Spectral Similarities between "H-450 " and Cytochrome P-450

The absorption spectrum of reduced H-450 was reversibly affected by a pH cnange; the Soret peak of the alkaline form was at 448 nm, and that of the acidic form (H-420) at 425 nm. The same spectral change of conversion of reduced H-450 to H-420 was also produced by the action of w-butanol, urea and p-chloromercuriben-zoate. These spectral properties of H-450 were similar to those of the conversion of cytochrome P-450 to cytochrome P-420, suggesting the similar heme environ-ments of these two hemoproteins. Reduced H-450 bound carbon monoxide to be transformed into a new spectral species having a Soret peak at 420 nm. Several years ago, Kim and Deal (/) found a new hemoprotein, "H-450, " in the soluble fraction ob-tained from a homogenate of pig livers. The prosthetic group of H-450 was protoheme, but its absorption spectra were atypical of a protoheme-containing hemoprotein in that the Soret peak of its reduced form was at around 450 nm. The 450 nm peak was similar to the Soret peak of the carbon monoxide compound of cytochrome P-450 (2). However, addition of carbon monoxide to oxidized or reduced H-450 did not produce any change in the absorption spectra, and they con-cluded that H-450 was not cytochrome P-450 (7). While studying the spectral properties of solu-ble hemoproteins in rat liver cytosol we noticed a significant spectral change of H-450 when the pH of the medium was altered. We further studied the effect of pH on the spectral properties of H-450 using purified preparations from rat livers. Reduced H-450 showed a prominent Soret peak at 448 nm as described by Kim and Deal (7) in 1 To whom reprint requests should be addressed. alkaline media, but the Soret peak was at 425 nm in acidic media. The latter form of H-450 will be called "H-420 " in this paper. We also found that n-butanol, urea and />-chloromercuribenzoate converted H-450 to H-420. Moreover, carbon monoxide was also effective in converting H-450 to a new spectral species having a Soret peak at 420 nm. These observations on the spectral con