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Galactosyltransferase activities in mitochondrial outer membrane: biosynthesis of dolichylmonophosphate-galactose
F Gasnier1, P Louisot, O Gateau
1Department of Biochemistry, INSERM CNRS U.189, Lyon-Sud Medical School, University of Lyon, Oullins, France.
Abstract:
Mitochondrial outer membranes were prepared from mouse liver homogenates by swelling purified mitochondria in phosphate buffer and were purified on a discontinuous sucrose gradient. Assays for marker enzymes and controls in electron microscopy confirmed the purity and homogeneity of this subfraction. Mitochondrial outer membranes had significant galactosyltransferase activity when incubated with UDP-[14C]galactose: 14C-labelling was found in products extractable with organic solvents and in a residual precipitate. Addition of exogenous dolichylmonophosphate loaded into phosphatidylcholine liposomes strongly enhanced the incorporation of [14C]galactose into chloroform/methanol (2:1, v/v) -extractable products. Thin-layer chromatography of these 2:1 extracts showed that the increase of [14C]galactose incorporation was attributable to the synthesis of a new galactosylated lipid, 'lipid L'. This 'lipid L' has been purified on silicic acid columns by elution with chloroform/methanol (1:1, v/v). The purified 'lipid L' was labile in acid and released [14C]galactose. It had the same chromatographic behaviour as dolichylmonophosphate-mannose in neutral, acid and alkaline solvent systems. Upon incubation in presence of [3H]dolichylmonophosphate and UDP-[14C]galactose, purified 'lipid L' contained both 3H- and 14C-labelling. 'Lipid L', synthesized by mitochondrial outer membranes, was therefore characterized as dolichylmonophosphate-galactose.
Insights
Mitochondrial outer membranes synthesize dolichylmonophosphate-galactose, a novel lipid involved in glycosylation. This discovery enhances our understanding of mitochondrial lipid metabolism and function.
Area of Science:
- Biochemistry
- Cell Biology
- Mitochondrial Research
Background:
- Mitochondria play crucial roles in cellular energy production and metabolism.
- The composition and function of the mitochondrial outer membrane are key to understanding mitochondrial processes.
- Glycosylation pathways are essential for protein modification and cellular signaling.
Purpose of the Study:
- To investigate the presence and activity of galactosyltransferase in mouse liver mitochondrial outer membranes.
- To identify and characterize novel lipid products synthesized by mitochondrial outer membranes.
- To elucidate the role of mitochondrial outer membranes in lipid-based glycosylation.
Main Methods:
- Isolation and purification of mouse liver mitochondrial outer membranes.
- Enzymatic assays using UDP-[14C]galactose to measure galactosyltransferase activity.
- Lipid extraction, thin-layer chromatography, and mass spectrometry for product identification.
- Acid lability and co-incubation studies to confirm lipid structure.
Main Results:
- Mitochondrial outer membranes exhibited significant galactosyltransferase activity.
- A novel galactosylated lipid, designated 'lipid L', was synthesized and identified.
- 'Lipid L' was characterized as dolichylmonophosphate-galactose, confirmed by its lability, chromatographic behavior, and incorporation of both galactose and dolichyl phosphate.
- Exogenous dolichylmonophosphate enhanced the synthesis of 'lipid L'.
Conclusions:
- Mitochondrial outer membranes are capable of synthesizing dolichylmonophosphate-galactose.
- This finding reveals a previously unrecognized role for mitochondria in the dolichol phosphate pathway.
- The synthesis of dolichylmonophosphate-galactose by mitochondria may have implications for glycosylation and mitochondrial function.