Apolipoprotein C-III/sphingomyelin recombinants: formation, isolation, and characterization.
The association of apolipoprotein C-III (apoC-III) from human very low density lipoprotein with sphingomyelin from egg yolk (EYSM) has been studied at the transition temperature (Tc) of the phospholipid. Upon incubation of aliquots of the apoprotein with increasing amounts of sphingomyelin, the alpha-helical content of the apoprotein increased from 20% in the absence of EYSM to a limiting value of 67% at a protein:lipid molar ratio of 1:200. The tryptophan fluorescence spectrum of the apoprotein exhibited a gradual blue shift from 356 nm in the absence of EYSM to 348 nm when the protein:lipid ratio in the complex had reached 1:50. Gel filtration chromatography of complexes formed by incubating the apoprotein and phospholipid at differing apoC-III:EYSM ratios demonstrated a disintegration of sphingomyelin vesicles into particles of decreasing size with increasing proportion of protein. This effect was confirmed by sedimentation velocity experiments in which the observed sedimentation coefficient of EYSM decreased from 14.0 S (for vesicles) to a limiting value of 7.0 S when the apoprotein:phospholipid ratio reached 1:50 in the complex. Electron micrographs of negatively stained EYSM vesicles showed spherical particles of 380-A diameter. Addition of apoC-III led to the formation of disk-shaped structures whose diameter decreased to a limiting value of 204 +/- 34 A at a protein:lipid ratio of 1:50. In contrast, the disk thickness was relatively constant at 51 +/- 2 A for all isolated complexes.(ABSTRACT TRUNCATED AT 250 WORDS)
D053305:Apolipoprotein C-III; D001056:Apolipoproteins C / Q000302:isolation & purification / Q000378:metabolism*; D002942:Circular Dichroism; D004530:Egg Yolk; D004578:Electron Spin Resonance Spectroscopy; D006801:Humans; D008081:Liposomes; D008854:Microscopy, Electron; D008968:Molecular Conformation; D011487:Protein Conformation; D013050:Spectrometry, Fluorescence; D013109:Sphingomyelins / Q000378:metabolism*; D013816:Thermodynamics
D053305:Apolipoprotein C-III; D001056:Apolipoproteins C; D008081:Liposomes; D013109:Sphingomyelins