Regulation of cystathionine-beta-synthase (CBS) mediated hydrogen sulfide (H2S) production and its biological function

同型半胱氨酸（Hcy）和硫化氢（H2S）是体内硫氨基酸（即蛋氨酸、半胱氨酸）代谢过程中产生的两种重要分子。Hcy是必需氨基酸蛋氨酸转化为半胱氨酸产生的中间氨基酸。Hcy既可以再生为蛋氨酸，也可以代谢为半胱氨酸或H2S。胱硫氨酸-合成酶（CBS）是调节Hcy代谢和H2S生成的关键酶。CBS酶活性降低导致(1)血液中Hcy升高和(2)H2S水平降低。CBS酶存在于包括肾脏在内的多个器官中，在调节硫氨基酸和Hcy代谢中起关键作用。人们早就知道，血液循环中过量的Hcy，即高同型半胱氨酸血症，会导致多器官损伤。高同型半胱氨酸血症常见于肾衰竭患者。肾功能衰竭的常见原因之一是缺血再灌注损伤。缺血是一种缺氧状态，减少或没有血液流向组织，再灌注是恢复血液流向缺血组织。受影响的组织经历氧化应激，在这种情况下，细胞被活性氧破坏。当活性氧的产生与抗氧化防御机制的解毒能力之间存在不平衡时，就会发生氧化应激。我们观察到缺血-再灌注导致CBS酶活性降低，导致肾中Hcy积累。反过来，Hcy刺激肾脏中活性氧的产生，导致氧化应激和细胞死亡。肾内H2S水平在缺血-再灌注时严重降低，与组织损伤呈负相关。在生理水平上，H2S作为血管扩张剂和神经递质。然而，H2S在肾脏中的生物学功能尚不清楚。在我们提出的研究中，我们旨在确定(1)H2S在肾脏中的作用(2)通过恢复CBS活性调节H2S合成是否对缺血-再灌注诱导的氧化应激和肾脏损伤具有保护作用。

Homocysteine (Hcy) and hydrogen sulfide (H2S) are two important molecules produced in the body during the metabolism of sulfur amino acids (i.e. methionine, cysteine). Hcy is an intermediate amino acid produced through the conversion of essential amino acid methionine to cysteine. Hcy can either be regenerated to methionine as well as metabolized to cysteine or H2S. Cystathionine beta-synthase (CBS) is a key enzyme that regulates Hcy metabolism and H2S generation. Reduced CBS enzyme activity leads to (1) increased Hcy and (2) decreased H2S levels in the blood. CBS enzyme is present in several organs including the kidney that plays a key role in regulating sulfur amino acid and Hcy metabolism. It has long been known that too much Hcy in the circulation, a condition called hyperhomocysteinemia, can cause multiple organ injury. Hyperhomocysteinemia is often found in persons with kidney failure. One of the common causes of kidney failure is ischemia-reperfusion injury. Ischemia is a hypoxic condition with decreased or no blood flow to the tissue and reperfusion is the restoration of blood flow to the ischemic tissue. The affected tissue undergoes oxidative stress, a condition in which cells are damaged by reactive oxygen species. Oxidative stress occurs when there is an imbalance between the production of reactive oxygen species and the detoxifying capability of antioxidant defense mechanisms. We have observed that ischemia-reperfusion causes a reduction of CBS enzyme activity leading to Hcy accumulation in the kidney. Hcy, in turn, stimulates the production of reactive oxygen species in the kidney causing oxidative stress and cell death. The H2S level in the kidney is severely decreased upon ischemia-reperfusion and is inversely associated with tissue damage. At physiological levels, H2S acts as a vasodilator and neurotransmitter. However, the biological function of H2S in the kidney is not well understood. In the proposed research, we aim to identify (1) the role that H2S plays in the kidney (2) whether regulation of H2S synthesis through restoration of CBS activity offers any protective effect against ischemia-reperfusion induced oxidative stress and kidney damage.