L-Ascorbic acid synthesis in a soluble enzyme system from rat-liver microsomes

Sterol carrier protein2 (SCP2) is known to stimulate utilization of cholesterol in enzymic reactions in which cholesterol is the substrate. Substantial recent experimental evidence indicates that SCP2: activates enzymic conversion of intermediates between lanosterol and cholesterol; stimulates the microsomal conversion of cholesterol into cholesterol ester in rat liver; and enhances mitochondrial utilization of cholesterol for pregnenolone formation in the adrenals. The conversion of cholesterol into 7 α-hydroxycholesterol is the rate-limiting step in bile-acid synthesis. We therefore investigated the effect of SCP2 on this physiologically critical reaction by using a gas-chromatography-mass-spectrometry procedure that measures the mass of 7 α-hydroxycholesterol formed. The results show that SCP2 enhances 7 α-hydroxycholesterol formation by rat liver microsomes (microsomal fractions), utilizing either endogenous membrane cholesterol, cholesterol supplied exogenously in serum or in the form of cholesterol/phospholipid liposomes. Microsomes immunotitrated with anti-SCP2 antibody exhibited considerably less capacity to synthesize 7 α-hydroxycholesterol, which was restored to control levels on addition of purified SCP2. These data are consistent with the suggestion that SCP2 may be of physiological significance in the overall metabolism of cholesterol.

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Effects of vitamins A and D on the biosynthesis of l-ascorbic acid by rat-liver microsomes

Biochemical Journal ◽

10.1042/bj0970247 ◽

1965 ◽

Vol 97 (1) ◽

pp. 247-249 ◽

Cited By ~ 9

Author(s):

NC Ghosh ◽

I Chatterjee ◽

GC Chatterjee

Keyword(s):

Vitamin D ◽

Ascorbic Acid ◽

Vitamin A ◽

Rat Liver ◽

Liver Microsomes ◽

Rat Liver Microsomes ◽

Stock Diet ◽

Hypervitaminosis A ◽

Hypervitaminosis D ◽

Liver Tissues

1. The synthesis of l-ascorbic acid from either d-glucuronolactone or l-gulonolactone by liver microsomes of rats is decreased under conditions of hypervitaminosis A; under hypervitaminosis D the synthesis from d-glucuronolactone is increased and that from l-gulonolactone is not affected. 2. The microsomal conversion of l-gulonolactone into l-ascorbic acid is impaired in liver tissues of rats made deficient with respect to either vitamin A or vitamin D when compared with the controls maintained on stock diet.

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The mechanism of initiation of lipid peroxidation. Evidence against a requirement for an iron(II)-iron(III) complex

Biochemical Journal ◽

10.1042/bj2580617 ◽

1989 ◽

Vol 258 (2) ◽

pp. 617-620 ◽

Cited By ~ 136

Author(s):

O I Aruoma ◽

B Halliwell ◽

M J Laughton ◽

G J Quinlan ◽

J M C Gutteridge

Keyword(s):

Lipid Peroxidation ◽

Ascorbic Acid ◽

Rat Liver ◽

Liver Microsomes ◽

Rat Liver Microsomes ◽

Brain Phospholipids ◽

Lag Period

When Fe2+ ions are added to rat-liver microsomes, lipid peroxidation begins after a short lag period. Fe2+-dependent peroxidation in the first few minutes of the incubation can be increased by adding Fe3+, ascorbic acid or Pb2+ ions; these stimulations are not additive. By contrast, Pb2+ ions inhibit peroxidation of microsomes in the presence of Fe3+/ascorbate or Fe3+-ADP/NADPH. In liposomes made from ox-brain phospholipids, Fe2+-dependent peroxidation is stimulated slightly by Fe3+, but much more so by ascorbic acid, Al3+ or Pb2+; these stimulations are not additive. Liposomal peroxidation in the presence of Fe3+/ascorbate is inhibited by Pb2+ or Al3+. These results argue against the participation of an Fe2+-Fe3+-O2 complex, or a critical 1:1 ratio of Fe2+ to Fe3+, in the initiation of lipid peroxidation in liposomes and rat-liver microsomes.

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