Saat Y A, Bloch K E
J Biol Chem. 1976 Sep 10;251(17):5155-60.
Squalene epoxidation catalyzed by rat liver microsomes requires oxygen NADPH, and the 105,000 x g supernatant (S105). The supernatant can be replaced by a partially purified S105 protein (SPF) and phospholipids (Tai, H., and Bloch, K. (1972) J. Biol. Chem. 247, 3767). When washed microsomes are preincubated anaerobically with [14C]squalene and S105 without NADPH, followed by centrifugation and washing to remove the unbound squalene and S105, epoxidation in the presence of O2 and NADPH occurs subsequently at the same rate as in direct assays containing all required components from the start. Partially purified SPF (65-fold) shows the same effect. Washed microsomes preincubated anaerobically with squalene alone, or with bovine serum albumin instead of S105, also take up large amounts of squalene, but the squalene so incorporated is only poorly converted to epoxide. The epoxidation of endogenous squalene formed in liver homogenates from [14C]mevalonate is also stimulated by S105. The incorporation of squalene into microsomes is temperature dependent. 2,3-Oxidosqualene-lanosterol cyclase (cyclase) also requires S105 for optimal activity. It is suggested that the S105 protein acts internally within the microsomal membrane system facilitating the access of substrate to specific enzyme sites.
大鼠肝脏微粒体催化的角鲨烯环氧化反应需要氧气、NADPH以及105,000×g上清液（S105）。该上清液可用部分纯化的S105蛋白（SPF）和磷脂替代（Tai, H.，和Bloch, K.（1972年）《生物化学杂志》247, 3767）。当洗涤后的微粒体在无氧条件下与[14C]角鲨烯和不含NADPH的S105预孵育，随后离心并洗涤以去除未结合的角鲨烯和S105，在氧气和NADPH存在的情况下，随后发生的环氧化反应速率与从一开始就包含所有所需成分的直接测定法相同。部分纯化的SPF（65倍）显示出相同的效果。仅与角鲨烯在无氧条件下预孵育的洗涤后微粒体，或用牛血清白蛋白代替S105的洗涤后微粒体，也会摄取大量角鲨烯，但如此掺入的角鲨烯仅能很差地转化为环氧化物。由[14C]甲羟戊酸在肝脏匀浆中形成的内源性角鲨烯的环氧化反应也受到S105的刺激。角鲨烯掺入微粒体的过程是温度依赖性的。2,3-氧化角鲨烯-羊毛甾醇环化酶（环化酶）的最佳活性也需要S105。有人提出，S105蛋白在微粒体膜系统内部起作用，促进底物进入特定酶位点。
