1. The urinary excretion of isomers I and III of coproporphyrin by fifty-nine patients with Dubin—Johnson syndrome has been examined, and compared with the results obtained for normal control subjects and patients with various types of jaundice.

2. The control subjects (with one exception) excreted less than 45% of the coproporphyrin as isomer I. Fifty-six patients with the Dubin—Johnson syndrome excreted more than 65% isomer I (mode, 87%). In three cases the relative content of isomer I was normal. These exceptional patients differed also in some other characteristics from typical cases of the Dubin—Johnson syndrome.

3. Patients with obstructive jaundice or infectious hepatitis showed an intermediate pattern of the isomer distribution. In Gilbert's disease the isomer pattern was normal (six cases). In Rotor syndrome the relative content of isomer I was increased (three cases).

4. The abnormal urinary excretion of coproporphyrins in the various types of jaundice is probably caused by different mechanisms. In obstructive jaundice and infectious hepatitis the absolute excretion of isomer I is raised and isomer III is normal or elevated. This pattern may be explained by a shift from the biliary to the urinary route of excretion affecting mainly isomer I. On the other hand, in Dubin—Johnson syndrome an increased excretion of isomer I was accompanied by a significant decrease of isomer III excretion in the urine. It is suggested that this abnormal pattern might be a result of a deficiency or inhibition of uroporphyrinogen isomerase (uroporphyrinogen III co-synthetase) in the liver of patients with the Dubin—Johnson syndrome.

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