A Function of Cytochrome b5 in Fatty Acid Desaturation by Rat Liver Microsomes

The distribution of cytochrome b5 in rat liver microsomes, and in two microsomal subfractions isolated by density equilibration in a linear sucrose gradient, was studied under the electron microscope by means of a ferritin-labeled hybrid anti-cytochrome b5/anti-ferritin antibody. Results of this study show that cytochrome b5 is present in essentially all microsomal vesicles derived from endoplasmic reticulum (ER), whether rough or smooth. Thus, the dissociation of ER constituents into two groups (b and c), achieved by subfractionating microsomes by isopycnic centrifugation (Beaufay, H., A. Amar-Costesec, D. Thines-Sempoux, M. Wibo, M. Robbi, and J. Berthet. 1974. J. Cell Biol. 61:213-231), does not reflect the association of each group with distinct microsomal particles but reflects rather an enzymatic heterogeneity of the ER: the ratio of group c to group b enzymes increasing with the density and ribosome load of the particles.

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Fluorescence polarization studies of rat liver microsomes with altered phospholipid desaturase activities

Canadian Journal of Biochemistry ◽

10.1139/o80-130 ◽

1980 ◽

Vol 58 (10) ◽

pp. 952-958 ◽

Cited By ~ 19

Author(s):

E. L. Pugh ◽

M. Kates ◽

A. G. Szabo

Keyword(s):

Fatty Acid ◽

Rat Liver ◽

Fluorescence Polarization ◽

Desaturase Activity ◽

Liver Microsomes ◽

Rat Liver Microsomes ◽

Polarization Ratio ◽

Parinaric Acid ◽

Cis And Trans ◽

Normal Controls

Phospholipid desaturase activity of rat liver microsomes can be varied by dietary manipulation: rats starved and refed a "fat-free" diet have two to three times the activity of normal controls whereas starved rats have no detectable activity. These changes were accompanied by changes in fatty acid composition of the microsomal phospholipids, resulting in a double bond:saturated fatty acid mole ratio (moles double bonds per mole saturated fatty acid) of 5.7 in control and starved rats and 4.7 in starved–refed rats. Fluorescence polarization ratio [Formula: see text] of cis- and trans-parinaric acid (PnA) showed no significant differences in physical state of the three microsomal preparations. However, the isolated microsomal phospholipids with trans-PnA as probe showed differences in the temperature at which onset of a change in polarization ratio occurred (starved–refed > normal > starved rats). With the cis-PnA as probe, the polarization ratio showed no change in the range 10–40 °C but was significantly higher (1.8) in starved–refed rats than in normal and starved rats (1.6 in both cases). These data indicate that the microsomal phospholipids of starved–refed animals were in a less fluid state than those from control and starved rats and that this decrease in fluidity was correlated with an increase in phospholipid desaturase activity.

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Characterization and biosynthesis of cytochrome b5 in rat liver microsomes

Biochemical Journal ◽

10.1042/bj1070839 ◽

1968 ◽

Vol 107 (6) ◽

pp. 839-849 ◽

Cited By ~ 24

Author(s):

J. R. Sargent ◽

B. P. Vadlamudi

Keyword(s):

Amino Acid ◽

Rat Liver ◽

Spectral Properties ◽

Proteolytic Enzymes ◽

Cytochrome B5 ◽

Liver Microsomes ◽

Rat Liver Microsomes ◽

Isolated Rat Liver ◽

Peptide Linkage ◽

Isolated Rat

1. Cytochrome b5 is released from rat liver microsomes by both proteolytic enzymes and by treatments that disrupt phospholipids. Cytochrome P-420 is only released to a marked extent by treatments that disrupt phospholipids. 2. Cytochrome b5 was isolated in a pure state from both the rough and smooth fractions of rat liver microsomes after treatment with trypsin, and was shown to contain two cytochrome components with identical spectral properties. 3. Amino acid analyses of the two components are presented, together with peptide ‘fingerprint’ patterns of tryptic digests of the two components. 4. Studies based on the direct isolation of cytochrome b5 after administration of a single dose of radioactive amino acid to rats demonstrate that the cytochrome is synthesized initially in the rough fraction of microsomes and only subsequently appears in the smooth fraction. 5. Isolated rat liver microsomes are capable of incorporating radioactive amino acids into cytochrome b5 under standard conditions. 6. Under these conditions the amino acid is incorporated into peptide linkage in the cytochrome.

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