Amino Acid Sequences of Tryptic Peptides of Cytochromes b5 from Microsomes of Human, Monkey, Porcine, and Chicken Liver

Liver microsomal apocytochromes b5 from man, monkey (Alouatta fusca), pig, and chicken were subjected to trypsin digestion, and all of the peptides were isolated and characterized. The sum of the residues present in these peptides equaled the total amino acid composition of the corresponding parent cytochrome. For all of these peptides a homologous segment was found in our previously established bovine and rabbit cytochrome sequence. This information provided sufficient evidence to construct a unique amino acid sequence for human, monkey, porcine, and chicken cytochrome b5. Comparison of the six sequences indicated a close similarity between these proteins. The human cytochrome differed from that of chicken in 15 positions but from that of monkey in only 2 positions. With the exception of the avian protein sequence, the amino acid replacements were confined predominantly to the NH2- and COOH-terminal segments of the cytochrome. A segment comprising residues 42 to 72 was invariant in all six cytochromes.