Purification and Characterization of Two Unique Forms of Cytochrome P-450 from Rabbit Nasal Microsomes

twoforms of cytochrome P-450, designated P-450NMa and P-450NMb, were purified to electrophoretic homogeneity from rabbit nasal microsomes. The purified cytochromes, which contained 14-16 nmol of P-450/mg of protein, exhibited apparent monomeric molecular weights of 49 500 and 51000, respectively. As indicated by several criteria, including the amino acid composition, absorption spectra, and peptide maps, the two nasal forms of P-450 are distinct from each other. Furthermore, as judged by the NH₂-terminal amino acid sequences, they are distinct from all other P-450 cytochromes described to date. In the ferric form, P-450NMa is in the low-spin state, whereas P-450NMb is predominantly in the high-spin state. When reconstituted with NADPH-cytochrome P-450 reductase and phospholipid, P-450NMa is very active in the oxidation of ethanol as well as several nasal procarcinogens, including the N-deethylation of N-nitrosodiethylamine, the O-deethylation of phenacetin, and the N-demethylation of hexamethylphosphoramide. P-450NMb also metabolizes these substrates, but at lower rates. Both nasal forms are also active with testosterone, with P-450NMa oxidizing the substrate in the 17-position to give androstenedione and P-450NMb catalyzing hydroxylation in the 15a-, 16a-, and 19-positions. The two cytochromes represent the major portion of the total P-450 in nasal microsomes, but the corresponding forms could not be detected in hepatic microsomes.