Specificity of the cytochrome P-450 interaction with cytochrome b₅

The specificity of the interaction of cytochrome b₅ with different forms of cytochrome P-450 was examined. Immunopurification of cytochromes P-450 1A1, 2B1 and 2E1 from rat liver microsomes resulted in co-purification of cytochrome b₅ with cytochrome P-450 forms 2B1 and 2EI but not 1A1. This specificity was evaluated in conjunction with multiple sequence alignment of the three cytochrome P-450s and a molecular model of the cytochrome P-450-cytochrome b₅ complex [(1989) Biochemistry 28, 8201-8205]. These analyses suggest two basic residues in the arginine cluster region of P-450, which are present in P-450s 2B1 and 2E1 but are absent in P-450 1A1, as potential binding sites for cytochrome b₅.