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Spectrophotometric examination of exogenous ligand complexes of iron iii cytochrome p 450 characterization of the axial ligand trans to thiolate in the native iron iii low spin form



Spectrophotometric examination of exogenous ligand complexes of iron iii cytochrome p 450 characterization of the axial ligand trans to thiolate in the native iron iii low spin form



Journal of Biochemistry (Tokyo) 91(5): 1651-1660



The .alpha. band of the native ferric low-spin form of cytochrome P-450 is more intense than the .beta. band and this is characteristic of the cytochrome. When the native sixth ligand of P-4501, a low-spin cytochrome P-450 of rabbit liver microsomes, was replaced with an exogenous ligand, the .alpha. band diminished and the cytochrome lost the above-mentioned spectral characteristics. This indicated that the intense .alpha. band of the cytochrome was related to the nature of its sixth ligand. P-4481, a rabbit liver microsomal cytochrome P-450 which is essentially in a high-spin state, was converted to low-spin complexes upon binding with various exogenous ligands. The intensity of the .alpha. band of low-spin P-4481 derivatives increased in the order of 1-methylimidazole and pyridine complexes, 2-methylimidazole and 2-methylpyridine complexes, and lower n-alkanol complexes. This order coincided with the decreasing order of field strength of the exogenous ligands, indicating that the intensity of the .alpha. band is inversely proportional to the field strength of the sixth ligand. The absorption spectrum of the 1-propanol complex of P-4481 was essentially identical with that of native low-spin cytochrome P-450, whereas spectra of other P-4481 derivatives were superimposable on those of the corresponding P-4501 derivatives. P-4501 and metmyoglobin showed an orderly red-shift of the Soret band upon binding with pyridine, 1-methylimidazole and cyanide. The order and the degree of the red-shift of the Soret band of P-4501 were essentially the same as those of metmyoglobin, indicating that the native sixth ligand of P-4501 may resemble that of metmyoglobin, a water molecule. The native sixth ligand of ferric low-spin cytochrome P-450 is probably an oxygenous one such as water molecule or an oxy-amino acid side chain from the apoprotein.

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