Some kinetic properties of human red cell uroporphyrinogen decarboxylase

Several kinetic properties of uroporphyrinogen decarboxylase (uroporphyrinogen-III carboxy-lyase, EC 4.1.1.37) from human hemoglobin-free hemolysates were studied, using substrates of both isomeric series I and III (uroporphyrinogen, hepta and pentacarboxyl porphyrinogens). Enzyme affinity for series II isomers was always found to be higher than for corresponding series I isomers. Mixed substrate experiments using porphyrinogen (both labelled with 14C and unlabelled) showed: (a) a reciprocal inhibition of decarboxylation of series III porphyrinogens by series I porphyrinogens with the same number of carboxylic groups; (b) no inhibition of hepta- and pentacarboxylic series III porphyrinogens decarboxylation by uroporphyrinogen III. It is demonstrated that porphyrinogens of both isomeric series with the same number of carboxylic groups are decarboxylated at the same active center; in contrast, the sequential decarboxylation of uroporphyrinogen III to coproporphyrinogen III occurs at four different active centers. Relationship between the kinetic properties of uroporphyrinogen decarboxylase and biological data of porphyria cutanea are discussed.