The spectral characterization of P450 dates back to the publication of Omura and Sato (1964). A protocol has been published by Guengerich et al.(2009).

Spectra of purified CYP6A1 from the house fly (from Andersen et al.1994). Absolute spectra (top)(this works best with clean, purified P450s) and the usual CO-difference spectrum (this works also with microsomal preparations from insect tissues).

P450 spectrum of CYP6A2 expressed in insect Sf21 cells with the baculovirus vector

P450 spectrum from Locusta migratoria Malphighian tubule microsomes, obtained in 1976 on a Cary 118c spectrophotometer. Note the P420 shoulder.