Lipid transport in the avian species. Part I. Isolation and characterization of apolipoproteins and major lipoprotein density classes of male turkey serum.
(1) Lipoproteins from the serum of male turkeys maintained on a normal diet were separated by sequential preparative ultracentrifugation into VLDL (d less than 1.006 g/ml), LDL (d = 1.006-1.063 g/ml), HDL (d = 1.063-1.21 g/ml) and VHDL (d greater than 1.21 g/ml). Lipoprotein density classes were characterized by analytical ultracentrifugation, agarose electrophoresis, immunodiffusion and immunoelectrophoresis, and by quantitative determination of protein, lipids and individual phosphatides. (2) HDL were the major density class representing 75% of the total lipoprotein content, LDL accounted for approximately 20% and VLDL for only 3-5% of the total lipoproteins. (3) VLDL were characterized by a relatively low content of glyceride (34%). Cholesterol esters were the major lipid (38%) of LDL, and the phospholipids (26%) of HDL. Glycerides of all major density classes consisted of equal amounts of triglycerides and diglycerides. (4) Phosphatidylcholine was the major phosphatide in all density classes. The composition of phosphatides was very similar in the VLDL and LDL, but it was different in the HDL. The ratio of phosphatidylcholine/sphingomyelin was higher in HDL than in VLDL and LD. (5) Immunological and electrophoretic studies showed that all three major density classes consisted of two lipoprotein families designated, in analogy to the human serum lipoprotein system [1], as LP-A and LP-B. The exception was HDL3 (d = 1.125-1.21 g/ml) which contained only LP-A. (6) ApoB was insoluble in aqueous buffers but could be solubilized after reduction and carboxymethylation. No C- or N-terminal amino acids were released by the usual chemical methods. The carbohydrate moiety of ApoB contained mannose, galactose and galactosamine. (7) ApoA consisted of a non-identical polypeptides designated in analogy to the human polypeptides as A-I and A-II. A-I was the major ApoA polypeptide and had a molecular weight of about 27,000. This polypeptide contained no half cystine, and the aspartic acid as the N-terminal and alanine as the C-terminal amino acids. A-II had a molecular weight of about 10,000, contained no half cystine and had alanine as the C-terminal amino acid. A-II showed no N-terminal amino acid by either dansylation, dinitrophenylation or Edman's procedure. Neither A-I nor A-II contained neutral sugars or hexosamines. (8) Concentrations of polypetides analogous to human ApoC, ApoD and "arginine-rich" polypeptide, if present, were too low for their unequivocal chemical characterization.