PHOTOCHEMISTRY OF NITRIC-OXIDE ADDUCTS OF WATER-SOLUBLE IRON(III) PORPHYRIN AND FERRIHEMOPROTEINS STUDIED BY NANOSECOND LASER PHOTOLYSIS

Water-soluble iron(III) porphyrin and ferrihemoproteins (methemoglobin , metmyoglobin, oxidized cytochrome c, and catalase) associate with NO to yield the nitric oxide adducts. The equilibrium constants for asso ciation of ferrihemoproteins and NO are 1 order of magnitude larger th an that of the water-soluble iron(III) porphyrin which is free from pr otein, suggesting that the proteins offset the forward and backward re action rates in the equilibrium reactions. Nanosecond laser photolysis studies of the nitric oxide adducts of metmyoglobin, oxidized cytochr ome c, and catalase, (NO)Mb(III), (NO)Cyt(III), and (NO)Cat(III), have been carried out. The transient detected after laser flash photolysis of (NO)Cat(III) is identified as Cat(III). However, the transients ob served for (NO)Mb(III) and (NO)Cyt(III) at 50 ns after laser pulsing a re ascribed to Mb(III)tr and Cyt(III)tr, respectively, with the absorp tion spectra different from those of uncomplexed Mb(III) and Cyt(III). In particular, the absorption spectrum of Cyt(III)tr markedly differs from that of the uncomplexed Cyt(III). The species Mb(III)tr and Cyt( III)tr are found to change to Mb(III) and Cyt(III), respectively, with in a few microseconds. The quantum yields for the photodissociation of NO from nitric oxide adducts of ferrihemoproteins are 1 order of magn itude less than that from the NO adduct of the water-soluble iron(III) porphyrin, probably due to fast geminate recombination reaction of NO and ferrihemoprotein in a heme pocket. The photochemistry of the nitr ic oxide adducts of hemoproteins and water-soluble iron(II) porphyrin is also described on the basis of laser phosolysis studies.