A Comparison of the Major Apolipoprotein from Pig and Human High Density Lipoproteins

High density lipoproteins (HDL) were isolated from pig plasma by ultracentrifugal flotation between densities 1.063 and 1.210 g per ml. After delipidation, the apolipoproteins were fractionated by chromatography on Sephadex G-150 and DEAE-cellulose in 5.4 m urea. One major apolipoprotein was isolated and characterized. From its chemical and physical properties, this major apolipoprotein appears very similar to apoLP-Gln-I from human HDL. Pig HDL appears to differ from human HDL in a marked decrease in the relative content of apoLP-Gln-II in the pig lipoprotein family. A distinct peak associated with apoLP-Gln-II was not identified by chromatography of pig apoHDL. A small quantity of a protein with the same mobility of human apoLPGln-II was detected in pig apoHDL after polyacrylamide gel electrophoresis in sodium dodecyl sulfate. The major apolipoprotein from pig HDL had an approximate molecular weight of 26,000 as determined by polyacrylamide gel electrophoresis in sodium dodecyl sulfate. Its calculated molecular weight from amino acid analysis was 29,800. Rabbit antisera prepared against the pig apolipoprotein formed precipitin lines of partial identity between this protein and apoLP-Gln-I from human HDL. Treatment of the pig apolipoprotein with carboxypeptidase established Leu-Gln as the COOH-terminal sequence. Pig and human apoLPGln-I had similar amino acid compositions, but the pig protein contained isoleucine and had 12 more residues of alanine than did human apoLP-Gln-I. Pig and human apoLP-Gln-I had indistinguishable absorption spectra and similar circular dichroism spectra indicating a relatively high content of α-helical conformation. Both proteins activated human lecithin : cholesterol acyltransferase. These studies demonstrate that the major apolipoprotein from human and pig HDL have similar chemical, immunological, physical, and physiological properties.