Lund-Katz S, Hammerschlag B, Phillips M C
Biochemistry. 1982 Jun 8;21(12):2964-9. doi: 10.1021/bi00541a025.
The mechanism of cholesterol and phosphatidylcholine (PC) exchange between human serum lipoproteins has been investigated by following the transfer of radiolabeled cholesterol and PC between high-density lipoprotein (HDL) and low-density lipoprotein (LDL). Initially, [14C]cholesterol was present in the donor lipoprotein particle which was either HDL2, HDL3, or LDL. After incubation in saline solution for various times, the HDL and LDL were separated by precipitation of the LDL with Mn2+-heparin reagent. More than 90% of the [14C]cholesterol in donor HDL3 is transferred to LDL in a first-order process whose half-time is 2.9 min at 37 degrees C. This indicates that transfer of cholesterol molecules from the cholesterol ester/triglyceride core of HDL to the phospholipid/apoprotein monolayer at the surface of the particle is not rate limiting for exchange. The half-time for dipalmitoyl-PC exchange from HDL3 to LDL is 5 +/- h, indicating that the flux of PC is much lower than that of cholesterol. The half-times for [14C]cholesterol exchange from HDL2 and LDL at 37 degrees C are 4 and 45 min, respectively. The interfacial fluxes at 37 degrees C from the various lipoproteins are 4, 15, and 10 cholesterol molecules/(10 nm2 h), respectively, for LDL, HDL2, and HDL3. The rate of labeled cholesterol transfer from HDL3 is not affected when the concentration of LDL acceptor is increased 40-fold. The activation energies of cholesterol transfer between 4 and 37 degrees C for HDL3, HDL2, and LDL are 70 +/- 3, 75 +/- 3, and 78 +/- 3 KJ/mol, respectively. The general characteristics of the process of exchange of cholesterol between lipoproteins resemble those for exchange between small unilamellar vesicles. The results are only consistent with a mechanism of exchange in which cholesterol molecules diffuse through the aqueous phase; the experimental activation energy is associated with desorption of lipid from the donor lipoprotein into the aqueous phase.
通过追踪放射性标记的胆固醇和磷脂酰胆碱（PC）在高密度脂蛋白（HDL）和低密度脂蛋白（LDL）之间的转移，对人血清脂蛋白之间胆固醇和PC的交换机制进行了研究。最初，[14C]胆固醇存在于供体脂蛋白颗粒中，该颗粒可以是HDL2、HDL3或LDL。在盐溶液中孵育不同时间后，用Mn2 + -肝素试剂沉淀LDL来分离HDL和LDL。供体HDL3中超过90%的[14C]胆固醇以一级过程转移到LDL中，在37℃时其半衰期为2.9分钟。这表明胆固醇分子从HDL的胆固醇酯/甘油三酯核心转移到颗粒表面的磷脂/载脂蛋白单层不是交换的限速步骤。二棕榈酰-PC从HDL3转移到LDL的半衰期为5±小时，表明PC的通量远低于胆固醇。在37℃时，[14C]胆固醇从HDL2和LDL交换的半衰期分别为4分钟和45分钟。在37℃时，来自各种脂蛋白的界面通量分别为4、15和10个胆固醇分子/（10 nm2·小时），分别对应LDL、HDL2和HDL3。当LDL受体浓度增加40倍时，HDL3中标记胆固醇的转移速率不受影响。HDL3、HDL2和LDL在4℃至37℃之间胆固醇转移的活化能分别为70±3、75±3和78±3 kJ/mol。脂蛋白之间胆固醇交换过程的一般特征类似于小单层囊泡之间的交换。结果仅与胆固醇分子通过水相扩散的交换机制一致；实验活化能与脂质从供体脂蛋白解吸到水相有关。
