Abstract:

Several properties of porphyrinogen carboxy-lyase from normal chicken erythrocytes were studied. 1. 1. The utilization of the substrate uroporphyrinogen (8-COOH), and the formation of intermediate products (porphyrinogens of 7-, 6- and 5-COOH) and the final product coproporphyrinogen (4-COOH) were investigated as function of time and substrate concentration. The results confirm a two-stage hypothesis involving firstly, the elimination of the first carboxyl group from uroporphyrinogen and secondly the elimination of the further three carboxyl groups to form coproporphyrinogen. The elimination of the first carboxyl group is not the rate-limiting step in this multiple decarboxylation because large amounts of 7 COOH porphyrinogen were accumulated. The effect of temperature on the stepwise decarboxylation process also suggests and easier elimination of the first carboxyl group. 2. 2. Cysteine and glutathione inhibited the decarboxylation process at low concentrations, but at higher concentrations cysteine continues to inhibit while glutathione allows the recovery of enzyme activity. 3. 3. Studies of the effect of uroporphyrinogen concentration on the first and second stages; and of 7-COOH porphyrinogen concentration on the second stage revealed both substrates as inhibitors of their own decarboxylations. Furthermore, when 7-COOH porphyrinogen was incubated in the presence of uroporphyrinogen, it further inhibited the first decarboxylation of uroporphyrinogen, 7-COOH porphyrinogen was a stronger inhibitor than 8 COOH porphyrinogen. © 1973.