High performance liquid chromatography/electron spin resonance/mass spectrometry analyses of radicals formed in an anaerobic reaction of 9- (or 13-) hydroperoxide octadecadienoic acids with ferrous ions

MitoCLox: A Novel Mitochondria-Targeted Fluorescent Probe for Tracing Lipid Peroxidation

Peroxidation of cardiolipin (CL) in the inner mitochondrial membrane plays a key role in the development of various pathologies and, probably, aging. The four fatty acid tails of CL are usually polyunsaturated, which makes CL particularly sensitive to peroxidation. Peroxidation of CL is involved in the initiation of apoptosis, as well as in some other important cellular signaling chains. However, the studies of CL peroxidation are strongly limited by the lack of methods for its tracing in living cells. We have synthesized a new mitochondria-targeted fluorescent probe sensitive to lipid peroxidation (dubbed MitoCLox), where the BODIPY fluorophore, carrying a diene-containing moiety (as in the C11-BODIPY (581/591) probe), is conjugated with a triphenylphosphonium cation (TPP⁺) via a long flexible linker that contains two amide bonds. The oxidation of MitoCLox could be measured either as a decrease of absorbance at 588 nm or as an increase of fluorescence in the ratiometric mode at 520/590 nm (emission). In CL-containing liposomes, MitoCLox oxidation was induced by cytochrome c and developed in parallel with cardiolipin oxidation. TPP⁺-based mitochondria-targeted antioxidant SkQ1, in its reduced form, inhibited oxidation of MitoCLox concurrently with the peroxidation of cardiolipin. Molecular dynamic simulations of MitoCLox in a cardiolipin-containing membrane showed affinity of positively charged MitoCLox to negatively charged CL molecules; the oxidizable diene moiety of MitoCLox resided on the same depth as the cardiolipin lipid peroxides. We suggest that MitoCLox could be used for monitoring CL oxidation in vivo and, owing to its flexible linker, also serve as a platform for producing peroxidation sensors with affinity to particular lipids.

A comparative study of the inhibitory effects by caffeic acid, catechins and their related compounds on the generation of radicals in the reaction mixture of linoleic acid with iron ions

Caffeic acid and (+)-catechin, which are abundantly contained in coffee and tea, are typical polyphenols. In order to know the relative magnitudes of antioxidant activity, effects by caffeic acid, (+)-catechin and their derivatives on the formation of 4-POBN/carbon-centered linoleic acid-derived radical adducts were examined in the control reaction mixture of linoleic acid with FeCl₃ at 30°C for 168 h. In the presence of 1.0 mM of the polyphenols, peak to peak heights of the third ESR signal resulted in 7.7 ± 2.4% (n = 3) (caffeic acid), 145 ± 13% (n = 3) (quinic acid), 4.4 ± 0.0% (n = 3) (chlorogenic acid), 104 ± 4.4% (n = 3) (ferulic acid), 4.3 ± 0.0% (n = 3) (noradrenaline), 12.5 ± 10.9% (n = 3) (gallic acid), 38.1 ± 7.1% (n = 3) [(+)-catechin], 47.9 ± 11.7% (n = 3) [(–)-epicatechin], 56.5 ± 1.6% (n = 3) (epigallocatechin), 13.5 ± 1.7% (n = 3) (catechol) and 83.7 ± 7.8% (n = 3) (resorcinol) of the control reaction mixture. All the compounds with catechol moiety exerted potent inhibitory effects on the radical formation except for (+)-catechin, (–)-epicatechin and epigallocatechin. (+)-Catechin, (–)-epicatechin and epigallocatechin may not exert the inhibitory effect as much possibly because they are less stable compared with caffeic acid. The resorcinol moiety in these molecules may also weaken their antioxidant activity.

Molecular mechanism of metal-independent decomposition of organic hydroperoxides by halogenated quinoid carcinogens and the potential biological implications

Halogenated quinones (XQ) are a class of carcinogenic intermediates and newly identified chlorination disinfection byproducts in drinking water. Organic hydroperoxides (ROOH) can be produced both by free radical reactions and enzymatic oxidation of polyunsaturated fatty acids. ROOH have been shown to decompose to alkoxyl radicals via catalysis by transition metal ions, which may initiate lipid peroxidation or transform further to the reactive aldehydes. However, it is not clear whether XQ react with ROOH in a similar manner to generate alkoxyl radicals metal-independently. By complementary applications of ESR spin-trapping, HPLC/high resolution mass spectrometric and other analytical methods, we found that 2,5-dichloro-1,4-benzoquinone (DCBQ) could significantly enhance the decomposition of a model ROOH tert-butylhydroperoxide, resulting in the formation of t-butoxyl radicals independent of transition metals. On the basis of the above findings, we detected and identified, for the first time, an unprecedented C-centered quinone ketoxy radical. Then, we extended our study to the more physiologically relevant endogenous ROOH 13-hydroperoxy-9,11-octadecadienoic acid and found that DCBQ could also markedly enhance its decomposition to generate the reactive lipid alkyl radicals and the genotoxic 4-hydroxy-2-nonenal (HNE). Similar results were observed with other XQ. In summary, these findings demonstrated that XQ can facilitate ROOH decomposition to produce reactive alkoxyl, quinone ketoxy, lipid alkyl radicals, and genotoxic HNE via a novel metal-independent mechanism, which may explain partly their potential genotoxicity and carcinogenicity.

Molecular mechanism of metal-independent decomposition of lipid hydroperoxide 13-HPODE by halogenated quinoid carcinogens

Halogenated quinones are a class of carcinogenic intermediates and newly identified chlorination disinfection by-products in drinking water. 13-Hydroperoxy-9,11-octadecadienoic acid (13-HPODE) is the most extensively studied endogenous lipid hydroperoxide. Although it is well known that the decomposition of 13-HPODE can be catalyzed by transition metal ions, it is not clear whether halogenated quinones could enhance its decomposition independent of metal ions and, if so, what the unique characteristics and similarities are. Here we show that 2,5-dichloro-1,4-benzoquinone (DCBQ) could markedly enhance the decomposition of 13-HPODE and formation of reactive lipid alkyl radicals such as pentyl and 7-carboxyheptyl radicals, and the genotoxic 4-hydroxy-2-nonenal (HNE), through the complementary application of ESR spin trapping, HPLC-MS, and GC-MS methods. Interestingly, two chloroquinone-lipid alkoxyl conjugates were also detected and identified from the reaction between DCBQ and 13-HPODE. Analogous results were observed with other halogenated quinones. This represents the first report that halogenated quinoid carcinogens can enhance the decomposition of the endogenous lipid hydroperoxide 13-HPODE and formation of reactive lipid alkyl radicals and genotoxic HNE via a novel metal-independent nucleophilic substitution coupled with homolytic decomposition mechanism, which may partly explain their potential genotoxicity and carcinogenicity.

Detection of the reduced forms of radical adducts on the ESR trace using HPLC-electrochemical detector-ultraviolet absorption detector-electron spin resonance-MS

To detect and identify the electron spin resonance (ESR) silent forms of the alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone (4-POBN) radical adducts, an electrochemical detector (ECD) was employed as a reactor in the HPLC-ECD-UV absorption detector-ESR-MS (HPLC-ECD-UV-ESR-MS). The ECD was employed to regenerate the radical forms from the reduced forms. The reduced forms of the 4-POBN/pentyl radical adducts were analyzed using the HPLC-ECD-UV-ESR-MS. On addition of the ECD applied potential of +0.3 V, a peak appeared on the ESR trace of the HPLC-ECD-UV-ESR-MS analyses, indicating that the radical forms are regenerated from the reduced forms. The HPLC-ECD-UV-ESR-MS analyses were also performed for the reaction mixtures of phenylhydrazine with CuCl(2). Two peaks (peaks I and II) were detected on the UV trace (300 nm) of the HPLC-ECD-UV-ESR-MS. The mass spectra showed that the peak I and peak II compounds are radical and reduced forms of the 4-POBN/phenyl radical adducts under the ECD applied potential of 0.0 V. Peak I was only detected on the ESR trace under the ECD applied potential of 0.0 V. In addition to peak I, peak II appeared on the ESR trace under the ECD applied potential of +0.3 V, indicating that the reduced forms are oxidized to the corresponding radical forms.

High performance liquid chromatography/electron spin resonance/mass spectrometry analyses of lipid-derived radicals

High performance liquid chromatography/electron spin resonance/mass spectrometry (HPLC/ EPR/MS) analyses of radicals is performed for the reaction mixture of 13-hydroperoxy-(9Z,11E)-octadeca-9,11-dienoic acid (13ZE-OOH) with ferrous ions under an aerobic condition, or an anaerobic condition. Radicals are identified from 13ZE-OOH by using high performance liquid chromatography/electron spin resonance spectrometry (HPLC/EPR) and HPLC/EPR/MS. The pentyl radical and isomers of epoxylinoleic acid radicals from 13ZE-OOH are identified under an anaerobic condition and the 7-carboxyheptyl radical and pentyl radical from 13ZE-OOH under an aerobic condition. These results suggest that the formation of the respective radical species depends to a great extent on oxygen concentration in the reaction mixtures.