The N-terminal 17% of apoB binds tightly and irreversibly to emulsions modeling nascent very low density lipoproteins

The N-terminal 17% of apolipoprotein B (apoB-17) readily associates with di myristoylphosphatidylcholine (DMPC) multilamellar vesicles (MLV) to form la rge (240-Angstrom diameter) discoidal particles. Because apoB is normally s ecreted with triacylglycerol (TAG)-rich lipoproteins, we studied the bindin g of apoB-17 to triolein-rich emulsions modeling nascent TAG-rich very low density-like lipoproteins. Emulsions with the following composition (by wei ght) were prepared: 85-89% triolein, 1.1-1.4% cholesterol, and 10-14% phosp hatidylcholines (PC) including either egg yolk (EY)-, dimyristoyl (DM)-, or dipalmitoyl (DP)-PC representing (at 25 degreesC), respectively, a fluid s urface, a surface at transition, and a mainly solid surface. The respective sizes were 1,260 +/- 500, 1,070 +/- 450, and 830 +/- 300 Angstrom mean dia meter. The emulsions were incubated with conditioned medium containing apoB -17, and then reisolated by ultracentrifugation. Analysis of the emulsion-b ound proteins by gel electrophoresis showed that all three emulsions bound primarily apoB-17. The DPPC emulsions bound more apoB-17 than EYPC or DMPC emulsions. Immunoaffinity-purified apoB-17 exhibited saturable, high affini ty binding to EYPC and DPPC emulsions. The respective K-d values were 32 +/ - 23 and 85 +/- 27 nM and capacities (N) were 10 and 58 molecules of apoB-1 7 per particle. When apoB-17 bound to emulsions was incubated with DMPC MLV at 26 degreesC for 18 h, it remained bound to the emulsions, indicating th at once bound to these emulsions it is unable to ex change off and solubili ze DMPC into discs. In contrast, apoE-3 bound to emulsions dissociated from the emulsions when incubated with DMPC MLV and formed discs. Thus, apoB-17 binds strongly and irreversibly to emulsions modeling nascent lipoproteins . It therefore may play an important role in the stabilization of nascent V LDL and chylomicrons.