Uroporphyrinogen decarboxylase (EC 4.1.1.37) has been purified 4419-fold to a specific activity of 58.3 nmol of coproporphyrinogen III formed/min per mg of protein (with pentacarboxyporphyrinogen III as substrate) from human erythrocytes by adsorption to DEAE-cellulose, (NH4)2SO4 fractionation, gel filtration, phenyl-Sepharose chromatography and polyacrylamide-gel electrophoresis. Progressive loss of activity towards uroporphyrinogens I and III occurred during purification. Experiments employing immunoprecipitation, immunoelectrophoresis and titration with solid-phase antibody indicated that all the uroporphyrinogen decarboxylase activity of human erythrocytes resides in one protein, and that the substrate specificity of this protein had changed during purification. The purified enzyme had a minimum mol.wt. of 39 500 on sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. Gel filtration gave a mol.wt. of 58 000 for the native enzyme. Isoelectric focusing showed a single band with a pI of 4.60. Reaction with N-ethylmaleimide abolished both catalytic activity and immunoreactivity. Incubation with substrates or porphyrins prevented inactivation by N-ethylmaleimide. An antiserum raised against purified erythrocyte enzyme precipitated more than 90% of the uroporphyrinogen decarboxylase activity from human liver. Quantitative immunoprecipitation and crossed immunoelectrophoresis showed that the erythrocyte and liver enzymes are very similar but not identical. The differences observed may reflect secondary modification of enzyme structure by proteolysis or oxidation of thiol groups, rather than a difference in primary structure.

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1. Erythrocyte uroporphyrinogen decarboxylase activity has been measured in 27 patients with porphyria cutanea tarda, of whom 11 had a family history of overt porphyria cutanea tarda. 2. Eight patients from six families had erythrocyte uroporphyrinogen decarboxylase activities that were decreased to about half of control values. This decrease was shown by family studies to be inherited as an autosomal dominant characteristic. Two of these patients had no family history of overt porphyria cutanea tarda. 3. Nineteen patients had uroporphyrinogen decarboxylase activities close to or within the range found in 18 control subjects. Of these, five patients had a family history of porphyria cutanea tarda. 4. Inheritance of an autosomal dominant gene which decreases uroporphyrinogen decarboxylase activity in erythrocytes and liver is an uncommon cause of porphyria cutanea tarda and may not explain all cases of familial porphyria cutanea tarda. The hepatic enzyme defect in the common type of porphyria cutanea tarda, in which erythrocyte uroporphyrinogen decarboxylase activity is normal, may be caused either by inheritance of a gene whose effect is restricted to the liver or by gene whose effect is restricted to the liver or by chemicals that selectively inhibit the hepatic enzyme.

Enzyme assays for phenylalanine mono-oxygenase have been performed on a variety of non-hepatic human cell types under varying conditions. The cell types studied were cultured fibroblasts, short-term lymphocyte cultures, long-term lymphoblastoid cultures, cultured amniotic fluid cells, hair roots and placental extracts. The cultured cells were assayed after growth under standard conditions and in the presence of a high concentration of substrate, a low concentration of end-product, added hydrocortisone or dexamethasone and under combinations of these conditions. In no instance was a significant enzyme activity obtained.