The primary structure of high density apolipoprotein glutamine I

The major protein constituent of human plasma high density lipoproteins was isolated and its complete amino acid sequence determined. The protein, designated apolipoprotein glutamine I by the presence of carboxyl terminal glutamine, is a single polypeptide chain of 245 amino acid residues, including 3 residues of methionine. The protein is devoid of cysteine, cystine, and isoleucine. Cleavage of apolipoprotein glutamine I with cyanogen bromide yields 4 fragments with 94, 90, 36, and 25 amino acids. The amino acid sequence of each fragment was determined by conventional methods, with proteolytic digestion with trypsin, chymotrypsin, and thermolysin. The alignment of the cyanogen bromide fragments was determined by the isolation of the methionine containing tryptic peptides from apolipoprotein glutamine I. Inspection of the sequence of apolipoprotein glutamine I suggests an interesting distribution of amino acids that may account for its helical structure and its ability to bind and transport lipid.