Mono‐N‐Methylation of 1,2,3,4‐Tetrahydro‐β‐Carbolines in Brain Cytosol: Absence of Indole Methylation
Mono‐N‐Methylation of 1,2,3,4‐Tetrahydro‐β‐Carbolines in Brain Cytosol: Absence of Indole Methylation
复制标题
脑细胞溶胶中 1,2,3,4-四氢-β-咔啉的单 N-甲基化：不存在吲哚甲基化
作者：
K. Matsubara;M. Collins;E. Neafsey
摘要：在随附的报告中，我们证明了豚鼠脑细胞核中的酶活性，该酶活性催化杂芳族β-咔啉（BC）在2[β]-氮上的S-腺苷甲硫氨酸（SAM）依赖性N-甲基化，随后在9[吲哚]-氮上进行N-甲基化，最终产生N2，N9-二甲基化BC。本文介绍了1，2，3，4-四氢-BC（THBCs）的N-甲基化的平行研究结果，THBCs通过色氨酸及其衍生的吲哚与羰基化合物缩合内源性形成，或者像它们的BC氧化产物一样，是环境成分和植物生物碱。在大鼠或豚鼠脑的未透析匀浆中，THBCs在2[β]-氮上被[3 H]-SAM酶促甲基化，但未观察到[3 H]甲基转移至9[吲哚]-氮。用毛细管气相色谱-质谱法验证了2[β]-甲基THBC产物的结构。此外，尽管BC N-甲基化主要是颗粒状的，并显示出BC底物的微摩尔Km值，但THBC 2[β]-N-甲基化活性是胞质的，并显示出THBC底物的相对较高（毫摩尔）Km。THBCs的N-甲基化可能是由于其他人已经使用不同的氮杂杂环研究的细胞溶质N-甲基转移酶。我们的总体研究表明，N2，N9-二甲基化的BCs可能是独特的神经毒性因子，通过BCs的连续N-甲基化在脑内被生物激活。这些结果表明，可能存在另一种途径产生假定的2，9-二甲基化毒素，作为第一步，在大脑和其他器官中对环境或内源性THBCs进行2[β]-N-甲基化。
Abstract: In an accompanying report we demonstrated enzyme activity in guinea pig brain cell nuclei that catalyzes S‐adenosylmethionine (SAM)‐dependent N‐methylations of heteroaromatic β‐carbolines (BCs) on the 2[β]‐nitrogen and subsequently on the 9[indole]‐nitrogen, ultimately yielding N2,N9‐dimethylated BCs. Presented here are the esults of a parallel study of the N‐methylation of 1,2,3,4‐tetrahydro‐BCs (THBCs), which form endogenously via condensations of tryptophan and its derived indoles with carbonyl compounds or, like their BC oxidation products, are environmental constituents and plant alkaloids. THBCs were enzymatically methylated on the 2[β]‐nitrogen by [3H]‐SAM in undialyzed homogenates of rat or guinea pig brain, but [3H]methyl transfer to the 9[indole]‐nitrogen was not observed. The structure of the 2[β]‐methyl THBC product was verified with capillary gas chromatography‐mass spectrometry. Furthermore, whereas BC N‐methylation was largely particulate and displayed micromolar Km values for BC substrate, THBC 2[β]‐N‐methylation activity was cytosolic and displayed a relatively high (millimolar) Km for THBC substrate. The N‐methylation of THBCs may be due to cytosolic N‐methyltransferases that others have studied using different azaheterocyclics. Our overall studies indicate that N2,N9‐dimethylated BCs could be unique neurotoxic factors that are bioactivated within brain by sequential N‐methylations of BCs. These results suggest the possibility of an additional route to the putative 2,9‐dimethylated toxins involving, as a first step, 2[β]‐N‐methylation of environmental or endogenously derived THBCs in the brain and perhaps other organs.