Unusual Kinetic Isotope Effects of Deuterium Reinforced Polyunsaturated Fatty Acids in Tocopherol-Mediated Free Radical Chain Oxidations

Abstract

Substitution of -CD 2 -at the reactive centers of linoleic and linolenic acids reduces the rate of abstraction of D by a tocopheryl radical by as much as 36-fold, compared to the abstraction of H from a corresponding -CH 2 -center. This H atom transfer reaction is the rate-determining step in the tocopherol-mediated peroxidation of lipids in human low-density lipoproteins, a process that has been linked to coronary artery disease. The unanticipated large kinetic isotope effects reported here for the tocopherol-mediated oxidation of linoleic and linolenic acids and esters suggests that tunneling makes this process favorable.The free radical oxidation of polyunsaturated fatty acids and sterols has attracted attention in recent years since lipid peroxidation is often associated with a variety of human pathologies. The formation of oxidized lipid products is frequently found to accompany heart disease, 1,2 environmental exposures, 3 and neurodegenerative disorders, 4,5 to cite a few examples. Natural antioxidants such as α-tocopherol (α-TOH) are excellent inhibitors of the free radical chain oxidation reaction in solution, 6 but antioxidant therapies for many diseases, Corresponding Authors. n.porter@vanderbilt.edu, misha@retrotope.com. ASSOCIATED CONTENT Supporting Information 1D and 2D NMR spectra, HPLC-MS data, and mass spectra of all the compounds are included in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.The authors declare the following competing financial interest(s): M. S. Shchepinov declares a competing financial interest as the Chief Scientific Officer of Retrotope, Inc. The other authors declare no competing financial interests. In free radical oxidation in the absence of antioxidants, the chain reaction propagates by the rate-limiting hydrogen atom transfer (Scheme 1, eq 1) and diffusion-controlled oxygen addition to the resulting carbon radical (eq 2). When tocopherol is used as an antioxidant in solution it serves as an H-atom donor to chain-carrying peroxyl radicals, generating a tocopheryl radical (α-TO • ) in the process (see Scheme 1, eq 3). The tocopheryl radical so generated normally traps a second peroxyl radical, terminating the chain sequence (Scheme 1, eq 4). The tocopheryl radical can maintain the chain sequence, however, by abstracting an H atom from lipid substrates if the rate of initiation is low and the concentration of tocopherol is high. 8 This reaction is slightly favored thermodynamically with the bond dissociation enthalpy of the phenolic O-H being 77.1 kcal/mol and the bis-allylic C-H being 76.4 kcal/mol. 9,10 This process, first recognized by Stocker, Ingold, and Bowry, 8,11,12 was designated as tocopherol-mediated peroxidation or TMP and is shown as eq 5 in Scheme 1. HHS Public AccessThe bis-allylic methylene group of polyunsaturated lipids is particularly vulnerable to radical attack, 13-15 and a recent strategy to diminish lipid peroxidation in vivo is based on deuterium substitution at the reactive cen...

Desmosterolosis and desmosterol homeostasis in the developing mouse brain

“…Detailed statistical analyses are shown in Figure S2. Our results also confirmed previously reported data that endogenous brain cholesterol synthesis in the developing brain starts around E12 …”

Deuterium‐reinforced polyunsaturated fatty acids improve cognition in a mouse model of sporadic Alzheimer's disease

“…Since oxidative stress is considered an important factor in the development of AD, and since D‐PUFAs have been shown to reduce LPO products in other models [], we investigated whether D‐PUFA treatment also reduces LPO in the Aldh2 − / − mouse model of sporadic AD. D‐PUFA treatment markedly decreased esterified F 2 ‐IsoPs by approximately 55% in both cortex and hippocampus, and prostaglandin F 2α (PGF 2α) levels by 20–25% (Fig.…”

“…Deuterium‐reinforced D‐PUFAs are resistant to nonenzymatic LPO and have been shown to reduce oxidative stress in several experimental models []. In the present study, we wished to evaluate whether alteration of lipid metabolism by a D‐PUFA‐enriched diet resulted in improvement of cognitive deficits in our model of oxidative stress‐induced cognitive impairment that exhibits AD‐like pathological changes.…”

Threshold protective effect of deuterated polyunsaturated fatty acids on peroxidation of lipid bilayers

“…The effect has been observed by monitoring oxygen consumption over the course of LPO in organic solutions of PUFAs ; while direct measurements of LPO end‐products revealed only a 12‐fold isotope effect along with some degree of LPO in the presence of 20% D‐PUFAs . In co‐oxidation experiments in organic solvents, a linear relationship was observed between the total amounts of oxidation products formed and the percentage of D‐PUFA, suggesting that D‐PUFA did not act as an antioxidant in the co‐oxidation reactions and only acted as a less reactive cosubstrate in the autoxidations carried out in solution .…”

“…Previous studies of this nonlinear effect have involved determination of the kinetics of D‐PUFA oxidation in organic solvents by monitoring oxygen consumption and peroxidation by‐products , thus likely bearing little relevance to processes in lipid bilayers. Studies have also been carried out in living cells , using cell survival and oxygen consumption rate (OCR) end points, but mechanistic interpretations are complicated by complex biological interactions following the initial LPO event.…”