Absorption spectral studies on heme ligand interactions of P-450nor.

Heme-external ligand interactions of P-450nor were examined spectrophotometrically and compared with those of other P-450s. Most nitrogenous ligands induced type II spectral changes on binding to ferric P-450nor, as did other P-450s. In contrast with other P-450s, 2-methylpyridine and 1-butanol induced type I changes in the spectrum of P-450nor. No spectral interaction of ferrous P-450nor with these ligands was observed. The absorption spectra of the alkyl isocyanide complexes of ferrous P-450nor exhibited the Soret peak at 427 nm with a slight shoulder at around 455 nm at neutral pH, and this shoulder was intensified as the pH was increased, suggesting that the isocyanide complexes of P-450nor existed in two states (the 430 and 455 nm states) which were in pH-dependent equilibrium in a similar manner to microsomal P-450s. However, the equilibrium was shifted mostly to the 430 nm state in the complexes of P-450nor. The findings suggest that P-450nor, especially its ferrous form, has some distinct features from P-450cam and microsomal P-450s in the distal heme environment.