Aryl hydrocarbon hydroxylase microsomes

Table III. Inhibition of monooxygenase (aryl hydrocarbon hydroxylase) activity in fish and mammalian hepatic microsomes (based on...

Bend, J. R., Hall, P., and Foureman, G. L. Comparison of benzo(a)pyrene hydroxylase (aryl hydrocarbon hydroxylase, AHH) activities in hepatic microsomes from untreated and 1,2,3,4-dibenzanthracene (DBA)-induced little skate (Raja erinacea). Bull. Mt. Desert Island Biol. Lab. (1976) 16. 3-5. 

No studies were located regarding the metabolic pathway of fuel oils in humans. In one animal study, fuel oil no. 2 applied to the skin of rats induced cutaneous aryl hydrocarbon hydroxylase activity in rat skin microsomal preparations by causing a three-fold induction of benzo(a)pyrene (BaP) 3-hydroxylase activity (Rahimtula et al. 1982). In addition, BaP 3-hydroxylase activity was selectively inhibited by -naphthoflavone, but not by metyrapone, suggesting that cytochrome P-448 enzymes are induced and may participate in the metabolism of this fuel oil (Rahimtula et al. 1982). 

Enzyme Differences. Variation in the nature and amount of constitutively expressed microsomal P450s have not been studied extensively in different strains of the same vertebrate. The only thorough investigations, those of the Ah Locus, which controls aryl hydrocarbon hydroxylase induction, have shown that in addition to quantitative differences in the amount of P450 after induction in different strains of mice, there may also be a qualitative difference in the P450 isozymes induced (see Section 9.5.2). 

Mechanism and Genetics of Induction in Mammals. Many different mechanisms may be involved in CYP induction. These include increased transcription of DNA, increased mRNA translation to protein, mRNA stabilization, and protein stabilization. Induction can only occur in intact cells and cannot be achieved by the addition of inducers directly to cell fractions such as microsomes. It has been known for some time that in most cases of increase in monooxygenase activity there is a true induction involving synthesis of new enzyme, and not the activation of enzyme already synthesized, since induction is generally prevented by inhibitors of protein synthesis. For example, the protein synthesis inhibitors such as puromycin, ethionine, and cyclo-heximide inhibit aryl hydrocarbon hydroxylase activity. A simplified scheme for gene expression and protein synthesis is shown in Figure 9.7. 

Table IV. Aryl Hydrocarbon Hydroxylase Activity in Hepatic Microsomes from Animals Consuming Various Levels and Types of Fat 0.3% BHT...

Aryl hydrocarbon hydroxylase (AHH) is part of the microsomal mixed-function oxidase system involved in the detoxification of polycyclic aromatic hydrocarbons. In the HPLC assay developed for the AHH activity, benzo[a]pyrene (BaP) is used as the substrate, and the activity is determined by measuring the unreacted BaP during the reaction. 

Rifkind AB, Tseng L, Hirsch MB, Lauersen NH. 1978. Aryl hydrocarbon hydroxylase activity and microsomal cytochrome content of human fetal tissues. Cancer Res. 38 1572-77... 

The induction of the microsomal enzymes has been demonstrated in many different species including humans, and in various different tissues as well as the liver. Induction usually results from repeated or chronic exposure although the extent of exposure is variable. The result of induction is an increase in the amount of an enzyme induction requires de novo protein synthesis, and therefore an increase in the apparent metabolic activity of a tissue in vitro or animal in vivo. Consequently, inhibitors of protein synthesis, such as cycloheximide, inhibit induction. It is a reversible, cellular response to exposure to a substance. Thus, it can be shown in isolated cells, such as hamster foetal cells in culture, that exposure to benzo[a] anthracene induces aryl hydrocarbon hydroxylase activity (AHH), one of the isoenzymes of cytochrome P-450. 

Dermal treatment of healthy volunteers with 10% coal tar for 4 days produced an 18-fold induction of CYP1A1 mRNA levels in coal-tar-treated skin (Li et al. 1995). In vitro incubation of DNA with coal tar fume concentrates in the presence of mouse and yeast microsomes expressing various cytochrome P450 isoforms or the aryl hydrocarbon hydroxylase receptor (AHR) demonstrated that coal tar fume condensates require metabolic activation to produce DNA adducts (Genevois et al. 1998). Both the AHR and CYP1A were involved in the metabolism of coal tar fume condensate. It was also shown that the reactive metabolites formed by CYP1A are substrates for microsomal epoxide hydrolase. 

Rats were dosed with soil suspensions, by gavage, either once or for four consecutive days. Total TCDD dosages administered to rats were either 10 ug TCDD/kg or 40 ug/ TCDD/kg. Rats were sacrificed 24 hours after the final dose, autopsied, and hepatic microsomal fractions were collected. Aryl hydrocarbon hydroxylase (AHH) levels were determined in the microsomes, using the fluorescent assay of the product of the metabolism of benzo(a)pyrene to 3-OH benzo(a)pyrene (16). 

Table III presents the results of aryl hydrocarbon hydroxylase (AHH) determinations in rat liver microsomes. AHH levels were roughly equivalent for males and females in each group except the negative control. Times Beach soil and Newark soil induced AHH activity in both sexes (Times Beach soil was ca. 2 to 4 times decontaminated soil, while Newark soil was ca. 2 to 5.5).

The comutagenic action of Norharman upon aryl hydrocarbon hydroxylase activity is shown to be cytochrome P-448 dependent by immunochemical analysis. The hydroxylated products of Norharman and Harman may play an important role in their comutagenic action by fluidizing the microsomal or nuclear membranes. 

In addition to direct oxygenation, e.g. by aryl hydrocarbon hydroxylase, oxidative N- or 0-dealkylation is another process catalyzed by components of the Cytochrome P-450 System (mixed-function oxidases). Reduction also occurs in this system NADPH-cytochrome P-450 reductase has an activity similar to microsomal nitroreductase, i.e. transformation of aronaatic nitro compounds into the corresponding arylamines takes place. The oxidation may be followed by other enzymic reactions, e.g. epoxides are hydrated to vicinal diols by microsomal epoxide hydratase or they are coupled with glutathione by glutathione-S-epoxide transferase. 

Wiebel, F. J., Leutz, J. C., Diamond, L., and Gelboin, H. V. Aryl hydrocarbon (benzo[a]pyrene) hydroxylase in microsomes from rat tissues differential inhibition and stimulation by benzoflavones and orqanic solvents. Arch. Biochem. Biophys. (1971) 144 78-86. 

Initial studies designed to obtain a valid subcellular fractionation scheme for rainbow trout liver illustrated the aryl-hydrocarbon (benzo[a]pyrene] hydroxylase activity separated with glucose-6-phosphatase (35). This observation indicated that the trout hemoprotein P-450-mediated monooxygenation system was located within the endoplasmic reticulum (microsomal fraction). 

Nagayama F, Yasuike T, Ikeru K, Kawamura C (1979) Lipase, carboxylesterase and catechol oxidase of the antartic krill. Trans Tokyo Univ Fish 3 153-159 Nasci C, Fossato VU (1982) Studies on physiology of mussels and their ability in accumulating hydrocarbons and chlorinated hydrocarbons. Environ Technol Lett 3 273-280 Nebert DW, Gelboin HV (1968) Substrate-inducible microsomal aryl hydroxylase in mammalian cell culture. J Biol Chem 243 107-116... 

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