Minimizing Tocopherol-Mediated Radical Phase Transfer in Low-Density Lipoprotein Oxidation with an Amphiphilic Unsymmetrical Azo Initiator

Abstract

The antioxidant alpha-tocopherol (alpha-TOH) has been found to act as a pro-oxidant under many in vitro conditions. The observed tocopoherol-mediated peroxidation (TMP) is dependent on two primary factors. (1) Chain transfer: alpha-TO. radical reacts with lipid to form lipid peroxyl radicals. (2) Phase transfer: alpha-TOH can transport radical character into the lipoprotein. Given the limitations of existing initiators, there is a need for new compounds that avoid the requirement for alpha-TOH to act as a phase-transfer agent. We report here a study showing that the new unsymmetrical azo compound, C-8, initiates LDL lipid peroxidation without requirement for alpha-TOH. This initiator provides a steady source of free amphiphilic peroxyl radicals that efficiently initiates oxidation of alpha-TOH-depleted LDL at a rate comparable to that reported for the very reactive hydroxyl radical (.OH). With other initiators tested, unsymmetrical C-12 and C-16 and symmetrical C-0 and MeOAMVN, alpha-TOH-depleted LDL displayed significant resistance to oxidation. Results indicate that the amphiphilic nature of the unsymmetrical initiators increases their partitioning into lipoprotein depending on the hydrocarbon chain length, and the symmetrical azo initiators C-0 and MeOAMVN primarily remain in the aqueous phase. Evidence suggests that even when the phase-transfer activity of alpha-TOH is limited, with the use of an initiator such as C-8, the mechanism of peroxidation remains controlled by TMP chain-transfer activity.

Preparation and Investigation of Vitamin B6‐Derived Aminopyridinol Antioxidants

“…The pyridinols that partition between both the lipid and aqueous phases ( 2 , 11 , 12 , and 14 ) were found to have lower n values (1.3–1.6), whereas those pyridinols that will partition more or less exclusively to the lipid phase had n values around the ideal value of 2.0 ( 13 , 15 , and 16 ). It has been noted by others that UA has little impact on radical chain oxidation processes of lipid bilayers,20, 21 although more lipophilic radical scavengers protect oxidizable substrates more efficiently,18b, 22 as we observe for the pyridinols (see above).…”

Role of Oxidative Modifications in Atherosclerosis

“…Under such conditions, the extent and ability of LDL lipid to oxidize is controlled primarily by ␣-TOH, as shown unambiguously by in vitro experiments. Thus so long as ␣-TOH remains in the particle, the process of LDL lipid peroxidation under mild oxidative conditions follows a model of tocopherolmediated peroxidation (77,81,169,433,443,496,893,968,1057,1058). In tocopherol-mediated peroxidation, ␣-TOH does not act as a classic chain-breaking antioxidant and the fate of ␣-TO⅐ determines whether vitamin E in LDL exhibits pro-or antioxidant activity.…”

New Insights Regarding the Autoxidation of Polyunsaturated Fatty Acids

“…The hydrophilic/hydrophobic character of the radicals generated from those amphiphilic azo initiators is an important factor for separating the two radicals. These radical initiators provide an advantage of free radical production, lipophilic access, and constant radical generation over lipophilic initiators and hydrophilic initiators when they are used to initiate the peroxidation of LDL oxidation for in vitro studies (15).…”