Interaction of Human Apolipoprotein a-I With Dipalmitoyl Phosphatidylcholine
Wetterau, John Rowley, Ii
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https://hdl.handle.net/2142/70515
Description
Title
Interaction of Human Apolipoprotein a-I With Dipalmitoyl Phosphatidylcholine
Author(s)
Wetterau, John Rowley, Ii
Issue Date
1982
Department of Study
Biochemistry
Discipline
Biochemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Biochemistry
Abstract
Dipalmitoylphosphatidylcholine (DPPC) bilayer lipid will react with human apolipoprotein A-I (apo A-I) to form lipid-protein micellar complexes. The reactivity of the lipid bilayer is dependent upon the physical state of the vesicle. Large vesicles with essentially planar bilayers do not form complexes except at temperatures near the gel to liquid-crystalline phase transition of the vesicles where they demonstrate a slow rate of complex formation. Small unilamellar vesicles which contain bilayers with a high degree of surface curvature are rapidly incorporated into micellar complexes at temperatures near and above the transition temperature. The highly reactive nature of the small vesicles is explained by the marked curvature of the lipid bilayer which affects the packing of the DPPC molecules in the bilayer and facilitate its penetration by apo A-I.
Dipalmitoylphosphatidylcholine vesicles and apo A-I will form micellar complexes with different sizes and compositions depending on the reaction conditions. A typical small complex has a DPPC to Apo A-I molar ratio of 75 (+OR-) 20:1 and contains 2 or 3 protein molecules per complex, while a typical large complex has a 290 (+OR-) 40:1 molar ratio and contains 3 or 4 proteins per complex. Physical, chemical, and spectroscopic characterization of the DPPC-apo A-I micellar complexes indicates that they are similar to the dimyristoylphosphatidylcholine-apo A-I micellar complexes previously characterized and reported in the literature. The large, lipid rich complexes are formed when the reaction mixtures contain a relatively high concentration of DPPC vesicles or the reaction temperature is low (below 40(DEGREES)C). The apo A-I protein must be covalently modified with dimethylaminonaphthalene sulfonyl (DNS) groups to the extent of 1 or 2 groups per protein to obtain a high yield of small micellar complexes from DPPC-apo A-I reaction mixtures. When reaction conditions are chosen which favor the formation of large complexes, it appears that the large complexes cannot be converted to small complexes because either there is insufficient protein or the lipid in the large micellar complexes is in a physical state which does not allow the conversion. By changing the reaction conditions (increasing the relative concentration of protein or the temperature of reaction), large micellar complexes can be converted to small; whereas small complexes do not readily convert to large complexes by a change in reaction conditions.
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