Interaction of Fe-protoporphyrin IX and heme analogues with purified recombinant heme oxygenase-2, the constitutive isozyme of the brain and testes.

Heme oxygenase-2 (HO-2) is the predominant form of heme oxygenase in the brain and testes. The enzyme is not readily amenable to isolation from mammalian tissues and has not been characterized for its kinetic properties and interaction with metalloporphyrins. Presently a rat HO-2 cDNA (Rotenberg, M.O., and Maines, M. D. (1990) J. Biol. Chem. 265, 7501-7506) was used to generate a construct with a neutral hydrophobicity profile at its COOH terminus for expression of nearly full-length HO-2 protein in Escherichia coli. The procedures used for HO-1 were of no utility in purification of HO-2. A multistep protocol developed for isolation of HO-2 resulted in a homogeneous protein with a specific activity up to 6,500 nmol of bilirubin/mg/h. Based on SDS-polyacrylamide gel electrophoresis and Western blot analyses, the protein had an apparent molecular mass of approximately 34 kDa. HO-2 binds Fe-protoporphyrin (heme) at near molar unity to give a complex with the absorption maximum at 403 nm. The Soret band has a blue shift to 430 nm when heme iron is reduced, with distinct alpha and beta bands at 485 and 550 nm, respectively. The Soret band of the CO complex of ferrous heme.HO-2 is at 420 nm, and alpha and beta bands are at 540 and 572 nm, respectively. The apparent Km for Fe-protoporphyrin is 0.33 microM, with a Vmax of 0.45 nmol of bilirubin/mg/h. Zn-protoporphyrin is a strong mixed inhibitor of enzyme activity, whereas Co-protoporphyrin is a poor competitive inhibitor of activity. When HO-2 was preincubated (10 min at 4 degrees C) with Fe-protoporphyrin, the cobalt complex did not inhibit enzyme activity, whereas the Zn-protoporphyrin effectively inhibited activity. Calorimetric measurements suggest that HO-2/heme interaction involves one type of association producing a single heat absorption peak upon melting of the complex and that the unfolding is not reversible. The association increases the enthalpy of HO-2 (130 kcal/mol versus 184 kcal/mol) and increases the stability to heat denaturation by 9 degrees C. Heat duration of zinc complex involves at least two stages of unfolding.