A study of the cystine-containing apolipoprotein of human plasma high density lipoproteins: Characterization of cyanogen bromide and tryptic fragments

A major protein (or apolipoprotein) constituent has been isolated from human plasma high density lipoproteins and shown to contain two monomeric units covalently linked by a single disulfide bond. The protein is designated apoLP-Gln-II, based on its carboxyl-terminal amino acid. It contains no histidine, arginine or tryptophan and is also devoid of carbohydrate. Amino acid analysis of reduced-aminoethylated apoLP-Gln-II indicates 77 residues and a single methionine per monomeric unit. Two unique cyanogen bromide fragments, CNBr III and IV, have been isolated from the reduced-aminoethylated protein and account for all of the 77 amino acids of the monomer. CNBr IV has 26 residues, a blocked amino-terminus, no isoleucine, one residue of aminoethylcysteine, carboxyl-terminal homoserine and corresponds therefore to the amino-terminal and cystine-containing portion of apoLP-Gln-II. CNBr III has 51 amino acids, including one residue of isoleucine, has carboxyl-terminal glutamine, has no aminoethylcysteine and corresponds to the carboxyl-terminus of apoLP-glutamine-II. CNBr III and IV have each been digested with trypsin and the resulting peptides isolated by a combination of gel filtration and ion-exchange chromatography. Twelve tryptic fragments accounted for the 77 amino acids present in apoLP-Gln-II. This finding and the tryptic fingerprint analysis of reduced aminoethylated apoLP-Gln-II indicate that apoLP-Gln-II must contain two identical or very nearly identical subunits.