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

肝细胞色素<b_5>是一种由埋在微粒体中的亲水催化区和疏水催化区组成的膜蛋白。红细胞中的细胞色素<b_5>存在于可溶性部分，其分子结构与肝酶的亲水结构域相似。这两种形式的细胞色素在不同的组织中起着不同的作用。(1)测定了人、猪、牛红细胞细胞色素<b_5>的完整氨基酸序列，以及肝蛋白的n端区和关节区序列。在红细胞细胞色素<b_5>的97个氨基酸残基中，残基1-96与同种肝蛋白的残基相同。然而，两种类型的残基97 （c端残基）不同，表明两种形式的细胞色素<b_5>是由两种不同但密切相关的mrna编码的。(2)从mRNA中构建编码兔肝细胞色素<b_5>的cDNA，分离并测序。cDNA全长727个核苷酸。5‘非翻译区包含22个碱基，其次是开放阅读框的402个碱基，3’非翻译区包含286个碱基和一个poly (a)。n端氨基酸残基5′侧为ATG，表明细胞色素不含疏水信号肽。(3)以兔细胞色素<b_5>的cDNA为探针，分离出人细胞色素<b_5>的基因组基因。基因测序目前正在进行中。虽然人与兔细胞色素<b_5>的氨基酸残基同源性超过90%，但只有不到50%的氨基酸残基密码子第3位的碱基相同。

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.