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Neutral lipids of the plasma membrane: composition of plasma membrane fractions isolated from ocular lens.

PURPOSE
To quantitatively examine and compare the neutral lipid composition of sedimenting and non-sedimenting ocular lens plasma membrane fractions isolated from the cortex and nucleus.


METHODS
Homogenates of bovine lens cortical and nuclear regions were subjected to centrifugation to sediment the water insoluble fractions, which were used as the sedimenting membrane fractions. The non-sedimenting membrane fractions were isolated from the water soluble fractions by flotation during overnight, high density centrifugation. Lipids were extracted from the membrane fractions and neutral lipids were separated by thin layer chromatography.


RESULTS
A greater concentration of triacylglycerol was found in the whole lens non-sedimenting membrane fraction (10.6 micrograms triacylglycerol/mg phospholipid) than in the sedimenting membrane (3.7 micrograms triacylglycerol/mg phospholipid). Triacylglycerol was found in the non-sedimenting membrane fraction isolated from both lens cortex and nucleus, but was not detected in the water insoluble fraction isolated from the lens nucleus. The higher triacylglycerol concentration of the non-sedimenting membrane correlates with its lower cholesterol level. Free fatty acids were found in both the whole lens non-sedimenting membrane fraction (66.2 micrograms free fatty acid/mg phospholipid) and water insoluble fraction (40.3 micrograms free fatty acid/mg phospholipid). Free fatty acids were more abundant in the nucleus. This could reflect phospholipase attack of nuclear phospholipids, a mechanism proposed to explain the lower glycerophospholipid concentration in this region. The fatty acid compositions of the phospholipids from the non-sedimenting and sedimenting membrane fractions were essentially identical. Triacylglycerols (from both non-sedimenting and sedimenting membrane fractions) contained a greater proportion of saturated fatty acids and a lesser proportion of mono-unsaturated fatty acids than the phospholipids. The triacylglycerols of the non-sedimenting membrane fraction contained a greater amount of palmitic and lesser amounts of myristic and stearic acids than the triacylglycerols of the sedimenting membrane fractions. The free fatty acids (of both non-sedimenting and sedimenting membrane fractions) were composed of a greater proportion of saturated fatty acids and a lesser proportion of mono-unsaturated fatty acids than found in the phospholipids. The non-sedimenting membrane fraction of both cortex and nucleus was enriched with an unidentified (presumed) lipid. These findings support earlier work indicating most membrane triacylglycerols are enriched in saturated fatty acids.


CONCLUSIONS
These results demonstrate differences in the neutral lipid composition between non-sedimenting and sedimenting lens membrane fractions and furthers our hypothesis that the non-sedimenting membrane could represent a specialized domain of the lens plasma membrane.