Comparative Study of Gene Structure for the Human liver-microsoml Type and Erythrocyte-soluble Type of Cytochrome <b_5> .

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Liver cytochrome <b_5> is a membrane protein consisting of a hydrophilic catalitic region and a hydrophobic region buried in microsomes. Cytochrome <b_5> in erythrocytes is present in soluble fraction and is similar in molecular structure to the hydrophilic domain of the liver enzyme. The two forms of the cytochrome play different functions in different tissues. (1) We have determined the complete amino acid sequence for human, porcine and bovine erythrocyte cytochrome <b_5> , and the sequences of the N-terminal region and joint region of the liver proteins. Of the 97 amino acid residues of erythrocyte cytochrome <b_5> , residues 1-96 were identical with those of the liver proteins of the same species. However, residue 97 (C-terminal residue) was different between the two types, indicating that the two forms of cytochrome <b_5> are encoded by two different but closely related mRNAs. (2) The cDNA encoding rabbit liver cytochrome <b_5> was constructed from mRNA, isolated and sequenced. The cDNA was found to be 727 nucleotides in length. The 5' untranslated region containd 22 bases and was followed by 402 bases of an open reading frame, 286 bases of 3' untranslated region and a poly (A). The 5' side of N-terminal amino acid residue was ATG indicating that the cytochrome had no hydrophobic signal peptide. (3) Using the cDNA of rabbit cytochrome <b_5> as a probe, genomic gene for the human cytochrome <b_5> has been isolated. The sequencing of the gene is now under progress. Although the homology in amino acid residues between human and rabbit cytochrome <b_5> is more than 90%, only less than 50% of the bases at the 3rd podition of the codons of the comparable amino acid residues were identical.