Subacute toxicity and mutagenicity of cis-9,10-epoxystearic acid

Rat liver glutathione S-transferase-catalyzed conjugation of glutathione to the endogenous epoxides of oleic acid and cholesterol

cis-9,10-Epoxy-octadecanoic acid (oleic acid epoxide, OAE) and 5α,6α-epoxy-cholesterol (ChE) are endogenous epoxides. Unlike other epoxides, the oxirane groups of OAE and ChE are relatively stable against nucleophiles. OAE lacks toxicity and mutagenicity, while ChE is considered harmful, mutagenic and cancerogenic to animals. In humans, ChE is associated with cancer. The metabolism of OAE and ChE includes hydrolysis by cytosolic and microsomal hydrolases to their diols and glutathione (GSH) conjugation by GSH S-transferases (GST) to form the GSH conjugates (R-SG; R, residue). The GST-catalyzed GSH conjugation of OAE and ChE is poorly investigated. This article reports on the GSH conjugation of OAE, its methyl ester (OAEMe) and of ChE by rat liver homogenate GST. The GSH conjugates of OAE, OAEMe and ChE, i.e., OAE-SG, OAEMe-SG and ChE-SG, respectively, were determined by pre-column derivatization with o-phthaldialdehyde (OPA)/2-mercaptoethanol, high-performance liquid chromatography (HPLC) and fluorescence detection. Complex biphasic kinetics were observed with substrate inhibition of GST activity by OAE, OAEMe and ChE, an optimum pH of about 8.3 for OAE, and no measurable chemical GSH conjugation, underlying the importance of GST for the biotransformation of these epoxides. The results confirm the substrate concentration-dependent kinetic mechanism of GST isoforms first reported by William B. Jakoby (J. Biol. Chem. 1974) for exogenous electrophiles including the epoxide 1,2-epoxy-3-(p-nitrophenoxy)propane and the organic nitrates. This mechanism allows for maximal GST activity that can be achieved under given concentrations of GSH, epoxides and other electrophiles.

Enzymatic Detoxification using Lipophilic Hollow-Fiber Membranes: IV. Glutathione Conjugation Reactions

A hollow-fiber technique was used in the enzymatic glutathione conjugation of lipophilic toxins. Native enzyme was circulated on the external side of a lipophilic hollow-fiber membrane while the toxin-containing media (blood, plasma or aqueous solution) were circulated inside the fiber. Glutathione conjugation reactions were catalyzed by rat liver cytosol, with a specific glutathione transferase activity of 40 nmol/min/mg protein (acceptor: 1,2-epoxy-3-(p-nitrophenoxy)propane). Clearance rates of 1,2-epoxy-3-(p-nitrophenoxy)propane, phenylglycidether, styrene oxide, cis-9, 10-epoxystearic acid, cis-9, 10-epoxystearic acid methyl ester, 5a, 6a-cholesterol oxide, 16-, 17a-pregnenolone oxide, and p-nitrobenzylchloride were 10.44, 13.37, 32.25, 7.60, 7.31, 3.92, 4.20 and 29.24 nmol/mg protein/h/cm2 hollow-fiber surface respectively. This technique makes possible glutathione conjugation reactions with crude enzyme preparations over long periods without loss of activity from covalent immobilization and without loss of cofactor from (auto)oxidation. The lipophilic membrane ensures the absence of hemolysis, immunological hazards and hormone loss, while elimination of the toxin is not impaired.

Human CYP4F3s are the main catalysts in the oxidation of fatty acid epoxides

CYP4F isoforms are involved in the oxidation of important cellular mediators such as leukotriene B4 (LTB4) and prostaglandins. The proinflammatory agent LTB4 and cytotoxic leukotoxins have been associated with several inflammatory diseases. We present evidence that the hydroxylation of Z 9(10)-epoxyoctadecanoic, Z 9(10)-epoxyoctadec-Z 12-enoic, and Z 12(13)-epoxyoctadec-Z 9-enoic acids and that of monoepoxides from arachidonic acid [epoxyeicosatrienoic acid (EET)] is important in the regulation of leukotoxin and EET activity. These three epoxidized derivatives from the C18 family (C18-epoxides) were converted to 18-hydroxy-C18-epoxides by human hepatic microsomes with apparent Km values of between 27.6 and 175 microM. Among recombinant P450 enzymes, CYP4F2 and CYP4F3B catalyzed mainly the omega-hydroxylation of C18-epoxides with an apparent Vmax of between 0.84 and 15.0 min(-1), whereas the apparent Vmax displayed by CYP4F3A, the isoform found in leukocytes, ranged from 3.0 to 21.2 min(-1). The rate of omega-hydroxylation by CYP4A11 was experimentally found to be between 0.3 and 2.7 min(-1). CYP4F2 and CYP4F3 exhibited preferences for omega-hydroxylation of Z 8(9)-EET, whereas human liver microsomes preferred Z 11(12)-EET and, to a lesser extent, Z 8(9)-EET. Moreover, vicinal diol from both C18-epoxides and EETs were omega-hydroxylated by liver microsomes and by CYP4F2 and CYP4F3. These data support the hypothesis that the human CYP4F subfamily is involved in the omega-hydroxylation of fatty acid epoxides. These findings demonstrate that another pathway besides conversion to vicinal diol or chain shortening by beta-oxidation exists for fatty acid epoxide inactivation.

Glutathione S-transferase-catalyzed conjugation of 9,10-epoxystearic acid with glutathione

The possible role of glutathione S-transferases (GST) in detoxification of fatty acid epoxides generated during lipid peroxidation has been evaluated. Present studies showed that cytosolic human glutathione S-transferases belonging to alpha, mu, and pi classes isolated from human liver and lung catalyzed the conjugation of glutathione and 9,10-epoxystearic acid. The product of enzymatic reaction, i.e., conjugate of GSH and epoxystearic acid, was isolated and characterized. The Michaelis constant (Km) values of the alpha, mu, and pi classes of GSTs for 9,10-epoxystearic acid were found to be 0.47, 0.32 and 0.80 mM, respectively, whereas the maximal velocity (V max) values for the alpha, mu, and pi classes of GSTs were found to be 142, 256, and 52 mol/min/mol, respectively. These results indicate that even though 9,10-epoxystearic acid is a substrate for all the three classes of GSTs, the mu class isozymes have maximum activity toward this substrate and may preferentially metabolize fatty acid epoxides more effectively as compared to the other classes of GSTs.