Importance of a novel oxidative mechanism for elimination of brain cholesterol. Turnover of cholesterol and 24(S)-hydroxycholesterol in rat brain as measured with 18O2 techniques in vivo and in vitro.

The brain is the most cholesterol-rich organ in the body. Brain cholesterol is characterized by a very low turnover with very little exchange with lipoproteins in the circulation. Very recently we showed that there is a continuous age-dependent flux of 24(S)-hydroxycholesterol from the human brain into the circulation (Lütjohann, D., Breuer, O., Ahlborg, G., Nennesmo, I., Sidén, A., Diczfalusy, U., and Björkhem, I. (1996) Proc. Natl. Acad. Sci. U. S. A. 93, 9799-9804). Here we measured the rate of synthesis of cholesterol as well as the conversion of cholesterol into 24(S)-hydroxycholesterol in rat brain in vivo with use of an 18O2 inhalation technique and mass isotopomer distribution analysis. Cholesterol synthesis was found to correspond to 0.03 +/- 0.01% of the pool per h. Conversion of cholesterol into 24(S)-hydroxycholesterol was of a similar magnitude, about 0.02% of the pool per h. Brain microsomes converted endogenous cholesterol into 24(S)-hydroxycholesterol at a similar rate when incubated in the presence of NADPH. When incubated with whole homogenate and subcellular fractions of rat brain, there was no significant conversion of tritium-labeled 24-hydroxycholesterol into more polar products. Plasma from 18O2-exposed rats contained 24(S)-hydroxycholesterol with an enrichment of 18O similar to that in 24(S)-hydroxycholesterol in the brain. The results suggest that the present 24(S)-hydroxylase mediated mechanism is most important for elimination of cholesterol from the brain of rats. There is a slow conversion of brain cholesterol into 24(S)-hydroxycholesterol with a rapid turnover of the small pool of the latter oxysterol due to leakage to the circulation (half-life of brain 24(S)-hydroxycholesterol is about 0.5 days as compared with 2-4 months for brain cholesterol). It is evident that the 24(S)-hydroxylation greatly facilitates transfer of cholesterol over the blood-brain barrier and that this hydroxylation may be critical for cholesterol homeostasis in the brain.