Porphobilinogen oxygenase. Purification and evidence of its hemoprotein structure

Porphobilinogen oxygenase oxidizes porphobilinogen to 2-hydroxy-5-oxo-porphobilinogen. This enzyme isolated from wheat germ has been purified to homogeneity, as judged by polyacrylamide gel electrophoresis under both nondenaturing and denaturing conditions. The molecular weight of the enzyme formed from two identical (or very similar) polypeptide chains is 70,000. It has a pI of 9.0 indicating its cationic nature. The pure enzyme contains 1 mol of high-spin heme and 2 mol of non-heme iron. It requires both of these as well as molecular O2 and a reducing agent for catalytic activity. Although the enzyme has many characteristics of a peroxidase, hydrogen peroxide cannot substitute for oxygen and dithionite for catalysis. The catalytic reaction is not affected by catalase, superoxide dismutase, or by hydroxyl radical scavengers. A comparison between porphobilinogen oxygenase and a commercial preparation of horseradish peroxidase was made. The latter also catalyzes aerobic porphobilinogen oxidation, with dithionite as electron donor. Here the oxidation of porphobilinogen is inhibited by superoxide dismutase and was not affected by catalase.