Zhao W Q, Latinwo L, Liu X X, Lee E S, Lamango N, Charlton C G
College of Pharmacy and Pharmaceutical Sciences, Florida A & M University, Tallahassee, Florida 32307, USA.
Exp Neurol. 2001 Sep;171(1):127-38. doi: 10.1006/exnr.2001.7726.
High nonphysiological doses of l-dopa are administered to Parkinson's disease (PD) patients, to replenish the depleted dopamine (DA). A large portion of the administered L-dopa and the newly formed DA undergoes methylation by reacting with S-adenosyl-L-methionine (SAM). In the process SAM, as well as L-dopa and DA, is utilized and great demands are placed on the transmethylation system. In this study we investigated whether L-dopa increases the transmethylation process by inducing methionine adenosyl transferase (MAT), the enzyme that produces SAM, and catechol-O-methyl transferase (COMT), the enzyme that transfers the methyl group from SAM to L-dopa and DA. Swiss Webster mice were injected with L-dopa, four times/day, for 1 to 16 days. Brain DA, 3-O-methyldopa (3-OMD), SAM, S-adenosylhomocysteine (SAH), MAT, and COMT were measured following a 24-h withdrawal period. An increase of 264% of brain DA occurred at days 2 and 3 after which it tapered to about 164% of control. The brain level of 3-OMD increased to 870% of the control. SAM was increased by 44% after the sixth day and SAH level was about double after the second day. After day 3, MAT activity was increased by about 35%. Western blot analysis showed that MAT is more clearly characterized in 10% mercaptoethanol reducing buffer in which 31.5-, 38- (beta), and 48-kDa (alpha1/alpha2) subunits were distinctly revealed. The induction of the 38-kDa and, more prominently, the 48-kDa subunits of MAT and the potential transactivator proteins of MAT, c-Jun/AP-1, was evident by day 6. The 31.5-kDa subunit was downregulated. COMT was detected as 24.7-, 30-, and 47.5-kDa bands in the brain, consistent with the membrane-bound COMT I (MB-COMT) and the dimeric COMT II. The 24.7- and the 30-kDa MB-COMT bands were induced in the brain by day 6 and peaked on day 9. The highlight of the study is the fact that L-dopa induces the enzymes MAT and COMT. In addition, the downturn in brain DA after the sixth day coincides with the increase in SAM and the 48-kDa MAT protein. Thus, during PD treatment with L-dopa the induction of MAT and COMT is likely to occur and in turn increase the methylation and reduction of L-dopa and DA that may help cause the tolerance or the wearing-off effect developed to L-dopa.
帕金森病（PD）患者需服用高剂量非生理性的左旋多巴（L-dopa），以补充耗尽的多巴胺（DA）。大部分给予的L-dopa以及新形成的DA会与S-腺苷-L-甲硫氨酸（SAM）反应而发生甲基化。在此过程中，SAM以及L-dopa和DA都会被利用，对转甲基化系统产生巨大需求。在本研究中，我们调查了L-dopa是否通过诱导产生SAM的蛋氨酸腺苷转移酶（MAT）以及将甲基从SAM转移至L-dopa和DA的儿茶酚-O-甲基转移酶（COMT）来增加转甲基化过程。给瑞士韦伯斯特小鼠每天注射4次L-dopa，持续1至16天。在停药24小时后测量脑内DA、3-O-甲基多巴（3-OMD）、SAM、S-腺苷高半胱氨酸（SAH）、MAT和COMT。在第2天和第3天，脑内DA增加了264%，之后逐渐降至约为对照的164%。脑内3-OMD水平增加至对照的870%。第6天后SAM增加了44%，第2天后SAH水平约为原来的两倍。第3天后，MAT活性增加了约35%。蛋白质免疫印迹分析表明，在10%巯基乙醇还原缓冲液中MAT的特征更明显，其中31.5 kDa、38 kDa（β）和48 kDa（α1/α2）亚基清晰可见。到第6天，MAT的38 kDa亚基，更显著的是48 kDa亚基以及MAT潜在的反式激活蛋白c-Jun/AP-1的诱导明显。31.5 kDa亚基下调。在脑内检测到COMT为24.7 kDa、30 kDa和47.5 kDa条带，与膜结合的COMT I（MB-COMT）和二聚体COMT II一致。到第6天，脑内24.7 kDa和30 kDa的MB-COMT条带被诱导，并在第9天达到峰值。该研究的重点是L-dopa诱导了MAT和COMT这两种酶。此外，第6天后脑内DA的下降与SAM和48 kDa MAT蛋白的增加同时出现。因此，在用L-dopa治疗PD期间，可能会发生MAT和COMT的诱导，进而增加L-dopa和DA的甲基化和降解，这可能有助于导致对L-dopa产生耐受性或药效减退效应。
