Purification and chemical modifications of porphobilinogen oxygenase

The regulation of porphobilinogen oxygenase and porphobilinogen deaminase activities in rat bone marrow under conditions of erythropoietic stress

Porphorbilinogen oxygenase (EC 4.2.1.24) was associated with the microsomal fraction of bone marrow in normal rats and in rats submitted to erythropoietic stress, while porphobilinogen deaminase (EC 4.3.1.8) of the same origin was present in the cytosol. An NADPH-dependent electron-donor system for the oxygenase was also present in the microsomes of the bone marrow. Under conditions of erythropoietic stress caused by hypoxia, the activities of both enzymes were found to be inversely correlated. While the oxygenase showed minimum activity between the 4th and 8th day of hypoxia, porphobilinogen deaminase reached its maximum activity during this period. After the 8th day of hypoxia, oxygenase activity increased while deaminase activity decreased. The NADPH-dependent electron-transport system necessary for the microsomal oxygenase activity was largely inactivated after the 10th day of hypoxia, while oxygenase activity was not affected. The particulate porphobilinogen oxygenase could be solubilized from the bone marrow microsomes with 1% deoxycholate or 0.5 M KCl. In addition, the oxygenase was also released by freezing and thawing the microsomes isolated from bone marrow of rats which had been submitted to an erythropoietic stress (hypoxia or phenylhydrazine). The enzyme solubilized with deoxycholate or KCl showed a high molecular weight form and a low molecular weight form (Mr 25 000). The former could be transformed into the latter either by treatment with 2 M KCl or by succinylation. When the oxygenase was solubilized by freezing and thawing a third molecular weight form (Mr 50 000) also appeared. The solubilized enzyme could be succinylated without loss of its catalytic activity, while the membrane-bound enzyme could not be succinylated.

Induction of porphobilinogen oxygenase and porphobilinogen deaminase in rat blood under conditions of erythropoietic stress

Porphobilinogen is the substrate of two enzymes: porphobilinogen deaminase and porphobilinogen-oxygenase. The first one transforms it into the metabolic precursors of heme and the second diverts it from this metabolic pathway by oxidizing porphobilinogen to 5-oxopyrrolinones. Rat blood is devoid of porphobilinogen-oxygenase under normal conditions while it carries porphobilinogen-deaminase activity. When the rats were submitted to hypoxia (pO2 = 0.42 atm) for 18 days, the activity of porphobilinogen-oxygenase appeared at the tenth day of hypoxia and reached the maximum at the 14-16th day. It decreased to a half after 2 days (half-life of the enzyme) and disappeared after 4 days of return to normal oxygen pressure. Porphobilinogen-deaminase activity increased after the first day of hypoxia, reached a maximum at the 14-16th day and did not decrease to normal values until the 15th day after return to normal oxygen pressure. The activities of both porphobilinogen-oxygenase and porphobilinogen-deaminase were induced by administration of erythropoietin. When rats were made anaemic with phenylhydrazine, porphobilinogen-oxygenase activity also appeared in the blood cells. Although the reticulocyte concentration was higher when compared to that obtained under hypoxia, the activities of the oxygenase obtained under both conditions were comparable. Porphobilinogen-deaminase activity was always closely related to the reticulocyte content. The appearance of porphobilinogenase-oxygenase under the described erythropoietic conditions was due to a de novo induction of the enzyme, as shown by its inhibition with actinomycin D and cycloheximide. Porphobilinogen-oxygenase as well as porphobilinogen-deaminase were present in the rat bone marrow under normal conditions. Their activities increased in phenylhydrazine treated rats. The properties and kinetics of porphobilinogen-oxygenase from the rat blood and bone marrow were determined and found it differ in several aspects.

Porphobilinogen oxygenase from human erythrocytes

Porphobilinogen oxygenase was isolated from red cells of healthy persons and of patients with disturbances in porphyrin metabolism. The hemolysates were purified by DEAE-cellulose chromatography and the oxygenase was eluted with either 3 mmol/l phosphate buffer (pH 7.4) or with 10 mmol/l Tris-HCl buffer (pH 7.6). The oxygenase can also be isolated by filtration of the hemolysate through Sephadex G-100. In healthy persons a mean value of 84.1 +/- 29.7 nmol of porphobilinogen consumed in 30 min per ml of blood was obtained. In patients with hepatic porphyrias (acute intermittent porphyria and porphyria cutanea tarda) the activity of porphobilinogen oxygenase was very low. In the case with acute intermittent porphyria the activity was increased when measured after storage at 4 degrees C but never reached normal values. In the cases of porphyria cutanea tarda, oxygenase activity increased after recovery and reached normal values. In patients with erythropoietic porphyria and in anemias the activity of porphobilinogen oxygenase gave high values. The erythrocyte enzyme was found to be heterogeneous when compared with the enzyme of other sources. It was only partially succinylated and was inactivated after storage for a few days at 4 degrees C. Some preparations showed the usual allosteric kinetics (n = 1.6--2.0), although Michaelian kinetics were also often observed. The enzyme was inhibited by alpha, alpha'-dipyridyl and EDTA as well as by several metals.