Rabbit esophagus metabolizes arachidonic acid predominantly via a lipoxygenase pathway

Eicosanoids modulate the response of gastrointestinal mucosa to noxious stimuli. Though these compounds have been extensively investigated in the stomach, their role in the esophagus has received less attention. Thus, the metabolism of 14C-arachidonic acid by homogenates of rabbit esophageal mucosa was investigated. The major metabolites formed and separated by TLC and HPLC had the chromatographic characteristics of (percent conversion follows each metabolite) 6-keto-prostaglandin F1 alpha (3.80 +/- 1.15), prostaglandin F2 alpha (2.05 +/- 0.37), prostaglandin E2 (5.92 +/- 1.65) and 12-hydroxyeicosatetraenoic acid (26.03 +/- 4.58). Indomethacin, a cyclooxygenase inhibitor, caused a significant decrease in prostaglandin formation without affecting 12-hydroxyeicosatetraenoic acid. BW755C, a combined cyclooxygenase-lipoxygenase inhibitor, dramatically decreased formation of all metabolites. It is concluded that esophageal mucosa metabolizes arachidonic acid primarily to a lipoxygenase derived product. This is the most abundantly produced eicosanoid yet described in the gastrointestinal tract. The importance of this compound to esophageal function is unknown but its presence suggests that future studies of eicosanoids in the esophagus should focus on lipoxygenase metabolites.

Renal cytochrome P-450-dependent metabolism of arachidonic acid in spontaneously hypertensive rats

Renal cytochrome P-450-dependent monooxygenases metabolize arachidonic acid to products some of which affect vascular tone and (Na+,K+)ATPase activity. We measured these metabolites in spontaneously hypertensive (SHR) and control normotensive Wister-Kyoto (WKY) rats. Systolic tail blood pressure in SHR increased from 112 to 202 mm Hg and in WKY from 97 to 136 mm Hg at 5 and 20 weeks respectively. Renal cortical and outer medullary microsomes were incubated with [14C]arachidonic acid; metabolites formed via the cytochrome P-450 pathway were defined as those dependent on NADPH, inhibited by SKF-525A, and unaffected by indomethacin. The P-450-dependent metabolites were higher in SHR vs WKY at 5, 7 and 11 weeks in the cortex and at 7 and 11 weeks in the outer medulla. In the outer medulla, the formation of these metabolites peaked at 7 weeks. Using reverse-phase HPLC, the cytochrome P-450-dependent metabolites were separated into three radioactive peaks: peak I had a retention time of 17.5 min and comigrated as 11,12-dihydroxyeicosatrienoic acid standard. Peak II had a retention time of 19 min and comigrated with omega-hydroxylation compounds. Peak III had a retention time of 27 min and comigrated with 11,12-epoxyeicosatrienoic acid. In the renal cortex, peak I was higher in SHR vs WKY at 5, 7, and 9 weeks and peak III at 5, 7, 9 and 11 weeks. In the outer medulla, peak I was higher in SHR at 5 and 7 weeks, and peaks II and III at 7 weeks. Cytochrome P-450 content in the renal cortex was always higher in SHR vs WKY. We conclude that renal cytochrome P-450-dependent metabolites of arachidonic acid may participate in the circulatory changes of SHR, particularly during the developmental stage.

Expression of heme oxygenase gene in rat and human liver

Developmental changes of microsomal heme oxygenase were studied. In human fetal liver, the enzyme activity was 8 times higher than that detected in the adult liver. On the other hand, adult livers contained 4 times more cytochrome P450 than fetal livers. Elevated heme oxygenase activity in fetal liver was not due to modulators present in the microsomes. Similar to human, rat fetal liver also contained high enzyme activity which appeared to be regulated at the transcriptional level. Hybridization analysis of rat liver RNAs with cDNA for rat heme oxygenase revealed that the level of mRNA for the enzyme was 3-fold higher in the fetus than in the adult. The low cytochrome P450 content may be due in part to the high heme oxygenase activity in fetal livers.

12(R)-hydroxyicosatetraenoic acid: a cytochrome-P450-dependent arachidonate metabolite that inhibits Na+,K+-ATPase in the cornea

When corneal microsomes were incubated with arachidonic acid in the presence of an NADPH-generating system, four polar metabolites (compounds A-D) were formed. Synthesis of these metabolites could be inhibited by carbon monoxide, SKF 525A, and anti-cytochrome c reductase antibodies. One of the metabolites, compound C, was found to inhibit partially purified Na+,K+-ATPase from the corneal epithelium in a dose-dependent manner with an ID50 of approximately 50 nM. After compound C was purified by TLC and HPLC, it was found to have a UV absorption spectrum with a maximum absorbance at 236 nm suggesting the presence of a conjugated diene. Mass spectrometric analysis using positive- and negative-ionization modes was carried out on derivatized compound C that had been synthesized from a mixture of specifically labeled ([5,6,8,9,11,12,14,15-2H8]arachidonic acid) and unlabeled arachidonic acid. Abundant fragment ions were consistent with compound C being a monooxygenated derivative of arachidonic acid with a hydroxyl substituent at carbon-12 of the icosanoid backbone; all deuterium atoms from [2H8]arachidonate were retained in the structure. Oxidative ozonolysis yielded products indicating double bonds between carbons at positions 10 and 11 and positions 14 and 15 of the 20-carbon chain. Compound C was, therefore, characterized as a 12-hydroxyicosatetraenoic acid. However, only 12(R) isomer was found to be an inhibitor of the Na+,K+-ATPase from the corneal epithelium, suggesting that the biologically active compound C was 12(R)-hydroxy-5,8,10,14-icosatetraenoic acid. Such an inhibitor of Na+,K+-ATPase synthesized in the cornea may have an important role in regulating ocular transparency and aqueous human secretion.

Presence of heme oxygenase and NADPH cytochrome P-450 (c) reductase in human corneal epithelium

The presence of heme oxygenase and NADPH cytochrome P-450 (c) reductase, the latter an integral component of heme oxygenase and cytochrome P-450-dependent drug metabolizing enzymes, was demonstrated in human corneal epithelium. We reported for the first time that human corneal epithelium contains heme oxygenase activity as high as 20% of that reported for the human liver. Using immunological techniques, we demonstrated that heme oxygenase proteins from human cornea and liver are very similar; both have a molecular weight of 32,000 as demonstrated by Western blot analysis. We also studied the presence of NADPH cytochrome P-450 (c) reductase. The human corneal epithelium contains significant amount of NADPH cytochrome P-450 (c) reductase activity, and this corneal protein is similar to the known liver protein; both have a molecular weight of 71,000 and react with antibodies prepared against purified liver NADPH cytochrome P-450 (c) reductase. As the heme oxygenase system is the rate limiting step in heme degradation, this system plays a pivotal role in regulation of cellular heme in corneal epithelium, thus modulating the activity of hemoproteins such as catalase, tryptophan pyrrolase and thromboxane synthetase.

Cytochrome P450, drug metabolizing enzymes and arachidonic acid metabolism in bovine ocular tissues

Little information is available on drug metabolizing enzymes in ocular tissues. We investigated the presence of various cytochrome P450 isozymes by measuring different drug metabolizing enzymes, i.e., aryl hydrocarbon hydroxylase, 7 ethoxycoumarin-o-deethylase and benzphetamine demethylase activities in ciliary body, corneal epithelium and endothelium, retina and retinal pigment epithelium. Our results demonstrate that the ciliary body and the retinal pigment epithelium possess the highest activities of cytochrome P450-dependent monooxygenases in the eye. The highest activity of drug metabolizing enzymes is accompanied by high activity of NADPH cytochrome P450 (C) reductase, an integral component of this enzyme system. Heme oxygenase, a key enzyme for the regulation of heme availability to hemoproteins such as cytochrome P450 also demonstrate high activity in these two ocular tissues. Although the corneal epithelium has a lower activity of drug metabolizing enzymes, it possesses the highest activity of cytochrome P450 species capable of metabolizing arachidonic acid to biologically active compounds, whereas the other ocular tissues possess cyclooxygenase as the main microsomal enzyme that metabolizes arachidonic acid. Whether the observed catalytic activities of drug metabolizing enzymes seen in ocular tissues are associated with major or minor forms of cytochrome P450 is not yet know. However, the specialized location of cytochrome P450 isozymes in ocular tissues suggests a physiological function related to activation of endogenous compounds such as arachidonic acid, in addition to detoxification of drugs.

Immunochemical studies on the contribution of NADPH cytochrome P-450 reductase to the cytochrome P-450-dependent metabolism of arachidonic acid

We have studied the role of NADPH cytochrome P-450 reductase in the metabolism of arachidonic acid and in two other monooxygenase systems: aryl hydrocarbon hydroxylase and 7-ethoxyresorufin-o-deethylase. Human liver NADPH cytochrome P-450 reductase was purified to homogeneity as evidenced by its migration as a single band on SDS gel electrophoresis, having a molecular weight of 71,000 Da. Rabbits were immunized with the purified enzyme and the resulting antibodies were used to evaluate the involvement of the reductase in cytochrome P-450-dependent arachidonic acid metabolism by bovine corneal epithelial and rabbit renal cortical microsomes. A highly sensitive immunoblotting method was used to identify the presence of NADPH cytochrome P-450 reductase in both tissues. We used these antibodies to demonstrate for the first time the presence of cytochrome c reductase in the cornea. Anti-NADPH cytochrome P-450 reductase IgG, but not anti-heme oxygenase IgG, inhibited the NADPH-dependent arachidonic acid metabolism in both renal and corneal microsomes. The inhibition was dependent on the ratio of IgG to microsomal protein where 50% inhibition of arachidonic acid conversion by cortical microsomes was achieved with a ratio of 1:1. A higher concentration of IgG was needed to achieve the same degree of inhibition in the corneal microsomes. The antibody also inhibited rabbit renal cortical 7-ethoxyresorufin-o-deethylase activity, a cytochrome P-450-dependent enzyme. However, the anti-NADPH cytochrome P-450 reductase IgG was much less effective in inhibiting rabbit cortical aryl hydrocarbon hydroxylase. Thus, the degree of inhibition of monooxygenases by anti-NADPH cytochrome P-450 reductase IgG is variable. However, with respect to arachidonic acid, NADPH cytochrome P-450 reductase appears to be an integral component for the electron transfer to cytochrome P-450 in the oxidation of arachidonic acid.

Regulation of arachidonic acid metabolism by cytochrome P-450 in rabbit kidney

Renal microsomal cytochrome P-450-dependent arachidonic acid metabolism was correlated with the level of cytochrome P-450 in the rabbit kidney. Cobalt, an inducer of haem oxygenase, reduced cytochrome P-450 in both the cortex and medulla in association with a 2-fold decrease in aryl-hydrocarbon hydroxylase, an index of cytochrome P-450 activity, and a similar decrease in the formation of cytochrome P-450-dependent arachidonic acid metabolites by renal microsomes (microsomal fractions). Formation of the latter was absolutely dependent on NADPH addition and was prevented by SKF-525A, an inhibitor of cytochrome P-450-dependent enzymes. Arachidonate metabolites of cortical microsomes were identified by g.c.-m.s. as 20- and 19-hydroxyeicosatetraenoic acid, 11,12-epoxyeicosatrienoic acid and 11,12-dihydroxyeicosatrienoic acid. The profile of arachidonic acid metabolites was the same for the medullary microsomes. Induction of cytochrome P-450 by 3-methylcholanthrene and beta-naphthoflavone increased cytochrome P-450 content and aryl-hydrocarbon hydroxylase activity by 2-fold in the cortex and medulla, and this correlated with a 2-fold increase in arachidonic acid metabolites via the cytochrome P-450 pathway. These changes can also be demonstrated in cells isolated from the medullary segment of the thick ascending limb of the loop of Henle, which previously have been shown to metabolize arachidonic acid specifically via the cytochrome P-450-dependent pathway. The specific activity for the formation of arachidonic acid metabolites by this pathway is higher in the kidney than in the liver, the highest activity being in the outer medulla, namely 7.9 microgram as against 2.5 micrograms of arachidonic acid transformed/30 min per nmol of cytochrome P-450 for microsomes obtained from outer medulla and liver respectively. These findings are consistent with high levels of cytochrome P-450 isoenzyme(s), specific for arachidonic acid metabolism, primarily localized in the outer medulla.

Teratocarcinoma cell MHC antigen expression is regulated in vitro by a soluble noninterferon factor

The 402AX murine teratocarcinoma is a spontaneous testicular tumor of 129 (H-2b) origin which does not express MHC encoded antigens. Rejection of this tumor is immunologically mediated and the tumor cells are induced in vivo to synthesize H-2b antigens when passaged in genetically resistant host mice. The present studies demonstrate that serum from tumor primed genetically resistant host mice can induce tumor cell MHC antigen expression in vitro as measured by indirect immunofluorescence using monoclonal antibodies. The inducing factor is specific for 402AX tumor cells and is not interferon as shown by the lack of response of the 402AX tumor to gamma interferon, and the absence of significant interferon activity in inducer serum. These studies demonstrate another factor independent of interferon that can induce MHC class I antigen expression on tumor cells.

Cytochrome P450 dependent metabolism of arachidonic acid in bovine corneal epithelium

Microsomes prepared from bovine corneal epithelium metabolized 14C-arachidonic acid into two unidentified products, separated by thin-layer chromatography and called Peaks I and II. Each peak was further separated by high performance liquid chromatography into two metabolites. The formation of these metabolites was dependent on the addition of NADPH and inhibited by carbon monoxide and SKF-525A, suggesting a cytochrome P450-dependent mechanism. The presence of cytochrome P450 in the corneal epithelium was assessed directly by measurement of the carbon monoxide reduced spectrum and indirectly by measuring aryl hydrocarbon hydroxylase activity. The activity of aryl hydrocarbon hydroxylase was protein- and NADPH-dependent and was inhibited by SKF-525A.