Spectral interactions of piperonyl butoxide and isocyanides with purified hepatic cytochrome P-450 from uninduced mice.

The binding of isocyanides and the metabolites of piperonyl butoxide (PBO) to reduced cytochrome P-450 in intact microsomes gives rise to the type III optical difference spectrum which is characterized by two pH dependent peaks in the Soret region. Each of the purified cytochrome P-450 isozymes (A1, B1, B2, B3) metabolized PBO and produced a spectrum in the Soret region. Only the A1 fraction produced the pH dependent type III spectrum. The B1 fraction produced a spectrum with only one peak at 430 nm while the spectrum produced by both the B2 and B3 fractions contained only the 455 nm peak. Each of the isozymes produce pH dependent type III spectra with both ethyl isocyanide and phenyl isocyanide dichloride.