Fleschner C R, Cenedella R J
Department of Biochemistry, Kirksville College of Osteopathic Medicine, MO 63501, USA.
Curr Eye Res. 1997 Mar;16(3):263-9. doi: 10.1076/ceyr.16.3.263.15402.
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.
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.
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.
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.
定量检测并比较从皮质和核中分离出的沉降性和非沉降性晶状体质膜组分的中性脂质组成。
对牛晶状体皮质和核区域的匀浆进行离心，以使水不溶性组分沉降，这些沉降的组分用作沉降性膜组分。通过过夜的高密度离心漂浮法从水溶性组分中分离出非沉降性膜组分。从膜组分中提取脂质，并通过薄层色谱法分离中性脂质。
在整个晶状体的非沉降性膜组分中发现的三酰甘油浓度（10.6微克三酰甘油/毫克磷脂）高于沉降性膜组分（3.7微克三酰甘油/毫克磷脂）。在从晶状体皮质和核中分离出的非沉降性膜组分中均发现了三酰甘油，但在从晶状体核中分离出的水不溶性组分中未检测到。非沉降性膜中较高的三酰甘油浓度与其较低的胆固醇水平相关。在整个晶状体的非沉降性膜组分（66.微克游离脂肪酸/毫克磷脂）和水不溶性组分（40.3微克游离脂肪酸/毫克磷脂）中均发现了游离脂肪酸。核中游离脂肪酸更为丰富。这可能反映了核磷脂的磷脂酶攻击，这是一种用来解释该区域甘油磷脂浓度较低的机制。非沉降性和沉降性膜组分中磷脂的脂肪酸组成基本相同。与磷脂相比，三酰甘油（来自非沉降性和沉降性膜组分）含有更大比例的饱和脂肪酸和更小比例的单不饱和脂肪酸。非沉降性膜组分的三酰甘油比沉降性膜组分的三酰甘油含有更多的棕榈酸和更少的肉豆蔻酸及硬脂酸。与磷脂相比，游离脂肪酸（非沉降性和沉降性膜组分中的）由更大比例的饱和脂肪酸和更小比例的单不饱和脂肪酸组成。皮质和核的非沉降性膜组分都富含一种未鉴定（推测）的脂质。这些发现支持了早期的研究工作，即大多数膜三酰甘油富含饱和脂肪酸。
这些结果证明了非沉降性和沉降性晶状体膜组分之间中性脂质组成的差异，并进一步支持了我们的假设，即非沉降性膜可能代表晶状体质膜的一个特殊区域。
