The School of Biomedical Sciences and Pharmacy and The Hunter Medical Research Institute, Faculty of Health, The University of Newcastle, Callaghan, NSW 2308, Australia.
Neurochem Int. 2013 Dec;63(8):726-31. doi: 10.1016/j.neuint.2013.09.016. Epub 2013 Sep 29.
Tyrosine hydroxylase (TH) is the first and rate-limiting enzyme in dopamine synthesis. Dopamine regulates TH as an end-product inhibitor through its binding to a high and low affinity site, the former being abolished by Ser40 phosphorylation only, and the latter able to bind and dissociate according to intracellular dopamine levels. Here, we have investigated TH inhibition by a dopamine metabolite found in dopaminergic brain regions, salsolinol (SAL). SAL is known to decrease dopamine in the nigrostriatal pathway and mediobasal hypothalamus, and to also decrease plasma catecholamines in rat stress models, however a target and mechanism for the effects of SAL have not been found. We found that SAL inhibits TH activity in the nanomolar range in vitro, by binding to both the high and low affinity dopamine binding sites. SAL produces the same level of inhibition as dopamine when TH is non-phosphorylated. However, it produces 3.7-fold greater inhibition of Ser40-phosphorylated TH compared to dopamine by competing more strongly with tetrahydrobiopterin, the cofactor of this enzymatic reaction. SAL's potent inhibition of phosphorylated TH would prevent TH from being fully activated to synthesise dopamine.
酪氨酸羟化酶（TH）是多巴胺合成的第一个限速酶。多巴胺通过结合高亲和力和低亲和力位点作为终产物抑制剂来调节 TH，其中前者仅被 Ser40 磷酸化所废除，而后者能够根据细胞内多巴胺水平结合和解离。在这里，我们研究了多巴胺代谢物 salsolinol（SAL）对 TH 的抑制作用，SAL 已知可减少多巴胺在黑质纹状体通路和中脑基底部的含量，并在大鼠应激模型中降低血浆儿茶酚胺水平，但尚未发现 SAL 作用的靶点和机制。我们发现，SAL 在体外以纳摩尔范围抑制 TH 活性，通过与高亲和力和低亲和力多巴胺结合位点结合。当 TH 未磷酸化时，SAL 产生与多巴胺相同水平的抑制作用。然而，与四氢生物蝶呤（该酶反应的辅助因子）竞争时，SAL 对 Ser40 磷酸化 TH 的抑制作用更强，是多巴胺的 3.7 倍。SAL 对磷酸化 TH 的强烈抑制作用会阻止 TH 被完全激活以合成多巴胺。
