Mast Natalia, Norcross Ryan, Andersson Ulla, Shou Magang, Nakayama Kazuo, Bjorkhem Ingemar, Pikuleva Irina A
Department of Pharmacology and Toxicology, University of Texas Medical Branch, Galveston, Texas 77555-1031, USA.
Biochemistry. 2003 Dec 9;42(48):14284-92. doi: 10.1021/bi035512f.
The known activity of cytochrome P450 46A1 (P450 46A1) is 24(S)-hydroxylation of cholesterol. This reaction produces biologically active oxysterol, 24(S)-hydroxycholesterol, and is also the first step in enzymatic degradation of cholesterol in the brain. We report here that P450 46A1 can further metabolize 24(S)-hydroxycholesterol, giving 24,25- and 24,27-dihydroxycholesterols in both the cell cultures transfected with P450 46A1 cDNA and the in vitro reconstituted system with recombinant enzyme. In addition, P450 46A1 was able to carry out side chain hydroxylations of two endogenous C27-steroids with and without a double bond between C5-C6 (7alpha-hydroxycholesterol and cholestanol, respectively) and introduce a hydroxyl group on the steroid nucleus of the C21-steroid hormones with the C4-C5 double bond (progesterone and testosterone). Also, P450 46A1 was found to metabolize xenobiotics carrying out dextromethorphan O- and N-demethylations, diclofenac 4'-hydroxylation, and phenacetin O-deethylation. Thus, substrate specificities of P450 46A1 are not limited to cholesterol and include a number of structurally diverse compounds. Activities of P450 46A1 suggest that, in addition to the involvement in cholesterol homeostasis in the brain, this enzyme may participate in metabolism of neurosteroids and drugs that can cross the blood-brain barrier and are targeted to the central nervous system.
细胞色素P450 46A1（P450 46A1）的已知活性是胆固醇的24（S）-羟基化。该反应产生具有生物活性的氧甾醇24（S）-羟基胆固醇，并且也是大脑中胆固醇酶促降解的第一步。我们在此报告，P450 46A1能够进一步代谢24（S）-羟基胆固醇，在转染了P450 46A1 cDNA的细胞培养物和重组酶体外重构系统中均产生24,25-二羟基胆固醇和24,27-二羟基胆固醇。此外，P450 46A1能够对两种内源性C27-类固醇进行侧链羟基化反应，这两种类固醇在C5-C6之间分别有或没有双键（分别为7α-羟基胆固醇和胆甾烷醇），并能在具有C4-C5双键的C21-类固醇激素（孕酮和睾酮）的类固醇核上引入羟基。而且，发现P450 46A1能够代谢外源性物质，进行右美沙芬的O-和N-去甲基化、双氯芬酸的4'-羟基化以及非那西丁的O-脱乙基化。因此，P450 46A1的底物特异性不仅限于胆固醇，还包括许多结构多样的化合物。P450 46A1的活性表明，除了参与大脑中的胆固醇稳态外，该酶还可能参与神经甾体和能够穿过血脑屏障并靶向中枢神经系统的药物的代谢。
