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.