Purification of uroporphyrinogen decarboxylase from human erythrocytes. Immunochemical evidence for a single protein with decarboxylase activity in human erythrocytes and liver

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Abstract
Urophorphyrinogen decarboxylase (EC 4.1.1.37) was purified 4419-fold to a specific activity of 58.3 nmol of coproporphyrinogen II 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 urophorphyrinogens 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 1 protein, and that the substrate specificity of this protein had changed during purification. The purified enzyme had a minimum MW of 39,500 on sodium dodecyl sulfate/polyacrylamide-gel electrophoresis. Gel filtration gave a MW of 58,000 for the native enzyme. Isoelectric focusing showed a single band with a pI [isoelectric point] of 4.60. Reaction with N-ethylamaleimide abolished both catalytic activity and immunoreactivity. Incubation with substrates of porphyrins prevented inactivation by N-ethylmalemide. 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. Quantitative immunoprecipitation and crossed immunoelectrophoresis showed that the erythrocyte and liver enzymes was 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.