Urinary total, isomer I, and isomer III coproporphyrin excretion was determined in 16 patients with the Dubin-Johnson syndrome, 73 phenotypically normal family members, and 20 unrelated normal controls of similar age. Normal subjects excreted 24.8 ± 1.3 per cent (mean ± S.E.M.) of urinary coproporphyrin as coproporphyrin I. Patients with the syndrome excreted 88.9 ± 1.3 per cent (mean ± S.E.M.) as coproporphyrin I (p less than 0.001). Twenty-five phenotypically normal parents and children of patients with the syndrome excreted 31.6 ± 1.2 per cent (mean ± S.E.M.) as coproporphyrin I, a value intermediate between results in patients and normal subjects (p less than 0.001). In 48 phenotypically normal more distant relatives, the percentage excretion of coproporphyrin I was distributed over a wide range. These results indicate that, with respect to urinary coproporphyrin excretion, the Dubin-Johnson syndrome is inherited as an autosomal recessive characteristic. THE Dubin-Johnson syndrome is characterized by a black liver and chronic nonhemolytic, predominantly conjugated, hyperbilirubinemia.1 2 3 4 Its familial nature has been described3 4 5 6 7 8 9 10 11 12 13; however, previous attempts to detect carriers of the syndrome have been unsuccessful. Normal serum bilirubin concentration, sulfobromophthalein retention (Mandema test),13 14 15 sulfobromophthalein relative storage capacity and transport maximum,31617 and absence of hepatic pigmentation have been observed in phenotypically normal but genetically identified obligate carriers.910 In the absence of a means of identifying carriers of the syndrome, its mode of inheritance remains uncertain. Koskelo et al.18 discovered, and Ben-Ezzer and her co-workers19 confirmed, that urinary coproporphyrin I excretion is increased.
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