There is reasonable evidence now to suggest that recombinants of dimyristoyl phosphatidylcholine (DMPC) and apolipoprotein A-I, the major apoprotein of high density lipoprotein (HDL), are in the form of a phospholipid bilayer disc containing an annulus of protein around its circumference. While nascent HDL has a discoid appearance by negative stain electron microscopy that is very similar to recombinant discoidal HDL (d-HDL), there is little additional structural evidence to support a topological similarity between these two forms of d-HDL. Because of the potential biological and medical al importance of nascent d-HDL, i.e., it may serve as a cholesterol sink in the mediation of reverse cholesterol transport from peripheral tissues, such as arterial wall, determination of the structure of nascent d-HDL is a highly significant problem. This is especially true since the basic annular protein-bilayer disc model for d-HDL is a simple structure that has self-evident structure-function modalities. Preliminary studies have provided evidence that (a) the NMR1H chemical shifts of the phospholipid are a sensitive probe of overall structure and detailed interactions, (b) within classes of DMPC: apo A-I complexes with two or three A-I molecules, there is a further quantization of size and protein-lipid ratio, presumably due to changes in the protein conformation, (c) discoidal apo A-I complexes prepared from egg PC using cholate dialysis reconstitution may have a structure that is significantly different from that of te DMPC:A-I complexes. The proposed studies include: (1) verification of the structures and apparent structural differences by techniques such as chemical cross-linking, low angle neutron scattering, circular dichroism, fluorescence, differential scanning calorimetry and further NMR studies, (2) a detailed investigation of the physical mechanisms for the observed chemical shifts, using a variety of systems, (3) extension of the studies on discoidal complexes to more complex and relevent model systems, including spheroidal complexes, (4) preliminary explorations of the effect of structure on parameters directly related to the physiological functions of HDL.
