Aromatic hydrocarbon hydroxylase

Ahokas, J.T., Pelkonen, 0. and K rki, N.T. The possible role of trout liver aryl hydrocarbon hydroxylase in activating aromatic polycyclic carcinogens. In Jollow, D.J. Kocsis, J.J., Snyder, R. and Vainio, H. (Eds.) Biological Reactive Intermediates. Formation, Toxicity, and inactivation (1977) Plenum Press, New York, pp 162-166. 

The marine environment acts as a sink for a large proportion of polyaromatic hydrocarbons (PAH) and these compounds have become a major area of interest in aquatic toxicology. Mixed function oxidases (MFO) are a class of microsomal enzymes involved in oxidative transformation, the primary biochemical process in hydrocarbon detoxification as well as mutagen-carcinogen activation (1,2). The reactions carried out by these enzymes are mediated by multiple forms of cytochrome P-450 which controls the substrate specificity of the system (3). One class of MFO, the aromatic hydrocarbon hydroxylases (AHH), has received considerable attention in relation to their role in hydrocarbon hydroxylation. AHH are found in various species of fish (4) and although limited data is available it appears that these enzymes may be present in a variety of aquatic animals (5,6,7,8). 

Perhaps the best understood example of induction involves induction of the aromatic hydrocarbon receptor (AhR) by compounds such as TCDD and 3-methylcholanthrene. The use of suitable inhibitors of RNA and DNA polymerase activity has shown that inhibitors of RNA synthesis such as actinomycin D and mercapto(pyridethyl)benzimida-zole block aryl hydrocarbon hydroxylase induction, whereas hydroxyurea, at levels that completely block the incorporation of thymidine into DNA, has no effect. Thus it appears that the inductive effect is at the level of transcription and that DNA synthesis is not required. 

Ah locus A gene(s) controlling the trait of responsiveness for induction of enzymes by aromatic hydrocarbons. In addition to aromatic hydrocarbons such as the polycyclics, the chlorinated dibenzo-p-dioxins, dibenzofurans, and biphenyls, as well as the brominated biphenyls, are involved. This trait, originally defined by studies of induction of hepatic aryl hydrocarbon hydroxylase activity following 3-methylcholanthrene treatment, is inherited by simple autosomal dominance in crosses and backcrosses between C57BL/6 (Ah-responsive) and DBA/2 (Ah-nonresponsive) mice. 

Aryl hydrocarbon hydroxylase (AHH) is a complex mixed function oxidase enzyme which converts polycyclic aromatic hydrocarbons such as DMBA to more hydrophilic and readily excretable products. During this process metabolites that are more carcinogenic than the parent compound can be produced. The distribution of MFO enzymes, their activity, and the balance between conversion of procarcinogens to active carcinogens and their detoxification is probably a... 

Mattison DR Thorgeirsson SS (1979) Ovarian aryl hydrocarbon hydroxylase activity and primordial oocyte toxicity of polycyclic aromatic hydrocarbons in mice. Cancer Res, 39 3471-3475. 

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. 

Kafafi SA, Afeefy HY, Said HK, Kafafi AG. 1993. Relationship between aryl hydrocarbon receptor binding, induction of aryl hydrocarbon hydroxylase and 7-ethyoxyresorufin o-deethylase enzymes and toxic activities of aromatic xenobi-otics in animals. A new model. Chem. Res. Toxicol. 6 328-34... 

AHH = aromatic hydrocarbon hydroxylase CHO = Chinese hamster ovary DNA = deoxyribonucleic acid FSHL = female sex hormone lutenizing NR = Not reported PAHs = polycyclic hydrocarbons SOS = DNA... 

Abe S, Nemoto N, Sasaki M. 1983a. Comparison of aryl hydrocarbon hydroxylase activity and inducibility of sister-chromatid exchanges by polycyclic aromatic hydrocarbons in mammalian cell lines. Mutat Res 122 47-51. 

For some drugs dozens of metabolites are possible and have frequently been found in various concentrations. In addition, an important oxidative enzyme, MAO, is encountered. Its distribution is more ubiquitous. It is found in intestinal and nerve tissue. Other more specific oxidative enzymes include aromatic hydrocarbon hydroxylases. These oxidize various polycyclic aromatic hydrocarbons and have been related to the carcinogenic nature of some of these compounds. This is a very dramatic example in which oxidative reactions may not result in detoxification but very likely in highly carcinogenic derivatives. 

Nearly 50 years ago, it was noted that rats exposed to 3-methylcholanthrene (3-MC) displayed a marked increase in metabolic capacity toward that substrate and other polycyclic aromatic hydrocarbons (PAHs) . This enhanced metabolic activity was referred to as aryl hydrocarbon hydroxylase (AHH) based on the ability of these enzymes to efficiently hydroxylate aromatic hydrocarbons ". It is now known that AHH activity is the collective activities of the CYPlAl, CYPl A2, and CYPIBI enzymes. 

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. 

Purification of fungal CYPs from cell extracts is a difficult task due to the usual low specific content, instability, and the presence of multiple forms. The fungal steroid hydroxylase CYPs have been studied and a polycyclic aromatic hydrocarbon hydroxylase . The emergence of genomes and the ability to express the CYPs present in... 

Jacobsen and Philippides " condensed a-hydroxyimino aromatic ketones with substituted benzaldehydes to prepare a series of metabolites of 2,5-diphenylox-azole, a noncarcinogenic, sensitive reagent used to assess aromatic hydrocarbon hydroxylase activity. In their work, acid-catalyzed cyclization of a series of... 

Hansen, A.R. and Fouts, J.R. Some problems in Michaelis-Menten kinetic analysis of benzpyrene hydroxylase in hepatic microsomes from polycyclic aromatic hydrocarbon-pretreated animals. Chem. Biol. Interactions. (1972) 5 167-182. 

Aromatic hydrocarbons are mainly hydroxylated to phenolic products. Complex (12) hydroxylated benzene in MeCN at 20 °C into phenol in ca. 55% yield, and no isotope effect was found for this reaction. Hydroxylation of toluene mainly occurs at the ring positions, with minor amounts of benzylic oxidation products. Hydroxylation of 4-deuterotoluene by (12) occurred with 70% retention and migration of deuterium in the formation of p-cresol. This high NIH shift value is in the same range as that found for liver microsome cytochrome P-450 hydroxylase, and suggests the transient formation of arene oxide intermediates. 

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