Erythrocyte porphyrinogen carboxy-lyase activity in porphyria cutanea tarda and certain other human porphyrias

Decreased leukocyte ferrochelatase activity in erythropoietic protoporphyria

We studied the porphyrin metabolism of a 7-year-old Japanese boy with erythropoietic protoporphyria (EPP) and his family members. Leukocyte ferrochelatase activity was markedly decreased in this patient, being approximately 12% of the mean value of normal controls (4 aged-matched healthy boys). In contrast, leukocyte delta-aminolevulinic acid (ALA) synthase activity was normal. The free protoporphyrin content of erythrocytes was greatly increased (4.3 mg/100 ml RBC), while erythrocyte ALA dehydratase and porphobilinogen (PBG) deaminase activities were 1.7- and 2.2-fold of respective control values. A survey of his family revealed that 12 of 19 members probably had manifest EPP or were EPP carriers. These results suggest that, in EPP, there might be an inherited impairement of ferrochelatase activity which gives rise to an elevation of erythroblast ALA dehydratase and PBG deaminase activities to compensate for a resultant decrease in heme production.

A model for testing compounds influencing porphyrin synthesis

The yeast Saccharomyces cerevisiae cultivated semi-anaerobically in a synthetic medium was used as a model to establish (a) total porphyrin synthesis, (b) ratio of intracellular to extracellular porphyrin concentrations. The antimalarials used for the therapy of porphyria cutanea tarda, chloroquine and pyrimethamine, reduced the total synthesis of porphyrins, pyrimethamine being more effective than chloroquine, like in porphyric patients. Both drugs exerted an antagonistic influence on the release of porphyrins from cells. Chloroquine reduced the concentration ratio of porphyrins while pyrimethamine increased it, apparently through inhibition of permeation of porphyrins. Combined treatment with the two compounds may hold promise for the therapy of porphyria cutanea tarda.

Immunoreactive uroporphyrinogen decarboxylase in the liver in porphyria cutanea tarda

Immunoreactive and catalytic uroporphyrinogen decarboxylase were measured in liver from 15 patients with sporadic porphyria cutanea tarda (PCT) and 4 patients with familial PCT at different stages of the disorder. In sporadic PCT, catalytic activity was lowest and immunoreactive enzyme concentration was highest when active skin lesions were present; this pattern was also seen in the one familial PCT patient who had skin lesions. During remission, the ratio of catalytic activity to immunoreactive enzyme concentration returned towards normal. Immunoreactive enzyme was increased by comparison with controls in sporadic patients with skin lesions; in familial PCT mean concentration was 59% of the overall sporadic value. In 4 sporadic patients in prolonged (4-8 years) remission (following venesection) enzyme activity and immunoreactive enzyme concentrations were normal. It is suggested that clinically overt PCT is precipitated by an iron-dependent process which inactivates the active centres of uroporphyrinogen decarboxylase molecules in the liver. Treatment by venesection eventually leads to complete reversal of this biochemical defect in at least some patients with sporadic PCT. The findings are consistent with the view that sporadic PCT is an acquired disorder.

Immunoreactive uroporphyrinogen decarboxylase in porphyria cutanea tarda

Immunoreactive uroporphyrinogen decarboxylase was measured by rocket immuno-electrophoresis in haemolysates from 7 unrelated patients with familial porphyria cutanea tarda (PCT), 6 patients with sporadic PCT, and 7 normal subjects. In all patients with familial PCT immunoreactive enzyme protein was decreased (51% of normal), to the same extent as catalytic activity (56% of normal), whereas in sporadic PCT both measurements were normal. These results show that familial PCT is commonly caused by a mutation which does not lead to the production of non-catalytic cross-reactive immunological material. Familial PCT can be distinguished from other types of PCT by a simple immunoelectrophoretic method that does not involve measurement of uroporphyrinogen decarboxylase activity and which is therefore likely to be suitable for routine diagnostic use.