Arimoto I, Saito H, Kawashima Y, Miyajima K, Handa T
Faculty of Pharmaceutical Sciences, Kyoto University, Japan.
J Lipid Res. 1998 Jan;39(1):143-51.
Lipoprotein lipase (LPL) hydrolyzes triacylglycerol (TG) of TG-rich lipoproteins. We investigated the effects of sphingomyelin (SM) and cholesterol (Chol) on the lipolysis of lipid emulsions by LPL using human apolipoprotein C-II (apoC-II) or plasma as an activator. Kinetic studies of the lipolysis rates clearly demonstrated that the primary effect of the activator on the LPL reaction was not to increase the affinity of LPL for the emulsion surface, but to enhance LPL catalytic activity. Incorporation of SM into the emulsion surface caused increases in Km(app) and decreases in Vmax(app), indicating that SM inhibited lipolysis by decreasing both affinity for substrates and catalytic activity of LPL. SM was also found to affect possible factors related to the lipolysis rates; that is, SM increased TG solubility in surface layers and decreased apoC-II binding to the emulsion surface. Interestingly, Chol did not affect the lipolysis rates even though it decreased TG solubility and apoC-II binding. These results indicated that neither TG solubility nor amount of apoC-II binding were determinate factors in LPL-mediated lipolysis under physiological conditions. Our results suggest that the content of SM in the lipoprotein surface plays an important role by controlling lipoprotein lipase-mediated lipolysis, and that cholesterol enrichment in the lipoprotein surface has no influence on lipolysis, but may affect other metabolic processes such as uptake by the liver through the selectivity of apolipoprotein binding.
脂蛋白脂肪酶（LPL）可水解富含三酰甘油（TG）的脂蛋白中的TG。我们使用人载脂蛋白C-II（apoC-II）或血浆作为激活剂，研究了鞘磷脂（SM）和胆固醇（Chol）对LPL介导的脂质乳剂脂解作用的影响。脂解速率的动力学研究清楚地表明，激活剂对LPL反应的主要作用不是增加LPL对乳剂表面的亲和力，而是增强LPL的催化活性。将SM掺入乳剂表面会导致表观米氏常数（Km(app)）增加，最大反应速度（Vmax(app)）降低，这表明SM通过降低对底物的亲和力和LPL的催化活性来抑制脂解作用。还发现SM会影响与脂解速率相关的可能因素；也就是说，SM增加了TG在表面层中的溶解度，并减少了apoC-II与乳剂表面的结合。有趣的是，尽管Chol降低了TG的溶解度和apoC-II的结合，但它并未影响脂解速率。这些结果表明，在生理条件下，TG的溶解度和apoC-II的结合量都不是LPL介导的脂解作用的决定性因素。我们的结果表明，脂蛋白表面的SM含量通过控制脂蛋白脂肪酶介导的脂解作用发挥重要作用，并且脂蛋白表面的胆固醇富集对脂解作用没有影响，但可能通过载脂蛋白结合的选择性影响其他代谢过程，如肝脏摄取。
