Additivity rule holds in the hydrogen transfer reactivity of unsaturated fatty acids with a peroxyl radical: mechanistic insight into lipoxygenase

Cis/Trans Isomerization of Unsaturated Fatty Acids in Polysorbate 80 During Light Exposure of a Monoclonal Antibody-Containing Formulation

Light exposure of a monoclonal antibody formulation containing polysorbate 80 (PS80) leads to cis/trans isomerization of monounsaturated and polyunsaturated fatty acids. This cis/trans isomerization was monitored by positive electrospray ionization mass spectrometry of intact PS80 components as well as by negative ion electrospray ionization mass spectrometry analysis of free fatty acids generated via esterase-catalyzed hydrolysis. The light-induced cis/trans isomerization of unsaturated fatty acids in PS80 required the presence of the monoclonal antibody, or, at a minimum (for mechanistic studies), a combination of N-acetyltryptophan amide and glutathione disulfide, suggesting the involvement of thiyl radicals generated by photoinduced electron transfer from Trp to the disulfide. Product analysis confirmed the conversion of PS80-bound oleic acid to elaidic acid; furthermore, together with linoleic acid, we detected conjugated linoleic acids in PS80, which underwent light-induced cis/trans isomerization.

Tunneling in tocopherol-mediated peroxidation of 7-dehydrocholesterol

The peroxidation of 7-dehydrocholesterol (7-DHC), a biosynthetic precursor to vitamin D3 and cholesterol, has been linked to the pathophysiology of Smith-Lemli-Optiz syndrome (SLOS), a devastating human disorder. In SLOS, 7-DHC plasma and tissue levels are elevated because of defects in the enzyme that convert it to cholesterol. α-Tocopherol can mediate the peroxidation of 7-DHC under certain circumstances and this prompted us to investigate the kinetic isotope effect (KIE) during this process. Thus, 9,14-d2-7-DHC was synthesized using a photochemical cyclization of deuterium-reinforced previtamin D3 (retro to its biosynthesis). Subsequently, we carried out co-oxidation of 9,14-h2-25,26,26,26,27,27,27-d7- and 9,14-d2-7-DHC in the presence of α-tocopherol under conditions that favor TMP. By monitoring the products formed from each precursor using mass spectrometry, the KIE for the hydrogen (deuterium) atom removal at C9 was found to be 21 ± 1. This large KIE value indicates that tunneling plays a role in the hydrogen atom transfer step in the tocopherol-mediated peroxidation of 7-DHC.

Novel insights into cyclooxygenases, linoleate diol synthases, and lipoxygenases from deuterium kinetic isotope effects and oxidation of substrate analogs

Cyclooxygenases (COX) and 8R-dioxygenase (8R-DOX) activities of linoleate diol synthases (LDS) are homologous heme-dependent enzymes that oxygenate fatty acids by a tyrosyl radical-mediated hydrogen abstraction and antarafacial insertion of O(2). Soybean lipoxygenase-1 (sLOX-1) contains non-heme iron and oxidizes 18:2n-6 with a large deuterium kinetic isotope effect (D-KIE). The aim of the present work was to obtain further mechanistic insight into the action of these enzymes by using a series of n-6 and n-9 fatty acids and by analysis of D-KIE. COX-1 oxidized C(20) and C(18) fatty acids in the following order of rates: 20:2n-6>20:1n-6>20:3n-9>20:1n-9 and 18:3n-3≥18:2n-6>18:1n-6. 18:2n-6 and its geometrical isomer (9E,12Z)18:2 were both mainly oxygenated at C-9 by COX-1, but the 9Z,12E isomer was mostly oxygenated at C-13. A cis-configured double bond in the n-6 position therefore seems important for substrate positioning. 8R-DOX oxidized (9Z,12E)18:2 at C-8 in analogy with 18:2n-6, but the 9E,12Z isomer was mainly subject to hydrogen abstraction at C-11 and oxygen insertion at C-9 by 8R-DOX of 5,8-LDS. sLOX-1 and 13R-MnLOX oxidized [11S-(2)H]18:2n-6 with similar D-KIE (~53), which implies that the catalytic metals did not alter the D-KIE. Oxygenation of 18:2n-6 by COX-1 and COX-2 took place with a D-KIE of 3-5 as probed by incubations of [11,11-(2)H(2)]- and [11S-(2)H]18:2n-6. In contrast, the more energetically demanding hydrogen abstractions of the allylic carbons of 20:1n-6 by COX-1 and 18:1n-9 by 8R-DOX were both accompanied by large D-KIE (>20).

Small amounts of isotope-reinforced polyunsaturated fatty acids suppress lipid autoxidation

Polyunsaturated fatty acids (PUFAs) undergo autoxidation and generate reactive carbonyl compounds that are toxic to cells and associated with apoptotic cell death, age-related neurodegenerative diseases, and atherosclerosis. PUFA autoxidation is initiated by the abstraction of bis-allylic hydrogen atoms. Replacement of the bis-allylic hydrogen atoms with deuterium atoms (termed site-specific isotope-reinforcement) arrests PUFA autoxidation due to the isotope effect. Kinetic competition experiments show that the kinetic isotope effect for the propagation rate constant of Lin autoxidation compared to that of 11,11-D(2)-Lin is 12.8 ± 0.6. We investigate the effects of different isotope-reinforced PUFAs and natural PUFAs on the viability of coenzyme Q-deficient Saccharomyces cerevisiae coq mutants and wild-type yeast subjected to copper stress. Cells treated with a C11-BODIPY fluorescent probe to monitor lipid oxidation products show that lipid peroxidation precedes the loss of viability due to H-PUFA toxicity. We show that replacement of just one bis-allylic hydrogen atom with deuterium is sufficient to arrest lipid autoxidation. In contrast, PUFAs reinforced with two deuterium atoms at mono-allylic sites remain susceptible to autoxidation. Surprisingly, yeast treated with a mixture of approximately 20%:80% isotope-reinforced D-PUFA:natural H-PUFA are protected from lipid autoxidation-mediated cell killing. The findings reported here show that inclusion of only a small fraction of PUFAs deuterated at the bis-allylic sites is sufficient to profoundly inhibit the chain reaction of nondeuterated PUFAs in yeast.

Isotope-reinforced polyunsaturated fatty acids protect yeast cells from oxidative stress

The facile abstraction of bis-allylic hydrogens from polyunsaturated fatty acids (PUFAs) is the hallmark chemistry responsible for initiation and propagation of autoxidation reactions. The products of these autoxidation reactions can form cross-links to other membrane components and damage proteins and nucleic acids. We report that PUFAs deuterated at bis-allylic sites are much more resistant to autoxidation reactions, because of the isotope effect. This is shown using coenzyme Q-deficient Saccharomyces cerevisiae coq mutants with defects in the biosynthesis of coenzyme Q (Q). Q functions in respiratory energy metabolism and also functions as a lipid-soluble antioxidant. Yeast coq mutants incubated in the presence of the PUFA α-linolenic or linoleic acid exhibit 99% loss of colony formation after 4h, demonstrating a profound loss of viability. In contrast, coq mutants treated with monounsaturated oleic acid or with one of the deuterated PUFAs, 11,11-D(2)-linoleic or 11,11,14,14-D(4)-α-linolenic acid, retain viability similar to wild-type yeast. Deuterated PUFAs also confer protection to wild-type yeast subjected to heat stress. These results indicate that isotope-reinforced PUFAs are stabilized compared to standard PUFAs, and they protect coq mutants and wild-type yeast cells against the toxic effects of lipid autoxidation products. These findings suggest new approaches to controlling ROS-inflicted cellular damage and oxidative stress.

Factors influencing the rearrangement of bis-allylic hydroperoxides by manganese lipoxygenase

Manganese lipoxygenase (Mn-LOX) catalyzes the rearrangement of bis-allylic S-hydroperoxides to allylic R-hydroperoxides, but little is known about the reaction mechanism. 1-Linoleoyl-lysoglycerophosphatidylcholine was oxidized in analogy with 18:2n-6 at the bis-allylic carbon with rearrangement to C-13 at the end of lipoxygenation, suggesting a "tail-first" model. The rearrangement of bis-allylic hydroperoxides was influenced by double bond configuration and the chain length of fatty acids. The Gly316Ala mutant changed the position of lipoxygenation toward the carboxyl group of 20:2n-6 and 20:3n-3 and prevented the bis-allylic hydroperoxide of 20:3n-3 but not 20:2n-6 to interact with the catalytic metal. The oxidized form, Mn(III)-LOX, likely accepts an electron from the bis-allylic hydroperoxide anion with the formation of the peroxyl radical, but rearrangement of 11-hydroperoxyoctadecatrienoic acid by Mn-LOX was not reduced in D(2)O (pD 7.5), and aqueous Fe(3+) did not transfer 11S-hydroperoxy-9Z,12Z,15Z-octadecatrienoic acid to allylic hydroperoxides. Mutants in the vicinity of the catalytic metal, Asn466Leu and Ser469Ala, had little influence on bis-allylic hydroperoxide rearrangement. In conclusion, Mn-LOX transforms bis-allylic hydroperoxides to allylic by a reaction likely based on the positioning of the hydroperoxide close to Mn(3+) and electron transfer to the metal, with the formation of a bis-allylic peroxyl radical, beta-fragmentation, and oxygenation under steric control by the protein.