The NH2 -Terminal Structures of Human and Rat Liver Microsomal NADH-Cytochrome b5 Reductases

Detergent-solubilized NADH-cytochrome b5 reductase was purified from human liver microsomes. Both the purified enzyme and the membrane-binding domain isolated from the purified enzyme were determined to be modified at the NH2,-terminal amino acid, glycine, by myristic acid in an amide form. Myristic acid was identified as a methyl ester by gas chromatography after the acid methanolysis of the purified enzyme and the NH2-terminal peptide. The NH2-terminal structure of the membrane-binding domain was determined to be CH3(CH2)12-CO-Gly-Ala-Gln-Leu-Ser-Thr-Leu-Gly-His-Met-Val-Leu-Phe-Val-Trp-Phe-Leu-Tyr-Ser-Leu-Leu-Met-Lys. The sequence from Leu-7 to Lys-24 completely coincided with that deduced from the base sequence of complementary DNA (cDNA) from human placenta (Yubisui, T. et at. (1987) Proc Natl Acad. Sci. U.S. 84, 3609–3613). The NH2-terminal structure of the detergent-solubilized enzyme from rat liver microsomes was also analyzed for, comparison with that of human liver microsomal enzyme. The NH2-terminal myristic acid and the first 7 amino acids of the membrane-binding domains of human, rat, and steer liver microsomal enzymes are completely conserved, and more than 70% homology was observed over the whole membrane-binding domains, implying the importance of the conserved structure as an anchor of the enzyme to the membrane.