Lamoril J, Martasek P, Deybach J C, Da Silva V, Grandchamp B, Nordmann Y
Centre Français des Porphyries, INSERM U.409, Hôpital Louis Mourier, Colombes, France.
Hum Mol Genet. 1995 Feb;4(2):275-8. doi: 10.1093/hmg/4.2.275.
Hereditary coproporphyria (HC) is an acute hepatic porphyria with autosomal dominant inheritance caused by a deficient activity of coproporphyrinogen IX oxidase (CPX). We previously described harderoporphyria, a homozygous variant form of coproporphyria in three siblings, characterized by a massive excretion of harderoporphyrin and a marked decrease of coproporphyrinogen IX oxidase activity. In this kindred, the transmission of the disease was autosomal recessive. In the present study, sequencing of cDNA and genomic DNA from these patients revealed a point mutation resulting in a lysine to glutamic acid substitution (K304E) in exon 6 of the gene and the absence of the normal allele, suggesting a homozygous state for the mutation. Expression studies of normal and mutated cDNAs in E. coli demonstrated that this amino acid substitution was responsible for the important decrease in the enzyme activity and for the accumulation of harderoporphyrin. The Michaelis constant of the mutated enzyme was 10-fold higher than normal suggesting that the lysine at position 304 is important for binding the substrate: a slightly increased sensitivity to thermal denaturation was also observed.
遗传性粪卟啉病（HC）是一种急性肝卟啉病，呈常染色体显性遗传，由粪卟啉原IX氧化酶（CPX）活性不足引起。我们之前描述过硬卟啉病，它是三个兄弟姐妹中出现的一种纯合子形式的卟啉病，其特征是大量排泄硬卟啉以及粪卟啉原IX氧化酶活性显著降低。在这个家族中，疾病的遗传方式为常染色体隐性遗传。在本研究中，对这些患者的cDNA和基因组DNA进行测序发现，一个点突变导致该基因第6外显子中的赖氨酸被谷氨酸取代（K304E），且不存在正常等位基因，提示该突变为纯合状态。在大肠杆菌中对正常和突变cDNA进行的表达研究表明，这种氨基酸取代导致了酶活性的显著降低以及硬卟啉的积累。突变酶的米氏常数比正常酶高十倍，这表明304位的赖氨酸对于结合底物很重要：还观察到对热变性的敏感性略有增加。
