The Carbon Monoxide-binding Pigment of Liver Microsomes. I. Evidence for Its Hemoprotein Nature.

The presence in mammalian liver microsomes of a carbon monoxide-binding pigment has been reported by Klingenberg (1) and by Garfinkel (2). The CO compound of the reduced pigment has an intense absorption band at 450 rnl.c and thus can be readily detected in dithionite-treated microsomes by difference spectrophotometry. The CO difference spectrum of reduced microsomes is, however, unusual in that it shows no peaks other than that at 450 rnp and, therefore, provides no clue to the nature of the pigment. The elucidation of its nature has further been hampered by the reported lability of the microsomal pigment to detergents, low pH, and enzymatic digestion (1,2). In addition, the CO compound has been reported as not photodissociable (1). In preliminary communications (3, 4), we have reported evidence for the hemoprotein nature of the microsomal CObinding pigment, provisionally called “P-450,” and shown that it can be converted into a solubilised form, which we term “P420,” by treatment of microsomes anaerobically with snake venom or deosycholate. The solubiliaation is accompanied by an unusual change in the spectral properties of the pigment. Further, the solubilized pigment has been partly purified, free from cytochrome 55, and shown to possess absorption spectra characteristic of hemoproteins (5). The present paper gives a detailed account of the investigations on rabbit liver microsomes and crude microsomal digests, which have led us to postulate the hemoprotein nature of the pigment. Purification and properties of the solubilized hemoprotein will be reported in the accompanying paper (6).