The reconstitution of purified apolipoprotein C-I and C-III2 with sn-3-dimyristoyl-lecithin and sn-3-dimyristoyl-lecithin:cholesterol (10:1) vesicles was studied by electron spin resonance spectroscopy using isomeric 5′-, 12′-, and 16′-(N-oxyl-4″,4″-dimethyloxazolidine)stearoyl spin-labelled lecithin probes. Results obtained from the temperature-induced changes of lipoprotein recombinants showed the hydrophilic nature of the lipid–protein interactions. The temperature-induced phospholipid phase transition, as measured by 5′-(N-oxyl-4″,4″-dimethyloxazolidine)stearoyl spin-labelled lecithin probe in recombinants containing apoprotein C-1 or apoprotein C-III2, is very broad and has a small cooperative unit indicative of extensive lipid–protein interactions occurring at the head group region of the phospholipid bilayer. When 12′- and 16′-(N-oxyl-4″,4″-dimethyloxazolidine)stearoyl spin-labelled lecithins are used as probes in the same system, similar sharper and more cooperative lipid phase changes are detected. These results indicate a surface location for both apoprotein C-I and apoprotein C-III2 with respect to the phospholipid bilayer in lipoprotein recombinants with and without cholesterol.
Regulation of UDP-glucuronosyltransferase by lipid-protein interactions
