Therapeutic and Metabolic Responses of Citrulline vs. Arginine Supplementation

描述(申请人提供)：精氨酸是一种条件性必需氨基酸，除了蛋白质合成外，还具有多种功能。精氨酸被用来合成肌酸、多胺和一氧化氮，因此在能量代谢、细胞增殖以及调节血压和免疫反应中发挥着核心作用。补充精氨酸在各种情况下都是有益的，如先兆子痫、高血压和脓毒症。然而，不良后果已有报道，精氨酸疗法的长期安全性和有效性仍未得到充分描述。瓜氨酸，它的前身 内源性精氨酸合成，是补充精氨酸的一种可行的替代方法。虽然精氨酸的第一次提取和水解率相当高(~40%，并且可能因补充而增加)，但瓜氨酸逃避肝脏新陈代谢。因此，补充瓜氨酸有可能比补充精氨酸本身向周围组织输送更多的精氨酸。此外，不同类型的细胞能够直接利用瓜氨酸来满足当地的精氨酸需求。细胞直接利用瓜氨酸的这种能力是癌症患者接受聚乙二醇化精氨酸脱亚氨酶(ADI-PEG)治疗的原因，聚乙二醇化精氨酸脱亚胺酶会导致精氨酸耗尽，但瓜氨酸水平较高，能够维持代谢平衡。补充瓜氨酸还有一个额外的优势，即当精氨酸在肠道和循环中的高活性(例如炎症性肠病、镰状细胞贫血溶血)导致精氨酸可获得性受到影响时，它可以提供精氨酸。为了解决补充瓜氨酸比补充精氨酸本身更好地获得内源性精氨酸这一假设，我们提出了以下具体目标。具体目的1：测定小鼠长期补充精氨酸或瓜氨酸时精氨酸的药代动力学、氧化和首过代谢。具体目标2：确定精氨酸限制时瓜氨酸利用率增加的分子基础，并测定ADI-PEG精氨酸耗竭小鼠不同组织的部分蛋白质合成率、全身一氧化氮产生和血压。具体目标3：确定不同水平的精氨酸和瓜氨酸对慢性高血压小鼠模型和先兆子痫小鼠模型的降压作用。将利用稳定同位素和血压遥测技术测量不同的代谢(通量、氧化、代谢物相互转化)和生理(血压、一氧化氮合成、蛋白质合成)终点。我们目前的知识只能占到作为血浆精氨酸前体产生的瓜氨酸的60%左右。由于瓜氨酸唯一已知的命运是将其转化为精氨酸，这种不明原因的瓜氨酸很可能为局部合成精氨酸提供前体。这些研究将填补我们对这种不明瓜氨酸及其生理效应的认识空白。此外，当对精氨酸的需求增加时，如高血压、先兆子痫、败血症、炎症性肠病和镰状细胞性贫血，这些信息将为新的人类治疗试验和补充精氨酸和瓜氨酸的建议提供基础。

DESCRIPTION (provided by applicant): Arginine is a conditionally essential amino acid that has many functional roles besides protein synthesis. Arginine is utilized for the synthesis of creatine, polyamines and nitric oxide and thus has a central role in energy metabolism, cell proliferation and in the regulation of blood pressure and immune response. Arginine supplementation has been beneficial in a variety of circumstances such as preeclampsia, hypertension, and sepsis. Detrimental outcomes, however, have been reported, and the long term safety and efficacy of arginine therapy remain poorly described. Citrulline, the precursor for endogenous arginine synthesis, is a viable alternative to arginine supplementation. Whereas first pass extraction and hydrolysis of arginine is considerable (~40%, and likely to increase due to supplementation) citrulline escapes liver metabolism. For this reason citrulline supplementation has the potential to deliver more arginine to peripheral tissues than arginine supplementation itself. Moreover, different cell types have the ability to directly utilize citrullne to meet their local arginine needs. This ability of cells to directly utilize citrulline is the reasonwhy cancer patients treated with pegylated arginine deiminase (ADI-PEG), which results in arginine depletion but high levels of citrulline, are able to maintain metabolic homeostasis. Citrulline supplementation has the added advantage that it can provide arginine when arginine availability is compromised due to high arginase activity in the gut and circulation (e.g., inflammatory bowel disease, sickle cell anemia hemolysis). To address the hypothesis that supplemental citrulline is a better source of endogenous arginine availability than arginine supplementation itself, we propose the following specific aims. Specific Aim 1: To determine the pharmacokinetics, oxidation and the first pass metabolism of arginine when arginine or citrulline are chronically supplemented in mice. Specific Aim 2: To determine the molecular basis for increased citrulline utilization when arginine is limiting and to determine fractional protein synthesis rate of differet tissues, whole body nitric oxide production and blood pressure in ADI-PEG arginine-depleted mice. Specific Aim 3: To determine the reduction in blood pressure in a chronic hypertensive mouse model and in a mouse model of preeclampsia supplemented with different levels of arginine and citrulline. Different metabolic (fluxes, oxidation, metabolite interconversion) and physiological (blood pressure, NO synthesis, protein synthesis) endpoints will be measured utilizing stable isotopes and blood pressure telemetry. Our current knowledge can only account for ~60% of the citrulline produced as the precursor for plasma arginine. Because the only known fate of citrulline is its conversion into arginine the unaccounted citrulline is likely to provide the precursor for local arginine synthesis. These studies will fill the gap in our knowledge regarding this unaccounted citrulline and its physiological effect. Moreover this information will provide the basis for new human therapeutic trials and recommendations on arginine and citrulline supplementation when the demand for arginine is increased as in hypertension, preeclampsia, sepsis, inflammatory bowel disease, and sickle cell anemia.