THERAPEUTIC AND STABLE ISOTOPIC INVESTIGATION OF THE UREA CYCLE DISORDERS

这个子项目是许多研究子项目中利用 资源由NIH/NCRR资助的中心拨款提供。子项目和 调查员(PI)可能从NIH的另一个来源获得了主要资金， 并因此可以在其他清晰的条目中表示。列出的机构是 该中心不一定是调查人员的机构。 尿素循环是排泄饮食摄入和蛋白质分解代谢产生的过量氮化合物所必需的(1)。人类尿素循环酶的遗传缺陷是众所周知的，通常出现在新生儿期或早期婴儿期，伴有代谢危机和随后的神经损伤。每种疾病的严重程度都有很大的差异。最常见的尿素循环障碍是鸟氨酸转氨酶(OTC)缺乏症。其他尿素循环障碍包括氨基甲酰磷酸合成酶(CPS)、精氨酸琥珀酸合成酶(ASS)、精氨酸琥珀酸裂解酶(ASL)和精氨酸酶缺乏症。精氨酸琥珀酸裂解酶障碍又称精氨酸琥珀酸尿症(ASA)。 AsA的发病率为每70000名活产儿中就有一名，通常出现在新生儿时期伴有高氨酸代谢危象。临床照片是一个看起来很健康的新生儿，在很短的一段时间内健康，出现呕吐、嗜睡和厌食症。如果不治疗，这些症状会迅速发展为昏迷和死亡。如果高氨血症持续时间延长，就会出现严重的永久性神经功能损害。血液透析和替代途径药物的“新生儿抢救”通常伴随着终生发作性高氨血症，通常是由轻微感染或饮食不平衡引起的。高氨血症复发会导致进一步的神经损伤。不太严重的ASA和其他尿素循环 周期紊乱可能出现在婴儿期、儿童期或成年期，是突变异质性的结果。 尿素循环障碍的治疗依赖于两种策略(2，3)。第一种是通过使用限制蛋白质的饮食来复制氮负荷。第二种方法使用肝脏的“替代”或潜伏酶途径，将氨基酸与载体分子(外源性药物)结合，并补充精氨酸，以增加含氮产品的尿液排泄。目前，ASA患者只接受饮食和精氨酸治疗。精氨酸疗法的原理是通过替换尿素循环中受损反应的下游产物(精氨酸琥珀酸被精氨酸琥珀酸裂解酶转化为精氨酸和富马酸)；循环被重新启动，以继续产生额外的精氨酸琥珀酸。由于其极高的肾脏清除率，它有效地充当了尿素的有效氮汇。 FDA批准的一种治疗尿素周期早期其他疾病的方法是苯丁酸钠(丁苯基)，即鸟氨酸转氨酶缺乏症和瓜氨酸血症。目前，关于苯丁酸钠对降低ASA高氨血症危象发生率、肝转氨酶水平和瓜氨酸/精氨酸琥珀酸水平的具体作用的定量信息很少。苯丁酸钠给药后迅速转化为苯乙酸乙酯。苯乙酸酯是一种具有代谢活性的化合物，它通过乙酰化与谷氨酰胺偶联，形成苯乙酰谷氨酰胺。这种化合物是水溶性的，然后从尿液中排出。通过这种方式，苯丁酸酯可作为氮素排泄的替代载体。 明显的肝肿大是ASA的一个特征，在其他尿素循环障碍中没有发现如此程度的肝脏肿大。肝纤维化已被这些患者的肝活检证实，通常在疾病的早期就开始(4，5)。其中大多数患者的肝脏转氨酶(ALT和AST)水平也升高到正常水平的2倍。这些海拔升高的原因尚不清楚，但已表明它们独立于氨控制而发生。肝纤维化的程度可能与转氨酶水平有关。随着患有这种疾病的儿童通过更好的医疗管理方法存活更长的时间，更好地控制转氨酶水平以避免纤维化变得更加重要，因为纤维化可能导致危及生命的肝硬变。肝移植已经在严重的肝硬变病例中进行过。由于在这种情况下唯一的代谢物是精氨酸丁二酸和/或其分解产物，我们推测精氨酸丁二酸和/或其代谢物可能是引起肝脏炎症的有害因素。具有讽刺意味的是，目前大剂量精氨酸治疗的目的是有效降低高氨血症的发生频率，从而保护大脑，它可能会增加肝脏炎症的发生。基于这些观察，我们假设，通过转移精氨酸丁二酸生产中的氮流量来刺激氮素的替代处理，我们可以观察到肝脏炎症减轻，证据是LFT降低，S，磁共振成像和临床可用的组织学测量的肝纤维化的稳定或改善。此外，我们还可以观察到对日粮外周氮的更大耐受性，从而减少高氨血症的频率和程度，降低AsA的稳态水平。

This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The urea cycle is required for excretion of excess nitrogen compounds generated by dietary intake and protein catabolism (1). Human genetic deficiencies of urea cycle enzymes are well known and usually present in the neonatal period or early infancy with metabolic crises and subsequent neurological impairment. Each disease has significant variability in severity. The most common urea cycle disorder is ornithine transcarbamylase (OTC) deficiency. The other urea cycle disorders are carbamyl phosphate synthetase (CPS), argininosuccinic acid synthetase (ASS), argininosuccinate lyase (ASL), and arginase deficiencies. Argininosuccinic acide lyase defiency is also known as Argininosuccinic aciduria (ASA). ASA has an incidence of one in seventy thousand live births and commonly presents in the neonatal period with hyperammonemic metabolic crisis. The clinic picture is that of a healthy appearing neonate who, after a short period of health, develops vomiting, lethargy and anorexia. These symptoms rapidly progress to coma and death if not treated. If the hyperammonemia is prolonged, there is severe and permanent neurolgical impairment. "Neonatal rescue" by heodialysis and alternative pathway drugs is typically followed by life long episodic hyperammonemia usually precipitated by minor infections or dietary imbalance. Recrudescence of hyperammonemia leads to further neurological injury2. Less severe forms of ASA and other urea cyc cycle disorders may present during infancy, childhood, or adulthood and are a consequence of mutation heterogeneity. Treatment of urea cycle disorders relies on two strategies(2,3). The first is reproduction of nitrogen load through the use of a protein-restricted diet. The second approach uses "alternate" or laten enzymatic pathways of the liver to conjugate amino acids to carrier molecules (exogenously administered drugs) and arginine supplementation to increase urinary excretion of nitrogenous products. Currently, ASA patients are treated only with diet and arginine therapy. The principle of arginine therapy is that by replacing the product of downstream of the impaired reaction in the urea cycle (argininosuccinic acid is converted to arginine and fumarate by argininosuccinic acid lyase); the cycle is "reprimed" to continue to produce additional argininosuccinic acid. Because of its extremely high renal clearance, it acts effectively as an efficient nitrogen sink in place of urea. A FDA approved therapy for other disorders earlier in the urea cycle, i.e., ornithine transcarbamylase deficiency and citrullinemia, is sodium phenylbutyrate (Buphenyl). At present there is little quantitative information as to the specific effect of sodium phenylbutyrate on the ability to reduce frequency of hyperammonemic crisis, hepatic transaminase levels, and citrulline/argininosuccinate levels in ASA. Sodium phenylbutyrate is rapidly converted to phenylacetate after administration. Phenylacetate is a metabolically active compound that conjugates with glutamine via acetylation to form phenylacetylglutamine. This compound is water-soluble and is then excreted in the urine. In this way, phenylbutyrate serves as an alternative vehicle for nitrogen excretion. Marked hepatomegaly is a hallmark of ASA and is not found to such a degree in the other urea cycle disorders. Hepatic fibrosis has been documented by liver biopsy of these patients and generally begins early in the disease (4,5). The majority of these patients also have elevations of hepatic transaminases (ALT and AST) to > 2x normal levels. The etiology of these elevations is not known but it has been shown that they occur independently of ammonia control. It is probable that the degree of liver fibrosis correlates to transaminase levels. As children with this disorder survive for longer periods of time with better methods of medical management, it will become more important to better control transaminase levels to avoid fibrosis, which may lead to life threatening cirrhosis. Liver transplantation has been performed in cases of severe cirrhosis. Since the unique metabolite in this condition is arininosuccinic acid and/or its breakdown products, we hypothesize that argininosuccinic acid and/or its metabolites may be the offending agent causing hepatic inflammation. Ironically, the current therapy of high dose arginine treatment is aimed at effectively decreases the frequency of hyperammonemia, hence protecting the brain, it may increase the occurrence of hepatic inflammation. Based on these observations, we hypothesize that by stimulating alternative disposal of nitrogen by diverting nitrogen flux away from the production of argininosuccinic acid, we may observe decreased hepatic inflammation as evidence by decreased LFT s, stabilization or improvement of hepatic fibrosis as measured by MRI and histology where clinically available. Moreover, we may also observe a greater tolerance for dietary peripheral nitrogen and hence decrease the frequency and magnitude of hyperammonemia and decreased steady state level of ASA.