Two distinct pathways of formation of 4-hydroxynonenal. Mechanisms of nonenzymatic transformation of the 9- and 13-hydroperoxides of linoleic acid to 4-hydroxyalkenals

The mechanism of formation of 4-hydroxy-2E-nonenal (4-HNE) has been a matter of debate since it was discovered as a major cytotoxic product of lipid peroxidation in 1980. Recent evidence points to 4-hydroperoxy-2E-nonenal (4-HPNE) as the immediate precursor of 4-HNE (Lee, S. H., and Blair, I. A. (2000) Chem. Res. Toxicol. 13, 698-702; Noordermeer, M. A., Feussner, I., Kolbe, A., Veldink, G. A., and Vliegenthart, J. F. G. (2000) Biochem. Biophys. Res. Commun. 277, 112-116), and a pathway via 9-hydroperoxylinoleic acid and 3Z-nonenal is recognized in plant extracts. Using the 9- and 13-hydroperoxides of linoleic acid as starting material, we find that two distinct mechanisms lead to the formation of 4-H(P)NE and the corresponding 4-hydro(pero)xyalkenal that retains the original carboxyl group (9-hydroperoxy-12-oxo-10E-dodecenoic acid). Chiral analysis revealed that 4-HPNE formed from 13S-hydroperoxy-9Z,11E-octadecadienoic acid (13S-HPODE) retains >90% S configuration, whereas it is nearly racemic from 9S-hydroperoxy-10E,12Z-octadecadienoic acid (9S-HPODE). 9-Hydroperoxy-12-oxo-10E-dodecenoic acid is >90% S when derived from 9S-HPODE and almost racemic from 13S-HPODE. Through analysis of intermediates and products, we provide evidence that (i) allylic hydrogen abstraction at C-8 of 13S-HPODE leads to a 10,13-dihydroperoxide that undergoes cleavage between C-9 and C-10 to give 4S-HPNE, whereas direct Hock cleavage of the 13S-HPODE gives 12-oxo-9Z-dodecenoic acid, which oxygenates to racemic 9-hydroperoxy-12-oxo-10E-dodecenoic acid; by contrast, (ii) 9S-HPODE cleaves directly to 3Z-nonenal as a precursor of racemic 4-HPNE, whereas allylic hydrogen abstraction at C-14 and oxygenation to a 9,12-dihydroperoxide leads to chiral 9S-hydroperoxy-12-oxo-10E-dodecenoic acid. Our results distinguish two major pathways to the formation of 4-HNE that should apply also to other fatty acid hydroperoxides. Slight ( approximately 10%) differences in the observed chiralities from those predicted in the above mechanisms suggest the existence of additional routes to the 4-hydroxyalkenals.

Oxygen-dependent DNA damage amplification involving 5,6-dihydrothymidin-5-yl in a structurally minimal system

5,6-Dihydrothymidin-5-yl (1) was independently generated in a dinucleotide from a phenyl selenide precursor (4). Under free radical chain propagation conditions, the products resulting from hydrogen atom donation and radical-pair reaction are the major observed products in the absence of O(2). The stereoselectivity of the trapping process is dependent on the structure of the hydrogen atom donor. No evidence for internucleotidyl hydrogen atom abstraction by 1 was detected. The tandem lesion (17) resulting from hydrogen atom abstraction from the C1' position of the adjacent 2'-deoxyuridine by the peroxyl radical derived from 1 (3) is observed under aerobic conditions. The structure of this product is confirmed by independent synthesis and its transformation into a second independently synthesized product (24). Internucleotidyl hydrogen atom abstraction is effected selectively by the 5S-diastereomer of the peroxyl radical. The formation of dinucleotide 17 provides further support for the novel O(2)-dependent DNA damage amplification mechanism involving 1 reported previously (Greenberg, M. M.; et al. J. Am. Chem. Soc. 1997, 119, 1828).

The reactivity of the 2-deoxyribonolactone lesion in single-stranded DNA and its implication in reaction mechanisms of DNA damage and repair

The formal C1'-oxidation product, 2-deoxyribonolactone, is formed as a result of DNA damage induced via a variety of agents, including gamma-radiolysis and the enediyne antitumor antibiotics. This alkaline labile lesion may also be an intermediate during DNA damage induced by copper-phenanthroline. Oligo-nucleotides containing this lesion at a defined site were formed via aerobic photolysis of oligonucleotides containing a photolabile ketone, and were characterized by gel electrophoresis and electrospray mass spectrometry (ESI-MS). Treatment of oligo-nucleotides containing the lesion with secondary amines produces strand breaks consisting of 3'-phosphate termini, and products which migrate more slowly in polyacrylamide gels. MALDI-TOF mass spectrometry analysis indicates that the slower moving products are formal adducts of the beta-elimination product resulting from 2-deoxyribonolactone and one molecule of amine. The addition of beta-mercapto-ethanol to the reaction mixture produces thiol adducts as well. The stability of these adducts suggests that they cannot be the labile species characterized by gel electrophoresis in copper-phenanthroline-mediated strand scission. The characterization of these adducts by mass spectrometry also provides, by analogy, affirmation of proposals regarding the reactivity of nucleophiles with the beta-elimination product of abasic sites. Finally, the effects of this lesion and the various adducts on DNA repair enzymes are unknown, but their facile generation from oligonucleotides containing a photolabile ketone suggests that such issues could be addressed.