Amino Acid Sequence of Rabbit Liver Microsomal Cytochrome b5

Cytochrome b5 from the rabbit liver microsomes was solubilized with pancreatic lipase. The apoprotein of the predominant form of cytochrome b5 was hydrolyzed with trypsin and chymotrypsin and the resulting peptides were isolated. The complete amino acid sequences of these peptides were determined by the Edman degradation procedure. For the first 90 residues, this amino acid sequence is essentially the same as that reported by Tsugita et al. (Tsugita, A., Kobayashi, M., Kajihara, T., and Hagihara, B., J. Biochem (Tokyo)), 64, 727, 1968) for a trypsin-solubilized preparation of crystalline cytochrome b5 from rabbit liver microsomes. Heterogeneity at residues 10 and 95 was observed. Comparison of this rabbit protein sequence with that of calf cytochrome, the only other cytochrome b5 with a known primary structure, shows that the rabbit cytochrome is 4 residues longer at the NH2 terminus and differs in 10 other residue positions.