Purification by hydrophobic chromatography of soluble cytochrome b5 of human erythrocytes

Direct preparation of giant proteo-liposomes by in vitro membrane protein synthesis

We investigated the direct constitution of membrane proteins into giant liposomes in cell-free (in vitro) protein synthesis. Giant liposomes were present in a translation reaction cocktail of a wheat germ cell-free protein translation system. Apo cytochrome b(5) (b5) and its fusion proteins were synthesized and directly localized in the liposomes. After the translation reaction, the proteo-liposomes were isolated by simplified discontinuous density-gradient centrifugation. Apo cytochrome b(5) conjugated dihydrofolate reductase (DHFR) was synthesized in the same procedure and the protein was directly displayed on the liposome surface. b5 acts as a "hydrophobic tag" for recruitment to the liposome surface.

One-electron reduction of quinones by the neuronal nitric-oxide synthase reductase domain

Flavin electron transferases can catalyze one- or two-electron reduction of quinones including bioreductive antitumor quinones. The recombinant neuronal nitric oxide synthase (nNOS) reductase domain, which contains the FAD-FMN prosthetic group pair and calmodulin-binding site, catalyzed aerobic NADPH-oxidation in the presence of the model quinone compound menadione (MD), including antitumor mitomycin C (Mit C) and adriamycin (Adr). Calcium/calmodulin (Ca2+/CaM) stimulated the NADPH oxidation of these quinones. The MD-mediated NADPH oxidation was inhibited in the presence of NAD(P)H:quinone oxidoreductase (QR), but Mit C- and Adr-mediated NADPH oxidations were not. In anaerobic conditions, cytochrome b5 as a scavenger for the menasemiquinone radical (MD*-) was stoichiometrically reduced by the nNOS reductase domain in the presence of MD, but not of QR. These results indicate that the nNOS reductase domain can catalyze a only one-electron reduction of bivalent quinones. In the presence or absence of Ca2+/CaM, the semiquinone radical species were major intermediates observed during the oxidation of the reduced enzyme by MD, but the fully reduced flavin species did not significantly accumulate under these conditions. Air-stable semiquinone did not react rapidly with MD, but the fully reduced species of both flavins, FAD and FMN, could donate one electron to MD. The intramolecular electron transfer between the two flavins is the rate-limiting step in the catalytic cycle [H. Matsuda, T. Iyanagi, Biochim. Biophys. Acta 1473 (1999) 345-355). These data suggest that the enzyme functions between the 1e- <==> 3e- level during one-electron reduction of MD, and that the rates of quinone reductions are stimulated by a rapid electron exchange between the two flavins in the presence of Ca2+/CaM.

Role of carboxyl residues surrounding heme of human cytochrome b5 in the electrostatic interaction with NADH-cytochrome b5 reductase

To identify the cytochrome b5 residues responsible for the electrostatic interaction with NADH-cytochrome b5 reductase (b5R), we prepared and characterized the cytochrome b5 mutants in which Glu41, Glu42, Glu63, Asp70, and Glu73 were replaced by Ala, utilizing site-directed mutagenesis and the expression system for cytochrome b5 in Escherichia coli. Apparent Km values of the wild type b5R for Glu42Ala cytochrome b5 and Asp70Ala cytochrome b5 were approximately three-fold and six-fold higher than that for the wild type cytochrome b5, respectively, while the kcat values for those mutants were not remarkably affected. In contrast, Glu41Ala, Glu63Ala, and Glu73Ala cytochrome b5 showed almost the same kinetic properties as the wild type cytochrome b5. Furthermore, kinetic studies on combinations of the cytochrome b5 and b5R mutants suggested the interaction between Glu42 and Asp70 of cytochrome b5 and Lys125 and Lys41 of b5R, respectively, in the reaction.

Electrostatic interaction between NADH-cytochrome b5 reductase and cytochrome b5 studied by site-directed mutagenesis

Electrostatic interaction between NADH-cytochrome b5 reductase and cytochrome b5 was studied by site-directed mutagenesis. The target residues for mutagenesis were selected on the basis of the previously reported chemical cross-linking study of these two proteins, which implicated possible charge-pair interactions between Lys-41, Lys-125, Lys-162, and Lys-163 of the enzyme, and Glu-47, Glu-48, Glu-52, Glu-60, Asp-64 (group A), and heme propionate of cytochrome b5. Mutant reductases that lost one of the above-listed Lys residues showed higher K(m) values for cytochrome b5 and lower kcat values than those of the wild type, suggesting that all of the examined Lys residues participate in binding with cytochrome b5 as reported previously. In contrast, a removal of one of (or even all of) the group A residues from cytochrome b5 by mutagenesis caused no significant effect on the catalytic properties of cytochrome b5. Additional elimination of another set of negative residues (Glu-41, Glu-42, Asp-57, and Glu-63 (Group B)), which are also located close to heme, elevated the K(m) value by more than five folds. These results suggest that there should be other acidic residue(s) than group A in cytochrome b5 which participate in binding with NADH-cytochrome b5 reductase.

Superoxide radical generation in peroxisomal membranes: evidence for the participation of the 18 kDa integral membrane polypeptide

Peroxisomes were isolated from pea (Pisum sativum L.) leaves and the peroxisomal membranes were purified by treatment with Na2CO3. The production of superoxide radicals (O2) induced by NADH was investigated in peroxisomal membranes from intact organelles incubated with proteases (pronase E and proteinase K). Under isoosmotic conditions, in the presence of pronase E, the production of O2-. radicals was inhibited by 80%. SDS-PAGE of peroxisomal membranes after protease treatment demonstrated a decrease in the 18-kDa PMP. This suggests that this polypeptide has a small fragment exposed to the cytosolic side of the peroxisomal membrane which is essential for O2-. production. The 18-kDa PMP was purified by preparative SDS-PAGE and in the reconstituted protein the NADH-driven production of O2-. radicals was investigated. The isolated polypeptide showed a high generation rate of superoxide (about 300 nmol O2-. x mg-1 protein x min-1) which was completely inhibited by 50 mM pyridine. The 18-kDa PMP was recognized by a polyclonal antibody against Cyt b5 from human erythrocytes. The presence of b-type cytochrome in peroxisomal membranes was demonstrated by difference spectroscopy. Results obtained show that in the NADH-dependent O2-. radical generating system of peroxisomal membranes, the 18-kDa integral membrane polypeptide, which appears to be Cyt b5, is clearly involved in superoxide radical production.

Characteristic properties of a retinoic acid synthetic cytochrome P-450 purified from liver microsomes of 3-methylcholanthrene-induced rats

An inducible cytochrome P-450 (P-450) catalyzing retinoic acid synthesis was purified from liver microsomes of 3-methylcholanthrene (3-MC)-treated rats, based on the activity of all-trans-retinoic acid formation from all-trans-retinal. We previously reported that the retinoic acid synthesis by microsomes was catalyzed by a cytochrome P-450-linked monooxygenase system (Tomita et al. (1993) Int. J. Biochem. 25, 1775-1784). This microsomal retinoic acid synthesis in rat liver was induced more than 8-fold by 3-MC. The purified P-450 electophoretically gave a single protein band and its minimum molecular weight was estimated to be 57.2 kDa on SDS-PAGE. The optical spectrum of the oxidized P-450 without retinal revealed it was the low-spin form, and the CO-complex exhibited a maximum peak at 447 nm. The specific activity of the reconstituted P-450-linked monooxygenase system was 29.5 nmol/min per nmol P-450 at pH 7.6 and 37 degrees C. The K(m) and Vmax values for all-trans-retinal were 11.6 microM and 38.5 nmol/min per nmol P-450, respectively. The amino-acid sequence of the N-terminal region of the P-450 was identical to that of rat P-450 1A1 (CYP 1A1). Xenobiotic activities, such as 7-ethoxycoumarin O-deethylase (7-ECOD) and 7-ethoxyresorufin O-deethylase (7-EROD) activities, of the P-450-linked monooxygenase system were specific to the P-450 1A1. The retinoic acid formation in the reconstituted monooxygenase system was specifically inhibited by alpha-naphthoflavone (alpha-NF), which is a P-450 1A1-specific inhibitor, citral, which is a retinoid analogue structurally, and an anti-rat P-450 1A1 antibody. These results further support that the purified P-450 is P-450 1A1. This paper describes that P-450 1A1 was purified and characterized as a retinoic acid synthetic P-450.

Mouse liver cytidine-5'-monophosphate-N-acetylneuraminic acid hydroxylase. Catalytic function and regulation

In this paper, we present the results of an investigation into the catalytic properties of CMP-Neu5Ac hydroxylase (Neu5Ac: N-acetylneuraminic acid) in high-speed supernatants of mouse liver. The enzyme was most active in Hepes/NaOH pH 7.4 and was markedly inhibited by relatively small increases in ionic strength, though the inhibition was abolished by desalting procedures. Several nonionic detergents could activate the hydroxylase to various degrees in a concentration-dependent manner. Ionic detergents and a number of phospholipids were, however, generally inert or inhibitory. The lack of inhibitory influence of a wide range of nucleotides revealed that CMP-Neu5Ac hydroxylase binds its sugar-nucleotide substrate with a high degree of specificity. Thus, even millimolar concentrations of several cytidine nucleotides elicited virtually negligible inhibition, though the reaction product, CMP-Neu5Gc (Neu5Gc: N-glycoloylneuraminic acid), was a weak inhibitor. The results also indicate that the enzyme is not regulated by any nucleotides or sugar-nucleotides. Dilution of high-speed supernatants with buffer gave rise to a decrease in the specific activity of the hydroxylase, implicating the involvement of more than one component in catalysis. Activity could be restored by the addition of a heat extract of the supernatant. The active principle in this extract was found to be a heat-stable protein with a molecular mass of about 17 kDa. Immunochemical studies allowed this protein to be identified as cytochrome b5 and it was shown that this electron carrier is essential for the activity of CMP-Neu5Ac hydroxylase. Inhibition studies using iron ligands and activation by exogenous iron salts suggest the involvement of a non-haem iron cofactor in the catalytic cycle of this hydroxylase. Cytochrome b5 may thus serve as an electron donor for this postulated cofactor.