US EPA

12704 

Journal Article 

The carbon monoxide-binding pigment of liver microsomes: I evidence for its hemoprotein nature 

Omura, T; Sato, R 

1964 

Yes 

Journal of Biological Chemistry
ISSN: 0021-9258
EISSN: 1083-351X 

239 

2370-2378 

English 

14209971 

CO-binding pigment, in rabbit liver microsomes is a new hemoprotein with unusual properties. Microsomal bound P-450, when reduced, binds CO and ethyl isocyanide. Although the CO difference spectrum of the reduced pigment is unusual for a hemoprotein, the isocyanide difference spectrum is characteristic of a hemoprotein compound. In microsomes P-450 is reducible by both NADH and NADPH under anaerobic conditions, but the reduced form is very rapidly reoxidizable in the presence of molecular oxygen. When microsomes are incubated anaerobically with heated snake venom or with deoxycholate, the pigment is converted to a soluhilized form called "P-420." The solubilization is, however, accompanied by profound changes in the spectral properties of the pigment. The CO compound of reduced P-420 shows a difference spectrum characterized by an intense Soret peak at 420 m'mu'. P-420 is labile to aeration, especially in the presence of dithionite. In microsomal digests it can be reduced by NADH and NADPH in the absence of oxygen, but most of the pigment is maintained in the oxidized form under aerobic conditions even in the presence of NADH or NADPH. Cytochrome bs in the digests, on the other hand, is fully reduced under comparable conditions. Taking advantage of such difference in oxidationreduction properties of P-420 and cytochrome bg, it is possible to measure a reduced minus oxidized difference spectrum of P-420 alone. The spectrum thus obtained suggests that P-420 is a cytochrome of the b type. Liver microsomes contain protoheme in excess of the amount associated with cytochrome b5. Most of the extra heme seems to be associated with the CO-binding pigment.