エマルション表面膜構造に依存するリポ蛋白質リパーゼ酵素反応

Abstract

The hydrolysis of triglycerides (TG) by lipoprotein lipase (LPL) is a crucial process in the metabolism of TG-rich lipoproteins and artificial lipid emulsions injected intravenously. In this study, we found that sphingomyelin (SM) at the emulsion surface inhibits LPL-mediated lipolysis both in vivo and in vitro. Incorporation of SM into the emulsion surface caused an increase in the apparent Michaelis-Menten constant (Km (app)) and a decrease in the apparent maximal lipolysis rate (Vmax (app)). SM was also found to affect factors which may be related to the kinetic parameters; that is, SM increased TG solubility in surface layers and decreased apoC-II binding to the emulsion surface. Interestingly, cholesterol (Chol) did not affect the lipolysis rates although it decreased TG solubility and apoC-II binding. These results indicated that neither TG solubility at the surface layer nor amount of apoC-II binding are determining factors in LPL-mediated lipolysis under physiological conditions. Furthermore, on the basis of kinetic studies, we showed that SM inhibits lipolysis by decreasing both the binding affinity for emulsions and the catalytic activity of LPL.The mechanism by which SM at the emulsion surface inhibits lipolysis was also discussed. SM strongly increased head group packing probably due to the high capacity of forming hydrogen bonds, whereas Chol had little effect on the head group structure of the emulsion surface. Decreasing the head group mobility by SM could inhibit the insertion of the binding region of LPL protein, resulting in increases in Km (app). In addition, SM stabilizes TG in the surface layer and retards the transfer of TG from the lipid particle surface to the catalytic pocket of LPL, resulting in decreases in LPL catalytic activity and Vmax (app). Our results suggested that head group packing significantly affects LPL binding to the lipid surface and that TG stability in the surface layer is important for the LPL catalytic activity.From these results, the content of SM in the lipoprotein surface is presumed to play an important role in controlling LPL-mediated lipolysis by the mechanism described above. Artificial lipid emulsions are used as drug carriers, and the control of TG hydrolysis of lipid carriers is important for development of better drug delivery systems. The lipolysis activity can be modulated by surface lipid properties, i.e., the surface lipid composition.