STUDIES OF SYNTHETIC PEPTIDES OF HUMAN APOLIPOPROTEIN-A-I CONTAINING TANDEM AMPHIPATHIC ALPHA-HELIXES

In mature human apolipoprotein A-I (apo A-I), the amino acid residues 1-43 are encoded by exon 3, whereas residues 44-243 are encoded by exo n 4 of the apo A-I gene. The region encoded by exon 4 of the apo A-I g ene contains 10 tandem amphipathic ct-helixes; their location and the class to which they belong are as follows: helix 1 (44-65, class Al), helix 2 (66-87, class Al), helix 3 (88-8, class Y), helix 4 (99-120, c lass Y), helix 5 (121-142, class Al), helix 6 (143-164, class Al), hel ix 7 (165-186, class Al), helix 8 (187-208, class Al), helix 9 (209-21 9, class Y), and helix 10 (220-241, class Y), To examine the effects o f multiple tandem amphipathic helixes compared to individual helixes o f apo A-I on lipid association, we have studied Lipid-associating prop erties of the following peptides: Ac-44-87-NH2 (peptide 1-2), Ac-66-98 -NH2 (peptide 2-3), Ac-66-120-NH2 (peptide 2-3-4), Ac-88-120-NH2 (pept ide 3-4), Ac-99-142-NH2 (peptide 4-5), Ac-121-164-NH2 (peptide 5-6), A c-143-186-NH2 (peptide 6-7), Ac-165-208-NH2 (peptide 7-8), Ac-187-219- NH2 (peptide 8-9), and Ac-209-241-NH2 (peptide 9-10). To study lipid-a ssociating properties of the region encoded by exon 3 of the apo A-I g ene, 1-33-NH2 (peptide G) has also been studied. The results of the p resent study indicate that, among the peptides studied, peptides 1-2 a nd 9-10 possess significantly higher lipid affinity than the other pep tides, with peptide 9-10 having higher lipid affinity than peptide 1-2 , as evidenced by (i) higher helical content in the presence of I,2-di myristoyl-sn-glycero-3-phosphocholine (DMPC), (ii) faster rare of asso ciation with DMPC multilamellar vesicles (MLV), (iii) greater reductio n in the enthalpy of gel to liquid-crystalline phase transition of DMP C MLV, (iv) higher exclusion pressure from an egg yolk phosphatidylcho line monolayer, and (v) higher partitioning into 1-palmitoyl-2-oleoyl- sn-glycero-3-phosphocholine MLV. A comparison of the free energies of lipid association (Delta G) of the peptides studied here with those st udied previously by us [Palgunachari, M. N., et al. (1996) Arterioscle r. Thromb. Vasc. Biol, 16, 328-338] indicates that, except for the pep tides 4-5 and 5-6, other peptides possess higher lipid affinities comp ared to constituent helixes. However, the lipid affinities of the pept ides studied here are neither higher than nor equal to the sum of the lipid affinities of the constituent helixes. This indicates the absenc e of cooperativity among the adjacent amphipathic helical domains of a po A-I for lipid association. As indicated by ac, the lipid affinity o f peptide 4-5 is higher than peptide 5 but lower than peptide 4; the l ipid affinity of peptide 5-6 is lower than both peptides 5 and 6. Impl ications of these results for the structure and function of apo A-I ar e discussed.