Porphyrin biosynthesis in normal and disease states

描述（由申请人提供）：迟发性皮肤卟啉症（PCT）是人类最常见的卟啉症，与肝脏铁储存过剩、酗酒、丙型肝炎和药物雌激素使用有关。PCT是由于肝脏中血红素生物合成酶-尿卟啉原脱羧酶（urod）的特定活性降低。我们已经确定，urod的催化活性受损是由一种名为uroporphoene的竞争性抑制剂引起的，这是一种八羧基四吡咯大环，在两个吡咯环之间有一个氧化的桥碳。桥碳的氧化是由P450和铁依赖反应介导的。我们的第一个具体目标是确定卟啉是由氧化羟基甲基二烷产生的，然后无酶环化，还是由氧化urod的完全还原底物urod卟啉原产生的。我们将通过用功能性人类P450系统转染野生型和突变型酵母，并通过改变胞质铁浓度，来验证产生卟啉需要特定的P450和铁的假设。我们的第二个具体目标是利用结构方法来定义卟啉介导的抑制机制，并确认我们的urod反应模型。我们的第三个具体目标是解决PCT中铁负荷的原因，我们将验证hepcidin表达的抑制是导致肝脏铁储存过剩的假设。总的来说，这些研究将明确PCT发病机制所涉及的分子机制，并解释在疾病相关危险因素高发的情况下PCT相对罕见的原因。公共卫生相关性：迟发性皮肤卟啉症（PCT）的特点是一种化合物在肝脏中积累，通过血液流动到皮肤。这种化合物被称为尿卟啉，它吸收太阳光并释放出伤害皮肤的能量粒子。这种疾病是由于肝脏中一种酶活性不足引起的。我们已经确定，缺乏酶活性是由一种抑制剂引起的，当过量的铁存在于肝脏中。许多PCT患者因酗酒、丙型肝炎或使用某些激素而出现肝损伤。我们建议确定环境和遗传因素如何结合以允许酶抑制剂发展。

DESCRIPTION (provided by applicant): Porphyria cutanea tarda (PCT), the most common form of porphyria in humans, is associated with excess liver iron stores, alcohol abuse, hepatitis C and medicinal estrogen use. PCT is due to a reduction in the specific activity of the heme biosynthetic enzyme uroporphyrinogen decarboxylase (URO-D) in the liver. We have determined that impaired catalytic activity of URO-D is caused by a competitive inhibitor designated uroporphomethene, an octacarboxylic tetrapyrrole macrocycle with a single oxidized bridge carbon between two of the pyrrole rings. Oxidation of the bridge carbon is mediated by P450 and iron dependent reactions. Our first specific aim is to determine if the porphomethene is generated by oxidation of hydroxymethyl bilane which then cyclizes non- enzymatically or by oxidation of uroporphyrinogen, the fully reduced substrate of URO-D. We will test the hypothesis that specific P450s and iron are required to generate the porphomethene by transfecting wild type and mutant yeast with functional human P450 systems and by altering cytosolic iron concentrations. Our second specific aim will utilize a structural approach to define the mechanism of porphomethene mediated inhibition and to confirm our model of the URO-D reaction. Our third specific aim will address the cause of iron loading in PCT. We will test the hypothesis that suppression of hepcidin expression is responsible for the excess liver iron stores. Collectively, these studies will define the molecular mechanisms involved in the pathogenesis of PCT and explain the relative rarity of PCT in face of the high incidence of disease associated risk factors. PUBLIC HEALTH RELEVANCE: Porphyria cutanea tarda (PCT) is characterized by accumulation in the liver of a compound that moves through the blood stream to the skin. The compound, called uroporphyrin, absorbs light from the sun and liberates energy particles that damage the skin. The disease is due to deficient activity of an enzyme in the liver. We have determined that deficient enzyme activity is caused by an inhibitor generated when excess iron is present in the liver. Many patients with PCT have liver damage from alcohol abuse, hepatitis C or from use of certain hormones. We propose to determine how environmental and genetic factors combine to allow the enzyme inhibitor to develop.