Alkylisocyanide adducts of microsomal P450 exist in two interconvertible forms, each giving the Soret maximum around 430 or 455 nm. This is demonstrated with a rabbit liver P450 2B4. Resonance Raman spectra of the 430-and 455-nm forms were examined for typical P450s of the two types as well as for P450 2B4 because the 430-nm form of P450 2B4 is liable to change into P420. P450cam and P450nor were selected as a model of the 430-and 455-nm forms, respectively. For the n-butyl isocyanide (CNBu)

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... uct, the Fe(II)-CNBu stretching band was observed for the first time at 480/467 cm ؊1 for P450cam and at 471/459 cm ؊1 for P450nor with their 12 CNBu/ 13 CNBu derivatives. For P450cam, but not P450nor, other 13 C isotope-sensitive bands were observed at 412/402, 844/835, and 940/926 cm ؊1 . The C-N stretching mode was identified by Fourier transform IR spectroscopy at 2116/2080 cm ؊1 for P450cam and at 2148/ 2108 cm ؊1 for P450nor for the 12 C/ 13 C derivatives. These findings suggest that the binding geometry of isocyanide differs between the two forms-bent and linear structures for P450cam-CNBu and P450nor-CNBu, respectively. In contrast, in the ferric state, the Raman 13 C isotopic frequency shifts, and the IR C-N stretching frequencies (2213/2170 and 2215/2172 cm ؊1 ) were similar between P450cam and P450nor, suggesting similar bent structures for both.