Aryl hydrocarbon monooxygenases

Mueller and Miller (33) and Brodie et al. (34) were the first to show that enzymes in the microsomal fraction of rat liver could effectively oxidize xenobiotics. Comparable enzymes (aryl hydrocarbon monooxygenases) were later reported in the hepatic tissues of fresh water and marine fish by Creaven et al. (35) and Buhler and Rasmusson (36). Reconstituted hepatic microsomal systems require cytochrome P-450 for monooxygenase activity in both mammals (37) and fish (38,39). Bend et al. 

Oesch, F. 1976. Differential control of rat microsomal aryl hydrocarbon monooxygenase and epoxide hydratase. J. Biol. Chem. 251(l) 79-87. 

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

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. 

The previous discussion has illustrated the role of monooxygenase systems in fish for the metabolism of xenobiotics. Measurements of this activity have therefore been used as a measure of the extent to which fish have been exposed to xenobiotics at the same time, of course, increased levels enable the fish to metabolize xenobiotics effectively (Kleinow et al. 1987). Although specific assays of cytochrome P-450 activity may be made by immunoblot methods (Monosson and Stegeman 1991), it may be expedient to measure specific enzyme activity using defined substrates. Two assays have been widely used (1) aryl hydrocarbon hydroxylase activity that may be assayed using benzo[a]pyrene as substrate, although this substrate has been replaced recently by the less hazardous 2,5-diphenyloxazole, and (2) activity for O-deethylation of 7-ethoxyresorufin (EROD) has been extensively used and is a simple and convenient assay. 

Poland, A.P, E. Glover, J.R. Robinson, and D.W. Nebert (1974). Genetic expression of aryl hydrocarbon hydroxylase activity. Induction of monooxygenase activities and cytochrome PI-450 formation by 2,3,7,8-tetrachlorodibenzo-p-dioxin in mice genetically nonresponsive to other aromatic hydrocarbons. J. Biol. Chem. 249, 5599-5606. 

Dioxin, PCB, DDT, and the like, i.e., chlorinated aromatic hydrocarbons are believed to bind to a special receptor called AH receptor (AH stands for aryl hydrocarbon). The AH receptor bound with dioxin (or PCB) enters into the nucleus of the cell, and binds to a specific site of a DNA. One of the effects caused by such a binding seems to be an induction of an enzyme dependent on cytochrome P-450. Cytochrome P-450-dependent monooxygenase was talked about in Sect. 6.2.2, and is involved in the metabolism of a wide variety of compounds including steroids and foreign substances (drugs). Probably the P-450 enzymes induced by dioxin, PCB, and others would then metabolize steroid hormones unnecessarily, and thus disrupt the endocrine system. The details are still not very well understood. However, you see that the processes are all chemical reactions at the deepest level. 

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