Conformational analysis of serum apolipoprotein AII in lipoprotein complexes with bifunctional crosslinking reagents.

Apolipoprotein AII isolated from human serum high density lipoproteins was recombined with phosphatidylcholine to yield homogeneous particles of 80-120 A diameter. The radioactive bifunctional crosslinkers dimethyl [1,1'-14C]suberimidate and dimethyl 4,4'-dithiobis([1-14C]butyrimidate) were reacted with these particles. The kinetics of the reactions and the localisation of the crosslinked lysines of the polypeptide chains were determined. Thermolysin hydrolysis followed by two-dimensional separation of the peptides and isolation of the mono- and bifunctionally modified peptides allowed the assignment of the crosslinked peptides of the apoprotein AII seqeunce. The crosslinking pattern indicates a close-neighbour relationship (13-15 A) of the peptide chains between amino acid residues 3, 23, 46 and 55 of the symmetrical halves of the apo AII molecule. A reconstruction of the secondary structure of Apo AII in the lipoprotein complex on the basis of theoretical calculations is given and correlated with the chemical data.