Nonspecific lipid transfer proteins as probes of membrane structure and function |
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Authors: | Richard C Crain |
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Affiliation: | (1) The Biological Sciences Group, U-125, The University of Connecticut, 06268 Storrs, CT |
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Abstract: | A protein that accelerates transfer of phospholipids of varying head group and fatty acid composition has been purified from
bovine liver. As previously found for other phospholipid transfer proteins, “nonspecific lipid transfer protein” stimulates
a kinetically biphasic transfer of radioactively labeled phospholipid from small unilamellar vesicles to unlabeled multilamellar
vesicles. The kinetics are consistent with rapid transfer of phospholipid from the outer monalyer and slow transfer of that
localized in the inner monolayer (half-times greater than 3 days for phosphatidylcholine, phosphatidylethanolamine, and phosphatidylinositol).
Protein catalyzed transfer is inhibited by high ionic strength and has an activation energy of 35 kJ/mol. The broad lipid
specificity and ease of large-scale purification make these proteins candidates for membrane phospholipid compositional modification.
The compositions of rat liver mitochondrial and microsomal membranes and Morris hepatoma 7288c mitochondrial membranes were
altered by incubation with lipid vesicles and nonspecific lipid transfer protein. Incubation with phosphatidylcholine vesicles
led to increased levels of phosphatidylcholine and decreased levels of other transferrable lipids (phosphatidylethanolamine,
phosphatidylinositol, and cholesterol) unless the latter were included in the vesicles. When vesicles containing dipalmitoylphosphatidylcholine
were incubated with microsomal membranes, a large increase in disaturated phosphatidylcholine was also observed. These changes
in composition were correlated with activities of membrane enzymes. It appears that microsomal glucose-6-phosphatase is inhibited
by increased phosphatidylcholine saturation. Moreover, this enzyme is also inhibited by decreases in the phosphatidylethanolamine/phosphatidylcholine
ratio whereas NADPH cytochrome c reductase is not. Likewise, decreased cholesterol to phospholipid ratios did not greatly
affect the abnormally low levels of hepatoma succinate cytochrome c reductase activity.
This paper was presented at the 73rd AOCS annual meeting, Toronto, Canada, May 1982. |
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