Coproporphyrinogen oxidase. I. Purification, properties, and activation by phospholipids

Coproporphyrinogen oxidase (EC 1.3.3.3), which converts coproporphyrinogen III into protoporphyrinogen IX, was purified about 3,200-fold from bovine liver. The most purified preparation had a specific activity of 6,920 units/mg of protein, the highest value so far reported, and was homogeneous in the polyacrylamide gel electrophoresis. The enzyme was monomeric and its molecular weight was approximately 71,600. The purified enzyme was analyzed for the amino acid composition and shown to have an abundance of aromatic amino acid residues amounting to 12% of the total residues. Spectral examination did not reveal the presence of heme and flavin. No metals were detected by atomic absorption spectroscopy either. Sulfhydryl reagents, metals, and metal chelators did not affect the enzyme activity to any significant extent. On the contrary, the purified enzyme was activated by crude phospholipid extracts from liver mitochondria and commercially available phospholipids about 2- to 5-fold. An increase in Vmax by the phospholipid extract as well as phosphatidylethanolamine accompanied a decrease in Km for coproporphyrinogen III from 48 microM to 18 to 25 microM. Synthetic nonionic detergents also exhibited an activation effect, although ionic detergents diminished the activity.