Differences in C-terminal amino acid sequences between erythrocyte and liver cytochrome b5 isolated from pig and human. Evidence for two tissue-specific forms of cytochrome b5

Loss of Ncb5or results in impaired fatty acid desaturation, lipoatrophy, and diabetes

Targeted ablation of the novel flavoheme reductase Ncb5or knock-out (KO) results in progressive loss of pancreatic beta-cells and white adipose tissue over time. Lipoatrophy persisted in KO animals in which the confounding metabolic effects of diabetes were eliminated by islet transplantation (transplanted knockout (TKO)). Lipid profiles in livers prepared from TKO animals were markedly deficient in triglycerides and diacylglycerides. Despite enhanced expression of stearoyl-Co-A desaturase-1, levels of palmitoleic and oleic acids (Delta9 fatty acid desaturation) were decreased in TKO relative to wild type controls. Treatment of KO hepatocytes with palmitic acid reduced cell viability and increased apoptosis, a response blunted by co-incubation with oleic acid. The results presented here support the hypothesis that Ncb5or supplies electrons for fatty acid desaturation, offer new insight into the regulation of a crucial step in fatty acid biosynthesis, and provide a plausible explanation for both the diabetic and the lipoatrophic phenotype in Ncb5or(-/-) mice.

Heterologous expression of Ascaris suum cytochrome b5 precursor protein: a histidine-tagged full-length presequence is correctly processed to transport the mature protein to the periplasm of Escherichia coli

The cytochrome b(5) of the body wall of adult Ascaris suum, a porcine parasitic nematode, is a novel type of cytochrome b(5). It is a soluble protein that lacks the COOH-terminal membrane-anchoring domain found in erythrocyte cytochrome b(5), but possesses an NH(2)-terminal extension (presequence) of 30 amino acids that are missing from the 82-residue protein purified from the nematode tissues [Yu, Y., Yamasaki, H., Kita, K., and Takamiya, S., 1996, Arch. Biochem. Biophys. 328, 165-172]. The nematode cytochrome b(5) is, therefore, probably synthesized as a precursor protein whose presequence is cleaved to form a mature protein, but the localization of the mature protein is still unknown. To investigate the processing of the putative precursor protein, the wild-type precursor of nematode cytochrome b(5) with a complete presequence (b5wt) and its NH(2) terminus-truncated derivatives, b5Delta18 and b5Delta28, with 18 and 28 residues deleted, respectively, were expressed using pET-28a(+) vector in Escherichia coli. As expected, all transformants, tb5wt, tb5Delta18, and tb5Delta28, produced recombinant proteins with a histidine-tagged NH(2)-terminal extension. However, only the recombinant protein with the full-length presequence, produced in tb5wt, was correctly processed and transported to the periplasm, from which the majority of the induced product was purified as a mature protein chemically and functionally identical to the native protein purified from the nematode body wall. These results clearly show that the nematode histidine-tagged presequence functions as a signal peptide in E. coli.

Differential expression of the mRNAs for the soluble and membrane-bound forms of rabbit cytochrome b5

Total RNA was extracted from a variety of rabbit tissues and reverse transcribed for use in the polymerase chain reaction technique. Using primers designed to amplify the membrane-bound liver cytochrome b5 cDNA, products of two sizes were observed. Both hybridized strongly to a radiolabelled liver cytochrome b5 probe. Sequencing confirmed that the two types of cDNA product encoded the membrane-bound and the soluble forms of b5. Messenger RNA corresponding to the soluble cytochrome was detected in the lung, gallbladder and the adrenal gland, as well as in reticulocytes and bone marrow. This was an unexpected finding since the protein has been isolated only from erythrocytes. In contrast, membrane-bound cytochrome b5 mRNA was detected in all tissues tested, suggesting that the corresponding protein is ubiquitous in tissue distribution.

The human liver and reticulocyte cytochrome b5 mRNAs are products from a single gene

Using a combination of standard cDNA library screening techniques and the Polymerase Chain Reaction (PCR) we have isolated and sequenced DNA fragments corresponding to the human reticulocyte cytochrome b5 mRNA. The reticulocyte specific sequence codes for amino acids 97 and 98 only, with a TAA stop codon. The reticulocyte specific 3'non-translated sequence has 15 new nucleotides then utilizes the liver mRNA sequence from amino acid 97 onward. This indicates that the reticulocyte specific exon has 24 base pairs (bp). In addition, we have isolated sequences that are derived from a transcribed cytochrome b5 pseudogene. This transcript contains multiple mutations which prevent the synthesis of any functional protein.

Soluble and membrane-bound forms of cytochrome b5 are the products of a single gene in chicken

In order to determine the relationship of the soluble cytochrome b5 found in erythrocytes to the membrane-bound form found in other tissues, a cDNA clone encoding cytochrome b5 in chicken erythrocytes was isolated by using mixed oligonucleotides based on a partial amino acid sequence of the protein. Complete nucleotide sequence identity between the erythrocyte cDNA and the sequence of a cDNA clone of the liver protein suggests that they are transcribed from the same gene. The isolation and structural analysis of genomic clones was also consistent with the presence of only one cytochrome b5 gene in chicken. These results suggest that the formation of soluble erythrocyte cytochrome b5 occurs by proteolytic processing of the membrane-bound form. Thus, previous reports indicating that the carboxyl terminal amino acid residue of the erythrocyte form differs from the corresponding residue of the membrane-bound form may suggest the existence of a novel post-translational modification.

Purification by hydrophobic chromatography of soluble cytochrome b5 of human erythrocytes

Soluble cytochrome b5 of human erythrocytes was purified very effectively by hydrophobic chromatography using a butyl-Toyopearl 650 column. Cytochrome b5 was adsorbed tightly on the column in the presence of 60% saturated ammonium sulfate, and was eluted at 40% saturation of ammonium sulfate in the elution buffer. The chromatography gave a good yield of cytochrome b5 of the highest purity so far reported as estimated from the 414 nm to 280 nm absorbance ratio of the oxidized form of the cytochrome b5. The value obtained with the cytochrome b5 purified in this study was 6.57, and is higher than the previously reported highest value of 6.4 (Hultquist, D.E., Dean, R.T. and Douglas, R.H. (1974) Biochem. Biophys. Res. Commun. 60, 28-34). Spectral properties including molecular absorption coefficients were determined using the cytochrome b5 purified by this method.

The primary structure of chicken liver cytochrome b5 deduced from the DNA sequence of a cDNA clone

A cDNA clone encoding the chicken liver cytochrome b5 was isolated by probing a library with synthetic oligonucleotides based on a partial amino acid sequence of the protein. Determination of the DNA sequence indicated a 414-nucleotide open reading frame which encodes a 138-amino acid residue polypeptide. The open reading frame contains 6 amino acids at the amino terminus which were not present on any of the cytochrome b5 polypeptides for which the amino acid sequence has been determined directly, suggesting that the protein is proteolytically processed to the mature form. The results of genomic Southern analysis were consistent with the presence of two structurally different genes in the chicken genome, raising the possibility that the soluble and membrane-bound forms of the protein are the products of separate genes.

Hamster hepatic cytochrome b5: purifications, immunochemical properties, and in vitro synthesis

Cytochrome b5 has been purified from hamster liver microsomes. Both Ouchterlony double-diffusion and rocket immunoelectrophoresis experiments indicate that no immuno-cross-reactivity exists between guinea-pig anti-rabbit cytochrome b5 antibody and hamster cytochrome b5. However, anti-rabbit b5 IgG inhibited both hamster microsomal NADH-cytochrome c reductase and NADPH-dependent 7-ethoxycoumarin-O-deethylase activities. Hamster cytochrome b5 stimulated several reconstituted hamster cytochrome P-450-dependent monooxygenase activities and this stimulatory effect could be inhibited by antibody against rabbit cytochrome b5. Two-dimensional iodinated tryptic peptide mapping experiments provided evidence that the polypeptide fingerprint of hamster cytochrome b5 is substantially different from the fingerprints of cytochrome b5 isolated from rabbit, rat and bovine. We also studied the in vitro synthesis of hamster cytochrome b5 from liver mRNA using a wheat germ lysate system. A 16 kDa polypeptide, which is the same size as hamster cytochrome b5, was immunoprecipitated by antibody against rabbit b5. This experiment suggested that in vitro synthesized hamster cytochrome b5 is recognized by a heterologous antibody. Thus, hamster and rabbit cytochrome b5 do share some common immuno-determinants which may be located close to the heme-binding active site.