Björkhem I, Lütjohann D, Breuer O, Sakinis A, Wennmalm A
Division of Clinical Chemistry, Karolinska Institutet, Huddinge Hospital, SE-141 86 Huddinge, Sweden.
J Biol Chem. 1997 Nov 28;272(48):30178-84. doi: 10.1074/jbc.272.48.30178.
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.
大脑是人体中胆固醇含量最高的器官。脑胆固醇的特点是周转率极低，与循环中的脂蛋白几乎没有交换。最近我们发现，人脑中有持续的、与年龄相关的24(S)-羟基胆固醇从大脑进入循环的通量（吕特约翰，D.，布勒尔，O.，阿尔伯格，G.，嫩内斯莫，I.，西登，A.，迪茨法卢西，U.，和比约克姆，I.（1996年）《美国国家科学院院刊》93，9799 - 9804）。在此，我们使用18O2吸入技术和质量同位素异构体分布分析，在体内测量了大鼠脑中胆固醇的合成速率以及胆固醇向24(S)-羟基胆固醇的转化。发现胆固醇合成相当于每小时池含量的0.03±0.01%。胆固醇向24(S)-羟基胆固醇的转化幅度相似，约为每小时池含量的0.02%。在NADPH存在的情况下孵育时，脑微粒体将内源性胆固醇转化为24(S)-羟基胆固醇的速率相似。当与大鼠脑的全匀浆和亚细胞组分一起孵育时，氚标记的24-羟基胆固醇没有显著转化为极性更强的产物。暴露于18O2的大鼠血浆中含有的24(S)-羟基胆固醇中18O的富集程度与脑中24(S)-羟基胆固醇中的相似。结果表明，目前由24(S)-羟化酶介导的机制对于大鼠脑中胆固醇的清除最为重要。脑胆固醇向24(S)-羟基胆固醇的转化缓慢，而后者氧化甾醇的小池由于泄漏到循环中而周转迅速（脑24(S)-羟基胆固醇的半衰期约为0.5天，而脑胆固醇的半衰期为2 - 4个月）。显然，24(S)-羟化极大地促进了胆固醇通过血脑屏障的转运，并且这种羟化可能对脑中胆固醇稳态至关重要。
