Specificity of heme oxygenase: a study with synthetic hemins

A large number of synthetic iron porphyrins were enzymatically oxidized by a microsomal heme oxygenase preparation from rat liver. They all had in common two vicinal propionic acid residues at C6 and C7. Iron porphyrins of type I were not substrates of the enzyme. Iron porphyrins that carried electron-withdrawing substituents (acyl residues) at C2 and C4 were substrates of heme oxygenase, although the product yields were reduced. Several iron porphyrins, such as hemin XIII (4) and hem III (5), were better substrates of heme oxygenase than the natural substrate hemin IX (1). The enzymatic oxidation was selective for the alpha-methine bridge, and the alpha-biliverdins obtained were reduced by Biliverdin reductase to the corresponding alpha-bilirubins. Preincubation of the enzymatic system with hemin IX (1) and hemin XIII (4) in the absence of NADPH resulted in an inhibition of their oxidation. The iron-free porphyrins which carried two vicinal propionic acid residues at C6 and C7 were also found to be inhibitors of the enzymatic system when preincubated with the latter. The presence of hematochemin IX (18) suppressed the enzymatic oxidation of hemin IX (1).