Renal microsomal cytochrome P-450 monooxygenasedependent metabolism of arachidonic acid generates a series of regioisomeric epoxyeicosatrienoic acids that can be further metabolized by soluble epoxide hydrolase to the corresponding dihydroxyeicosatrienoic acids. Evidence exists that these metabolites affect renal function and, in particular, blood pressure regulation. To examine this possibility, blood pressure and renal arachidonic acid metabolism were examined in mice with a targeted disruption of the soluble epoxide hydrolase gene. Systolic blood pressure of male soluble epoxide hydrolase-null mice was lower compared with wild-type mice in both the absence and presence of dietary salt loading. Both female soluble epoxide hydrolase-null and wild-type female mice also had significantly lower systolic blood pressure than male wild-type mice. Renal formation of epoxyeicosatrienoic and dihydroxyeicosatrienoic acids was markedly lower for soluble epoxide hydrolase-null versus wild-type mice of both sexes. Although disruption of soluble epoxide hydrolase in female mice had minimal effects on blood pressure, deletion of this gene feminized male mice by lowering systolic blood pressure and altering arachidonic acid metabolism. These data provide the first direct evidence for a role for soluble epoxide hydrolase in blood pressure regulation and identify this enzyme as a novel and attractive target for therapeutic intervention in hypertension. Mammalian soluble epoxide hydrolase (sEH)1 is a cytosolic enzyme expressed in multiple tissues that catalyzes the conversion of a diverse group of epoxides to their corresponding diols (1, 2). The broad spectrum of xenobiotic epoxides metabolized by this enzyme suggests a role in the protection of cells against the potentially harmful effects of these compounds (3). However, sEH hydrates fatty acid epoxides most effectively, with epoxides of arachidonic acid (AA) being among the best substrates (4 -6). This metabolism may result in the formation of metabolites with greater or less biological activity, indicating that sEH may have important physiological functions. Intracellular metabolism of AA by prostaglandin H2 synthase, lipoxygenases, and the cytochrome P-450 (CYP) monooxygenase system generates a number of metabolites, collectively termed eicosanoids, with important biological and cell signaling roles (7-12). CYP-dependent metabolism of AA (Fig. 1) generates three primary classes of metabolites: mid-chain cis-trans-conjugated dienols (5-, 8-, 9-, 11-, 12-and 15-hydroxyeicosatetraenoic acids (HETEs)); -terminal alcohols (16-through 20-hydroxyeicosatetraenoic acids); and, cis-epoxyeicosatrienoic acids (6-, 8,9-, 11,12-, and 14,15 epoxyeicosatrienoic acids (EETs)) (13). EETs have been shown in vitro to be efficiently hydrated to their corresponding dihydroxyeicosatrienoic acids (DHETs) by sEH (5).Evidence exists for a variety of renal functions attributable to EETs and DHETs generated by CYP-dependent epoxygenase activity and sEH, respectively. Most notably, evidence exists for ...

Heme metabolism normally involves enzymatic conversion to biliverdin and subsequently to bilirubin, catalyzed by heme oxygenase and biliverdin reductase, respectively. We examined the ability of exogenously added hemin, biliverdin, or bilirubin to regulate Cyp1a1, an enzyme that may be active in bilirubin elimination. A substantial dose-dependent increase in Cyp1a1 mRNA occurred after treatment of Hepa 1c1c7 cells with either of the three compounds. This increase was readily apparent 1 hr after treatment with biliverdin or bilirubin but required >/=2 hr with hemin. Treatment of Hepa 1c1c7 cells with these compounds also caused a dose-dependent increase in Cyp1a1-dependent 7-ethoxyresorufin-O-deethylase (EROD) activity. Of the three compounds, bilirubin produced the greatest maximal increase in Cyp1a1 mRNA and EROD (5.5-, 10.5-, and 15-fold for 100 microM hemin, biliverdin, and bilirubin, respectively) activity. The RNA polymerase inhibitor actinomycin D completely blocked Cyp1a1 induction by these compounds, indicating a requirement for de novo RNA synthesis via transcriptional activation. The protein synthesis inhibitor cycloheximide did not affect Cyp1a1 mRNA induction, indicating a lack of requirement for labile protein factors. In contrast, EROD induction by hemin, biliverdin, or bilirubin was completely blocked by cycloheximide treatment, indicating that the increase in enzyme activity is dependent on increased Cyp1a1 apoprotein synthesis. Aryl hydrocarbon receptor (AHR)- and AHR nuclear translocator-deficient mutant Hepa 1c1c7 cells did not exhibit increased Cyp1a1 mRNA or EROD activity after treatment with these compounds, indicating the requirement for a functional AHR for this response. Consistent with this, hemin, biliverdin, and bilirubin were able to induce expression of the dioxin-response element/luciferase reporter plasmid pGudLuc1.1 after transient transfection into wild-type Hepa 1c1c7 cells. Gel retardation assays demonstrated that bilirubin, but not hemin or biliverdin, was able to transform the AHR to a form capable of specifically binding to a 32P-labeled oligonucleotide containing a dioxin-response element sequence. These data indicate that bilirubin induces Cyp1a1 gene transcription through direct interaction with the AHR. In contrast, hemin and biliverdin seem to induce Cyp1a1 indirectly by serving as precursors to the endogenous formation of bilirubin via normal heme metabolism pathways. This is the first direct demonstration that the endogenous heme metabolite bilirubin can directly regulate Cyp1a1 gene expression and enzymatic activity in an AHR-dependent manner.

Naringin and 6',7'-dihydroxybergamottin are not the major active ingredients, although they may contribute to the grapefruit juice-felodipine interaction. The variable effect with the particulate fraction may result from erratic bioavailability of unidentified primary active substances. The findings show the importance of in vivo testing to determine the ingredients in grapefruit juice responsible for inhibition of cytochrome P450 3A4 in humans.

No abstract

Unprocessed grapefruit can cause a drug interaction with felodipine. The active ingredients are naturally occurring in the grapefruit. Bergamottin is likely important in drug interactions with commercial grapefruit juice. 6',7'-Dihydroxybergamottin and naringin may be more important in grapefruit segments because they are present in higher concentrations. Any therapeutic concern for a drug interaction with commercial grapefruit juice should now be extended to include whole fruit and possibly confectioneries made from grapefruit peel.

Bergamottin and reversible inhibition are not the primary substance and mechanism responsible for inhibition of CYP3A4 activity clinically. Red wine can cause dose dumping of extended-release felodipine in certain individuals.

No abstract

Erythromycin produced an important pharmacokinetic interaction with felodipine by inhibition of drug metabolism. Although erythromycin and grapefruit juice shared a common mechanism, erythromycin likely reduced felodipine biotransformation at the gut wall and liver, whereas single-dose grapefruit juice had an effect mainly at the gut wall.