Differences in C‐terminal amino acid sequences between erythrocyte and liver cytochrome b 5 isolated from pig and human

Two forms of cytochrome b 5 , a soluble erythrocyte form and a membrane‐bound liver form, were purified from pig and human, and structural differences between them were analyzed. Porcine and human erythrocyte cytochrome b 5 consisted of 97 amino acid residues and contained the same catalytic domain structure (residue 1–96) as that of the corresponding liver cytochrome b 5 , but had one amino acid replacement at the C‐terminus (residue 97). These results suggest that erythrocyte cytochrome b 5 is not derived from the liver protein by proteolysis but a translational product from another distinct mRNA of cytochrome b 5 .