Tissue concentrations of both homocysteine (Hcy) and cysteine (Cys) are maintained at low levels by regulated production and efficient removal of these thiols. The regulation of the metabolism of methionine and Cys is discussed from the standpoint of maintaining low levels of Hcy and Cys while, at the same time, ensuring an adequate supply of these thiols for their essential functions. S-Adenosylmethionine coordinately regulates the flux through remethylation and transsulfuration, and glycine N-methyltransferase regulates flux through transmethylation and hence the S-adenosylmethionine/S-adenosylhomocysteine ratio. Cystathionine beta-synthase activity is also regulated in response to the redox environment, and transcription of the gene is hormonally regulated in response to fuel supply (insulin, glucagon, and glucocorticoids). The H2S-producing capacity of cystathionine gamma-lyase may be regulated in response to nitric oxide. Cys is substrate for a variety of anabolic and catabolic enzymes. Its concentration is regulated primarily by hepatic Cys dioxygenase; the level of Cys dioxygenase is upregulated in a Cys-responsive manner via a decrease in the rate of polyubiquitination and, hence, degradation by the 26S proteasome.

中文翻译：

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硫氨基酸代谢：高半胱氨酸和半胱氨酸的产生和去除途径。

通过调节产量和有效去除这些硫醇，可将高半胱氨酸（Hcy）和半胱氨酸（Cys）的组织浓度维持在较低水平。从维持低水平的Hcy和Cys的角度讨论蛋氨酸和Cys的代谢调控，同时确保这些硫醇充分供应以实现其基本功能。S-腺苷甲硫氨酸通过再甲基化和转硫来协调调节通量，而甘氨酸N-甲基转移酶通过反甲基化调节通量，因此，S-腺苷甲硫氨酸/ S-腺苷同型半胱氨酸比率。胱硫醚β-合酶活性也响应于氧化还原环境而被调节，并且该基因的转录响应于燃料供应（胰岛素，胰高血糖素和糖皮质激素）而被激素调节。可以响应一氧化氮调节胱硫醚γ-裂合酶的硫化氢生产能力。半胱氨酸是多种合成代谢和分解代谢酶的底物。它的浓度主要受肝脏半胱氨酸二加氧酶的调节。Cys双加氧酶的水平通过多聚泛素化率的降低，从而被26S蛋白酶体降解而以Cys响应方式上调。