On the Products of Cholesterol Autoxidation in Phospholipid Bilayers and the Formation of Secosterols Derived Therefrom

Abstract

In homogenous solution, cholesterol autoxidation leads to am ixture of epimers of 5 primary products,whose concentrations vary in the presence/absence of antioxidants,such as vitamin E. Tw oo ft he products (5a-OOH and 6b-OOH) undergo Hockf ragmentation to yield electrophilic secosterols implicated in disease.H erein, we show that the product distribution is similar in phospholipid bilayers,i nt hat the 7-OOHs are the major products,but the presence/absence of vitamin Ehas no effect on the distribution. Cholesterol 7a-OOH, but not 7b-OOH, undergoes Hockfragmentation to yield am ixture of unprecedented A-ring cleavage products and 6,7-epoxides.W hen subjected to typical derivatization conditions,7 a-OOH yields products with essentially indistinguishable chromatographic and spectroscopic features from the previously identified secosterols,c asting further doubt on their controversial origin from endogenous O 3 .

Reactive Sterol Electrophiles: Mechanisms of Formation and Reactions with Proteins and Amino Acid Nucleophiles

“…Recent studies showed that C7α-OOH undergoes Hock fragmentation readily, while C7β-OOH is unreactive [40]. This rearrangement does not follow the typical Hock mechanism shown in Figure 7A; instead, an intermediate epoxy carbocation is formed, followed by water entrapment or fragmentation, to give an allylic epoxide that should be highly electrophilic.…”

“…The autoxidation of cholesterol incorporated into phosphatidylcholine liposomes gives the same set of products that are found in solution, but the dynamics of reactions in membrane-like vesicles have a significant effect on the product profiles observed when compared to the product distributions found in isotropic solutions. One notable feature of cholesterol autoxidation in phosphatidylcholine liposome is that no evidence of C5α-OOH formation was found when α-tocopherol or PMC was incorporated into the vesicles [40]. This can be understood when consideration is given to the fact that α-tocopherol is a much poorer H-atom donor in phospholipid bilayers (k inh = 4.7 × 10 3 M −1 s −1 ) compared to organic solutions (k inh = 3.6 × 10 6 M −1 s −1 ) due to phenolic hydrogen bonding with the polar headgroup of the phospholipid (see Figure 5A) [87].…”

“…In contrast to the results obtained with α-tocopherol, the use of aromatic amine RTAs, such as phenoxazine [87], in the oxidation of cholesterol in liposomes leads to the formation of C5α-OOH (see Figure 5B). Phenoxazine has an apparent rate constant for reactions with peroxyl radicals in liposomes (kinh= 2.4 × 10 5 M −1 s −1 ) that is some 50 times greater than αtocopherol, making H-atom transfer from the amine competitive with β-fragmentation of the C5α peroxyl radical [40]. In competition with H-abstraction from the allylic positions in cholesterol, peroxyl radicals add to the C5-C6 double bond.…”

“…The fragmentation of cholesterol hydroperoxides leads to highly electrophilic aldehydic species. Both C5α-OOH and C6β-OOH readily undergo acid-catalyzed (Hock) fragmentation [103][104][105] to give the B-ring cleavage product secosterol A, and its aldolized product, secosterol B (see Figure 6) [39][40][41]106]. These derivatives are precisely the same compounds as those formed from the reaction of cholesterol with ozone [70,71,74,107,108]; their character as reactive electrophiles will be subsequently Dendrogenin A is a product of histamine and the α-epoxide.…”

“…At the time that reactive lipid-derived electrophiles like MDA were being associated with lipid oxidation reactions in animal tissues, the chemistry of free radical chain reactions responsible for their formation was also drawing significant attention. While early work on the mechanism of autoxidation was principally centered on the degradation of commercially important hydrocarbons, lipid peroxidation drew increased interest during the latter half of the 20th century, with efforts to describe mechanisms for the autoxidation of polyunsaturated fatty acids (PUFAs) [23][24][25][26][27][28][29][30] and sterols [27,[31][32][33][34][35][36][37][38][39][40][41][42] attracting interest that has continued to this day.…”