An alternate apoprotein conformation in high density apolipoprotein discoidal complexes. A Fourier transform infra-red study.

Discoidal complexes have been prepared from 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and the apoproteins of HDL3 (apo HDL3) or purified apo A-I. Gel electrophoresis established that apo HDL3 contained 74% apo A-I. Deconvolution and curve-fitting of the infra-red amide I band of the apoprotein in the lipid-protein complex revealed a secondary structure containing approximately 40% alpha-helix and 50% beta-structure. This contrasted with the results from circular dichroism studies (Surewicz et al. (1986) J. Biol. Chem., 261, 16191) of apo A-I/DMPC complexes which predicted 68% alpha-helix and 7% beta-structure. The discrepancy between the two methods and limitations of the two techniques for lipoproteins is discussed.