FURTHER CHARACTERIZATION OF THE HUMAN SERUM D 1.063-1.21, α1-LIPOPROTEIN*

The high density lipoprotein class floating at a solvent density between 1.063 and 1.21 g per ml can be fractionated by flotation in solvents of in- termediate density into two major subfractions (1, 2), which according to Shore (3) have a protein moiety with identical amino acid composition and Cand N-terminal amino acids. Homogeneity of the D 1.063-1.21 lipoprotein in terms of protein moiety has also been indicated by immunochemical studies (4). It would appear, therefore, that the human serum high density lipoprotein of D 1.063- 1.21 constitutes a group of molecules identi- cal as to protein moiety and possibly differing only in lipid complement. The experiments reported below were designed to test this hypothesis. The serum D 1.063-1.21 lipoprotein was arbitrarily fractionated into three fractions floating respectively at solvent densities of D 1.063-1.125, 1.125-1.168, and 1.168-1.21. The results dealing with some of the physicochemical and bio- logical properties of these lipoprotein subfractions form the object of this report.