Heterogeneity in the conformation of apo A-I on the surface of HDL particles

Kinetics and mechanism of transfer of reduced and carboxymethylated apolipoprotein A-II between phospholipid vesicles

The transfer of 14C-labeled, reduced and carboxymethylated human apolipoprotein A-II (RCM-AII) between small unilamellar vesicles (SUV) has been investigated. Ion-exchange chromatography was used for rapid separation of negatively charged egg phosphatidylcholine (PC)/dicetyl phosphate donor SUV containing bound 14C-labeled RCM-AII from neutral egg PC acceptor SUV present in 10-fold molar excess. The kinetics of 14C-labeled RCM-AII transfer in incubations of up to 12 h at 37 degrees C are consistent with the existence of fast, slow, and apparently "nontransferrable" pools of SUV-associated apolipoprotein; the transfers from these pools occur on the time scales of seconds or less, hours, and days/weeks, respectively. For donor SUV (0.15 mg of phospholipid/mL reaction mixture) containing about 15 RCM-AII molecules per vesicle, the sizes of the fast, slow, and nontransferrable pools are 13, 69, and 18%, respectively. The transfer of RCM-AII from the slow kinetic pool follows first-order kinetics, and the half-time (t 1/2) is about 3 h. The different kinetic pools of SUV-associated RCM-AII probably reflect apoprotein in different conformations of the SUV surface. Increasing the number of RCM-AII per donor SUV enlarges the size of the fast pool and increases the t 1/2 of transfer from the slow pool. In contrast, raising the incubation temperature reduces the t 1/2 of slow transfer. The t 1/2 of RCM-AII transfer from the slow kinetic pool is inversely proportional to the acceptor/donor SUV ratio which suggests that the transfer of apoprotein molecules in this kinetic pool is mediated by SUV collisions.(ABSTRACT TRUNCATED AT 250 WORDS)