Importance of a Novel Oxidative Mechanism for Elimination of Brain Cholesterol

Abstract

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)-hydroxycholes-terol from the human brain into the circulation (Lü tjo-hann, D., Breuer, O., Ahlborg, G., Nennesmo, I., Sidé n, Å., Diczfalusy, U., and Bjö rkhem, 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 cho-lesterol into 24(S)-hydroxycholesterol in rat brain in vivo with use of an 18 O 2 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)-hydroxycholes-terol was of a similar magnitude, about 0.02% of the pool per h. Brain microsomes converted endogenous choles-terol into 24(S)-hydroxycholesterol at a similar rate when incubated in the presence of NADPH. When incu-bated with whole homogenate and subcellular fractions of rat brain, there was no significant conversion of tri-tium-labeled 24-hydroxycholesterol into more polar products. Plasma from 18 O 2 -exposed rats contained 24(S)-hydroxycholesterol with an enrichment of 18 O sim-ilar 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)-hydroxycho-lesterol 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. The largest pool and concentration of cholesterol in the body is found in the brain. Being a constituent of myelin and cell membranes cholesterol is important for the function of this organ and an inborn defect in cholesterol synthesis is associ-ated with serious neurological and mental dysfunctions (1). Brain cholesterol is efficiently protected from exchange with circulating lipoproteins by the blood-brain barrier (2). In ac-cordance with this, most recent studies have favored the view that the majority of brain cholesterol is synthesized locally although at a low rate (for a review, see Ref. 3). Using in vivo and in vitro techniques the estimated half-life of cholesterol has been found to be 4 – 6 months in rats (4, 5). The protection of brain cholesterol from exchange with cir-culating lipoprotein by the blood-brain barrier is not absolute (6, 7) but this exchange seems to be low in relation to the local synthesis of cholesterol. To what extent there is a flux of un-metabolized cholesterol in the opposite direction from the brain into the circulation, is not known. If the blood-brain barrier is equally effective in both directions to prevent flux of choles-terol, there is a need for another more specific mechanism for elimination of cholesterol from the brain to compensate for the synthesis under steady-state conditions. Very recently we described an age-dependent net flux of 24(S)-hydroxycholesterol from the human brain into the circu-lation (8) and suggested that it could be of importance for cholesterol homeostasis in the brain. The concentration of 24(S)-hydroxycholesterol was found to be 30 –1500 times higher in the brain than in any other organ except the adre-nals, consistent with the possibility that most circulating 24(S)-hydroxycholesterol originates from the brain. To evaluate the importance of this new mechanism for elim-ination of cholesterol from the brain, we have now used an animal model for studies on synthesis of cholesterol and oxys-terols in vivo with use of an 18 O 2 inhalation technique (9). This technique utilizes the fact that synthesis of cholesterol as well as subsequent oxidation of cholesterol into oxysterols involves participation of a mixed function oxidase that incorporates one oxygen atom into the product.