Burns C P, Welshman I R, Scallen T J, Spector A A
Biochim Biophys Acta. 1982 Dec 13;713(3):519-28. doi: 10.1016/0005-2760(82)90312-5.
When human lymphocytes, granulocytes and monocyte-enriched cell preparations were incubated with [2-14C]acetate, only 10-36% of the radioactivity incorporated into the nonsaponifiable lipid fraction was present as digitonin-precipitable sterols. This percentage is considerably less than that observed for rat hepatocytes (95%) or human liver slices (68%). Even though a marked increase in the incorporation of labeled acetate into both nonsaponifiable lipids and digitonin-precipitable sterols resulted from stimulation of lymphocytes with concananvalin A or granulocytes by phagocytosis, the proportion of nonsaponifiable lipid radioactivity that was digitonin-precipitable remained low. These findings suggest that rate-limiting steps beyond the 3-hydroxy-3-methylglutaryl coenzyme A reductase reaction exist in the sterol synthetic pathway of leukocytes. Pulse-chase experiments demonstrated a precursor product relationship in leukocytes between lanosterol and cholesterol, but some squalene appears to be in a pool that is not further metabolized. A subcellular fraction prepared from mixed leukocytes was incapable of converting appreciable amounts of [3H]squalene to lanosterol or cholesterol, suggesting an enzyme deficiency in this segment of the sterol synthetic pathway. With isolated liver microsomes, 50% of the nonsaponifiable lipid radioactivity synthesized from [3H]squalene was recovered in cholesterol when the system contained added liver cytosol. By contrast, if the liver cytosol was replaced by leukocyte cytosol, 14-fold less radioactivity was incorporated into nonsaponifiable lipids, and only 17% of the radioactivity was recovered in cholesterol. When sterol-carrier protein 1, partially purified from rat liver, was added to leukocyte cytosol, [3H]squalene incorporation into lanosterol increased more than 3-fold. With the addition of both sterol-carrier protein 1 and sterol-carrier protein 2, cholesterol synthesis increased 2-fold. These results suggest that the low cholesterol synthetic activity in human leukocytes is due to a defect in one or more of the microsomal enzymes that operate between squalene and cholesterol, as well as to a deficiency of sterol-carrier proteins in leukocyte cytosol.
当人淋巴细胞、粒细胞和富含单核细胞的细胞制剂与[2-¹⁴C]乙酸一起孵育时，掺入非皂化脂质部分的放射性中，只有10%-36%以洋地黄皂苷可沉淀的甾醇形式存在。该百分比远低于大鼠肝细胞（95%）或人肝切片（68%）中观察到的比例。尽管用伴刀豆球蛋白A刺激淋巴细胞或通过吞噬作用刺激粒细胞会导致标记乙酸掺入非皂化脂质和洋地黄皂苷可沉淀甾醇的量显著增加，但洋地黄皂苷可沉淀的非皂化脂质放射性比例仍然很低。这些发现表明，白细胞甾醇合成途径中存在3-羟基-3-甲基戊二酰辅酶A还原酶反应之后的限速步骤。脉冲追踪实验证明了白细胞中羊毛甾醇和胆固醇之间的前体-产物关系，但一些角鲨烯似乎处于一个不再进一步代谢的池中。从混合白细胞制备的亚细胞部分无法将大量的[³H]角鲨烯转化为羊毛甾醇或胆固醇，这表明甾醇合成途径的这一阶段存在酶缺陷。对于分离的肝微粒体，当系统中添加肝胞质溶胶时，由[³H]角鲨烯合成的非皂化脂质放射性中有50%在胆固醇中回收。相比之下，如果用白细胞胞质溶胶代替肝胞质溶胶，则掺入非皂化脂质的放射性减少14倍，并且只有17%的放射性在胆固醇中回收。当从大鼠肝脏中部分纯化的甾醇载体蛋白1添加到白细胞胞质溶胶中时，[³H]角鲨烯掺入羊毛甾醇的量增加了3倍多。同时添加甾醇载体蛋白1和甾醇载体蛋白2时，胆固醇合成增加了2倍。这些结果表明，人白细胞中胆固醇合成活性低是由于角鲨烯和胆固醇之间起作用的一种或多种微粒体酶存在缺陷，以及白细胞胞质溶胶中甾醇载体蛋白缺乏。
