Bloj B, Zilversmit D B
J Biol Chem. 1982 Jul 10;257(13):7608-14.
Nonspecific lipid transfer protein accelerated cholesterol exchange from brush border vesicles according to a biphasic time course, but sonicated vesicles made from brush border phospholipids and glycosphingolipids showed a single phase exchange. Removal of surface protein with papain or opening brush border vesicles with deoxycholate did not abolish the biphasic exchange pattern. In brush border vesicles treated with cholesterol oxidase, 21 +/- 10% of the free cholesterol was oxidized rapidly, and the remaining cholesterol was oxidized at a slower rate. Opening vesicles with sodium deoxycholate or treatment with phospholipase C, which degraded 55% of the phospholipids, did not increase the size of the rapidly oxidizable cholesterol pool. The rapidly exchangeable and the rapidly oxidizable cholesterol pools appear to represent the same fraction. In double-labeled brush border vesicles 27 +/- 9% of the cholesterol is present in a readily accessible pool, which slowly equilibrates with the remaining membrane cholesterol. The fractional turnover rate of cholesterol in the readily accessible pool equals 0.07 +/- 0.04 h-1 and is increased to 3.35 h-2 by 12 micrograms/ml of nonspecific lipid transfer protein. The heterogeneous distribution of cholesterol in the intact brush border vesicles may not reflect an inside-outside distribution or interaction of cholesterol with membrane lipids but rather an association of more than two-thirds of the membrane cholesterol with a membrane protein fraction.
非特异性脂质转运蛋白按照双相时间进程加速了刷状缘小泡中胆固醇的交换，但由刷状缘磷脂和糖鞘脂制成的超声处理小泡显示出单相交换。用木瓜蛋白酶去除表面蛋白或用脱氧胆酸盐打开刷状缘小泡并没有消除双相交换模式。在用胆固醇氧化酶处理的刷状缘小泡中，21±10%的游离胆固醇被快速氧化，其余胆固醇以较慢速率被氧化。用脱氧胆酸钠打开小泡或用磷脂酶C处理（降解55%的磷脂）并没有增加可快速氧化的胆固醇池的大小。快速可交换和快速可氧化的胆固醇池似乎代表相同的部分。在双标记的刷状缘小泡中，27±9%的胆固醇存在于一个易于接近的池中，该池与其余膜胆固醇缓慢平衡。易于接近池中胆固醇的分数周转率等于0.07±0.04 h⁻¹，并且通过12微克/毫升的非特异性脂质转运蛋白增加到3.35 h⁻²。完整刷状缘小泡中胆固醇的异质分布可能并不反映胆固醇的内外分布或与膜脂质的相互作用，而是超过三分之二的膜胆固醇与膜蛋白部分的关联。
