Phase I Oxidations

1a Cytochrome P450. A majority of enzymes involved in xenobiotic detoxication can also catalyze the formation of reactive metabolic intermediates however, oxidation reactions have the highest propensity to produce reactive metabolites. The CYPs are the major enzyme family responsible for the oxidation 

A number of families consisting of one or more distinct proteins comprise this enzyme group. Many xenobiotics are preferentially metabolized by a particular CYP or CYP family however, they may also be substrates for other CYPs. In these instances, the differences in enzyme activity are in the rate at which the oxidation occurs, the site of the modification on the parent compound (regioselectivity), and the steric configuration of the product (stereoselectivity). 

Cytochrome P450 Flavin-Containing Monooxygenase Prostaglandin Synthetase 

A-Dimethyl-4-aminoazobenzene 7,12-Dimethylbenzanthracene Ethanol Halothane A-Nitrosodimethylamine Vinyl chloride Thiocarbamides Thalidomide 

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Reflecting the increasing importance of drug transporters in pharmacokinetics, we need to extend the historical two-phase concept for the metabolism of xenobiotics. As shown in Figure 15.1, the metabolic phases I (oxidation) and II (conjugation) are flanked by drug transporter phases 0 (uptake) and III (export). Phase 0 is the first step... 

Specific examples of phase I oxidative reactions are shown in Figure 6.29. 

Rates of hepatic enzyme processes are either unchanged or slightly increased in obesity. Phase I oxidative processes and conjugation to glucuronides -Phase II - are commonly enhanced and account for some of the observed increases in overall systemic drug clearance. 

As with adults, the primary organ responsible for drug metabolism in children is the liver. Although the cytochrome P450 system is fully developed at birth, it functions more slowly than in adults. Phase I oxidation reactions and demethylation enzyme systems are significantly reduced at birth. However, the reductive enzyme systems approach adult levels and the methylation pathways are enhanced at birth. This often contributes to the production of different metabolites in newborns from those in adults. For example, newborns metabolize approximately 30% of theophylline to caffeine rather than to uric acid derivatives, as occurs in adults. While most phase I enzymes have reached adult levels by 6 months of age, alcohol dehydrogenase activity appears around 2 months of age and approaches adult levels only by age 5 years. 

The BZs are all metabolized in the liver via the hepatic cytochrome P450 (CYP) enzymes through one or both of the following pathways phase I oxidation and dealkylation, and/or phase II conjugation to glucuron-ides, sulfates, and acetylated compounds. Diazepam, chlordiazepoxide, and flurazepam all undergo both phase 1 and phase 11 metabolism. Lorazepam, lorme-tazepam, oxazepam, and temazepam are all metabolized by phase 11 alone and are better tolerated by patients with liver impairment. 

Genetically determined defects in the phase I oxidative metabolism of debrisoquin, phenacetin, guanoxan, sparteine, phenformin, warfarin, and others have been reported (Table 4-4). The defects are apparently transmitted as autosomal recessive traits and may be expressed at any one of the multiple metabolic transformations that a chemical might undergo. 

A small percentage of acetaminophen is metabolized by phase I oxidation with A-hydroxylation of the amide (8.81) (Scheme 8.22). Loss of water affords A-acetyl-p-benzoquinone imine (8.82). Benzoquinones are electron-deficient and strongly electrophilic, a trait of many toxic... 

In addition to cytochrome P-450 enzymes, another enzyme that mediates phase I oxidations is flavin-containing monooxygenase (FMO), likewise contained in the endoplasmic reticulum. It is especially effective in oxidizing primary, secondary, and tertiary amines. Additionally, it catalyzes oxidation of other nitrogen-containing xenobiotic compounds, as well as those that contain sulfur and phosphorus, but does not bring about hydroxylation of carbon atoms. 

Most Phase I oxidations are performed by cytochrome P-450. "Cytochrome," derived from Greek, literally means "colored substance in the cell." The color is derived from the properties of the outer electrons of the transition element iron. "P-450" denotes a reddish pigment with the unusual property of having its major optical absorption peak (Soret maximum) at about 450 nm, when it has been reduced and combined with carbon monoxide.330 Although the name "P-450" was intended to be temporary (until more was known about the substance), the terminology has persisted for 18 yr because of the increasing complexity of this enzyme system and because of the lack of agreement on new nomenclature. 

Hines, R. N., and McCarver, D. G. The ontogeny of human drug-metabolizing enzymes Phase I oxidative enzymes. J. Pharmacol. Exp. Ther. 300, 355-360, 2002. 

Figure 11-17 shows the results of automated, unattended MRM and subsequent MS/MS analysis of buspirone, one of the four substrates to be rapidly metabolized by liver microsomes. A total of 42 MRMs were monitored in this particular MS/MS analysis. The MRMs pre-selected represented the common phase I oxidative metabohtes, including mono- and d-hydroxylation, A-oxide formation, and A-dealkylation, to name a few. For 21 of the MRM transitions, the mass corresponding to the expected oxidative metabolite was added to the mass of the precursor ion (e.g., buspirone MW = 386, mono-hydroxylated buspirone MW = 402), keeping the mass of the product ion of the MRM pair unchanged. For the remaining 21 MRM transitions, the mass of the expected... 

Phase I oxidation generally is described as the addition of an oxygen atom (e.g., as an hydroxyl moiety) to the parent molecule. Phase I oxidation is carried out by multiple enzyme pathways, including the various isoforms of the cytochrome P450 (CYP) family and the non-P450 biotransformation enzymes such as flavin-containing monooxygenase (FMO) and monamine oxidase (MAO). 

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