A-Naphthoflavone

An obvious difference was also noted between control and induced skate hepaticdnicrosomal AHH activity in the presence of a-naphthoflavone (10 M). This compound, when added in vitro at this or higher concentrations, caused significant stimulation of AHH activity in control animals (about 3-fold) but inhibition (80%) was found in DBA-pretreated skates. Similar results were earlier reported for control and 3-methylcholanthrene-treated rats (23), where it appears that the response is due to differential effects of a-naphthoflavone on hepatic microsomal cytochrome P-450 (stimulated) and cytochrome P-448 (inhibited) (24). Our data suggests that there may be a novel form of cytochrome P-450 synthesized in skate liver in response to polycyclic hydrocarbon administration, even though there was no hypsochromic shift in the carbon monoxide difference spectrum of dithionite reduced hepatic microsomes from DBA-treated skates (relative to hepatic microsomes from control fish). 

One of our more interesting observations is illustrated in Table III. The administration of DBA to winter flounder increased hepatic microsomal AHH and 7-ethoxyresorufin deethylase activities as expected, and AHH activity was strongly inhibited in the DBA-treated flounder by 10" M a-naphthoflavone as we have previously reported for both little skate (4) and sheepshead (9). However, the presence of high AHH and 7-ethoxyresorufin deethylase activities in one control flounder, and the inhibition of AHH activity by a-naphthoflavone in this animal suggested that the hepatic microsomal MFO system of this fish was already induced. 

AHH Activity (FU/min/mg protein) Without a-Naphthoflavone With a-Naphtboflavone (10 M) 7-ERF Activity (pmol/min/mg protein)... 

Addition of a-naphthoflavone, metyrapone or glutathione to incubations decreased the covalent binding of 1,1,2,2-tetrachloroethane to olfactory and hepatic tissues in vitro (Eriksson Brittebo, 1991). 

In addition to being prone to homotropic activation, CYP3A4 is also prone to heterotropic activation. The CYP1A2 inhibitor, a-naphthoflavone is an activator of certain CYP3A4-dependent reactions [a factor that complicates the use of this flavonoid in CYP inhibition studies (discussed later)]. CYP3A4-catalyzed... 

One of the complicating factors with chemical inhibitors is that a chemical that inhibits one CYP enzyme may activate another enzyme. If both enzymes contribute to metabolite formation, the inhibitory effect of the chemical on one enzyme may be offset by its activating effect on the other enzyme. a-Naphthoflavone is an inhibitor of CYP1A2 but an activator of CYP3A4, whereas quinidine is an inhibitor of CYP2D6 but, in certain cases, an activator of CYP3 A4 (184-186). a-Naphthoflavone and quinidine both appear on the list of FDA preferred and acceptable chemical inhibitors, so their ability to inhibit one enzyme but activate another are relevant to reaction phenotyping. 

A2 Ethoxyresorufin, Phenacetin Caffeine (low turnover), Theophylline (low turnover), Acetanilide (mostly applied in hepatocytes) Furafylline a-Naphthoflavone (but also inhibits 3A4)... 

A number of K-region arene oxides have been detected as intermediates in the metabolism of the corresponding PAHs in liver systems for example, phenanthrene, benz[a]anthracene, pyrene, benzo [a]pyrene, and dibenz(a,h)anthracene. These K-region arene-oxide metabolites were generally only detected by trapping the radiolabeled intermediate. The arene-oxide metabolite 102 obtained from a-naphthoflavone was found to be sufficiently stable with respect to isomerization and resistant to attack by epoxide hydrolase so that it could be isolated and identified spectroscopically. ... 

The role of xenobiotic metabolism in the cytotoxicity of an ecotoxicant to fish cell lines has been best studied with BaP, but even with this PAH the story is incomplete (Table 3). Two types of cytotoxic responses to BaP have been documented. One has been a transitory decline in metabolism, but not in cell viability, after BaP exposures of 24-48 h. This was observed in the rainbow trout liver cell lines, RTL-W1 and Rl174. Metabolism appeared necessary for this effect as the response was blocked in RTL-W1 and Rl by, respectively, the CYP1A inhibitor, a-naphthoflavone (ANF) and the prostaglandin-H-synthase inhibitor, indomethacin. BaP quinones were thought to be the BaP metabolites responsible for this effect. 

Gasiewicz, T.A. and G. Rucci. a-Naphthoflavone acts as an antagonist of 2,3,7,8-tetrachlorodibenzo-p-dioxin by forming an inactive complex with the Ah receptor. Mol. Pharmacol. 40 607-612, 1991. 

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