Phase I enzymes

The metabolism of foreign compounds (xenobiotics) often takes place in two consecutive reactions, classically referred to as phases one and two. Phase I is a functionalization of the lipophilic compound that can be used to attach a conjugate in Phase II. The conjugated product is usually sufficiently water-soluble to be excretable into the urine. The most important biotransformations of Phase I are aromatic and aliphatic hydroxylations catalyzed by cytochromes P450. Other Phase I enzymes are for example epoxide hydrolases or carboxylesterases. Typical Phase II enzymes are UDP-glucuronosyltrans-ferases, sulfotransferases, N-acetyltransferases and methyltransferases e.g. thiopurin S-methyltransferase. 

It is not presently known what phase I enzymes metabolize methyl parathion, and consequently, whether metabolism differs between children and human adults. There is some suggestive evidence for age-related differences in metabolism of methyl parathion in rats (Benke and Murphy 1975). 

The human body deploys an array of enzymes that metabolise potentially toxic compounds derived from the environment. Phase I enzymes such as the... 

Recently, P-carotene has been shown to act as an activator of phase I enzymes in the human liver via PXR-mediated mechanism (Ruhl, 2005). 

Buhler, D.R. and Williams, D.E. 1989, Enzymes involved in metabolism of PAHs by fishes and other aquatic animals Oxidative enzymes (or Phase I enzymes). In Metabolism of Polycyclic Aromatic Hydrocarbons in the Aquatic Environment. Varanasi, U., Ed. CRC Press Boca Raton, El pp. 151-184. 

First-pass clearance can be tracked to gut-stability or metabolism by phase I and then either direct clearance or clearance of the metabolite by phase It enzymes or biliary, renal or plasma clearance. Metabolite stability by phase I enzymes include inhibition, induction, regiospecificity, lability or affinity toward several cytochrome... 

Tab. 7.1 Division of enzymes into phase I and phase II. Phase I enzymes are normally oxidative and phase II conjugative.

As an example, acetaminophen (APAP) in overdose has been used by several groups to identify hepatotoxicity biomarkers in mice. APAP-induced hepatotoxicity is characterized by hepatic centrilobular necrosis and hepatitis. APAP biotransformation by Phase I enzymes leads to the formation of the reactive metabolite N-acetyl-p-benzoquinone (NAPQI), which can deplete glutathione and form adducts with hepatic proteins (see Section 15.2). Protein adduction primes the hepatocytes for cytokines released by activated macrophages (Kupffer cells) and/or destructive insults by reactive nitrogen species. Although necrosis is recognized as the mode of cell death in APAP overdose, the precise mechanisms are still being elucidated [152]. 

The cytochrome P450 (CYP450) enzyme superfamily is the primary phase I enzyme system involved in the oxidative metabolism of drugs and other chemicals. These enzymes also are responsible for all or part of the metabolism and synthesis of a number of endogenous compounds, such as steroid hormones and prostaglandins. 

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. 

Phase I enzymes are located primarily in the endoplasmic reticulum of cells. These enzymes are membrane bound within a lipoprotein matrix and are referred to as microsomal enzymes. This is in reference to the subcellular fraction isolated by differential centrifugation of a liver homoge-... 

Many exogenous and endogenous carcinogens undergo oxidative metabolism by phase I enzymes, especially by the members of the cytochrome P450 (CYP)... 

There are no adequate data to evaluate whether pharmacokinetics of DEHP in children are different from adults. It is not known whether DEHP (or metabolites) can cross the placenta in humans, although it has been detected in breast milk (FDA 200 lh). Studies in animals have shown that DEHP (or metabolites) crosses the placenta and can be transferred to offspring via mother s milk however, quantitative data are lacking. There is no information to evaluate whether metabolism of DEHP is different in children than in adults since the specific phase I enzymes involved in DEHP metabolism have not been identified. It is known that phase II metabolism involves conjugation with glucuronic acid, but the specific isoform of glucuronosyltransferase is not known. 

Intestinal Caco-2 cells provide useful models for assessing the effect of xeno-biotics on enzyme activity. Thus, Faist et al.291 have studied the effect of melanoidins both on a Phase-I enzyme, NADPH cytochrome c reductase, and a Phase-II enzyme, glutathione S-transferase. Where the results achieved a significant level, activity was reduced, thus implying that detoxification would be less efficient in the presence of the melanoidins tested. However, some other results pointed the other way (see Chapter 6). 

Biochemical part of the barrier contains alkaline phosphatase, y-glutamytranspeptidase, dipeptidylpep-tidase IV, aminopeptidase N, endopeptidase 24-11 and disaccharidases saccharidase, isomaltase, lactase. Among metabolising enzymes Phase I enzymes CYP 1A1 and 3A4 and Phase II enzymes glutathion-S-transferase, sulfotransferase and glucuronidase are present. 

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