Cholesterol has been exploited with great advantage to detect any oxidation process in cell membranes. In contrast with unsaturated fatty acids, cholesterol exists as a single molecular species, its oxidation products are thus much less complicated to isolate and characterize (Smith LL, Cholesterol autoxidation, Plenum Press, NY 1981).

Cholesterol may undergo autoxidation and photo-oxidation, both processes give rise to oxysterols of various structures depending on the type of oxidation and the physical state of the substrate. Thus, the identification of cholesterol oxidation products may be used as a mechanistic proof in various oxidant systems. When cholesterol esters are oxidized, the structure and the yield of the formed oxysterols depend on the fatty acid species.
An extensive review on oxysterols, including their analysis, formation, occurrence, metabolism and physiological actions, may be consulted for further information (Schroepfer GJ, Physiol Rev 2000, 80, 362).

1 - AUTOXIDATION PRODUCTS

Cholesterol molecule

In simplifying the very complicated situation, it can be considered that the reaction of cholesterol with free radicals gives rise mainly to:

- C-7 oxygenated molecules:

7a-OH or 7b-OH cholesterol

These molecules are further transformed into hydroxylated derivatives or into:

7-keto cholesterol

These molecules are further transformed into a triol derivative:

2 - PHOTO-OXIDATION PRODUCTS

Peroxidation of cholesterol by singlet oxygen produces primarily a C-5 oxygenated molecule:

3 - ENZYMATIC OXIDATION

Further complications arise from the possible in vivo formation of some other oxysterols by enzymatic reaction.

If these reactions form mainly oxygenated derivatives in C-7 position, other products result from reactions on the lateral chain at C-20, C-22, C-24, C-25 and C-27.
24S-Hydroxycholesterol (cerebrosterol) is an enzymatically oxidized product of cholesterol mainly synthesized in the brain (Ercoli A et al., J Am Chem Soc 1953, 75, 3284). An overview of several studies on cerebrosterol has been presented with a discussion about its possible connection with neurodegenerative diseases (Bjorkhem I, Lipids 2007, 42, 5).

It must be noticed that the later compound was found in high concentration in atheroma plates.