Abstract
To understand the catalytic sequence of LPL reaction and the mechanism by which apo C-II increases the LPL activity, the role of phospholipid vesicles and the effect of apo C-II on esterase activity of LPL which hydrolized short chain fatty acid ester (p-nitrophenyl butyrate) were investigated. Esterase activity was enhanced by the addition of dipalmitoyl phosphatidyl choline vesicles (DPPC) and the enhanced rate was dependent on the physicochemical properties of the phospholipids; below transitional temperature [DPPC 42°C, dimyristoyl phosphatidyl choline (DMPC), 24°C], both phospholipid vesicle enhanced esterase activity of LPL at the same extent (800%). The addition of DPPC vesicles increased the Vmax value of LPL for PNPB, but did not change Km values.
The DPPC-triolein vesicles which contained 0.5% tri[1-14C]oleoyl glycerol were prepared and the effect of apo C-II on the rates of hydrolysis of trioleoyl glycerol and of PNPB were simultaneously determined. The rate of PNPB hydrolysis decreased, and the rate of trioleoyl glycerol hydrolysis increased reciprocally as the added amount of apo C-II increased.
These results suggest that LPL undergoes conformational change on the surface of phospholipid so that esterase activity increased, and then apo C-II increases the affinity of catalytic site for trioleoyl glycerol, such that reciprocal effects of apo C-II on lipase and esterase activity of LPL were observed.
