The lipid transport system in the mouse, Mus musculus: isolation and characterization of apolipoproteins B, A-I, A-II, and C-III

Abstract

Four of the principle apolipoproteins of murine serum have been isolated and characterized. On the basis of their physicochemical properties, they are homologous with the human and rat apoA-I, A-II, B, and C-III. The group of apolipoproteins of middle to low molecular weight, i.e., A-I, A-II and C-III, were separated from the protein moiety of high density lipoproteins (HDL) by gel filtration chromatography, followed by electrophoresis in alkaline-urea polyacrylamide gel with electrophoretic elution. Murine apoA-I, the major protein of HDL (60-80%) displayed an Mr of approximately 27,000, and was polymorphic (four prominent isoproteins with isoelectric points in the range of pH 5.5-5.7). The amino acid profiles of mouse, rat, and human apoA-I generally resembled each other, the former being distinguished by a content of one isoleucine residue per mole. Amino terminal sequence analysis revealed marked homology between the mouse, rat, dog, and human proteins; mouse and rat apoA-I differed at residues 9 and 18 with potential dissimilarities at residues 5 and 15, while the murine and canine sequences were distinct at residues 6, 9, 13, 15, and 30. Apolipoprotein A-II was a monomer, exhibiting an Mr approximately 11,000 in SDS gels; in addition, it was polymorphic (three apparent isoproteins with pI in the pH range 5.05-5.2), and resembled its human and rat counterparts in amino acid composition. ApoC-III, an acidic peptide of pI 4.74 and of Mr approximately 9,600, possessed an amino acid composition very like that of the homologous human and rat proteins. The homology of mouse apoC-III with the human protein was confirmed by NH2-terminal sequence analysis, which revealed identical amino acids in six positions (1, 2, 4, 8, 9, and 13). As shown earlier (Camus et al. 1983. J. Lipid Res. 24: 1210-1228), two forms of immunologically reacting apoB predominated in mouse VLDL and LDL. After isolation of these lipoproteins in the presence of 1 mM PMSF, the apparent sizes of the high and low Mr forms, apoBH and apoBL, were in the ranges approximately 400,000-530,000 and approximately 250,000-280,000, respectively, according to the SDS gel system. We observed that inclusion of 1 mM PMSF was essential to retard degradation of the high Mr form apoBH. The murine B proteins were isolated from apoVLDL and apoLDL by gel filtration chromatography on Sephadex G150 in anionic detergent, and displayed apparent Mr values of 460,000 (apoBH) and 250,000 (apoBL) in 3% SDS gels.(ABSTRACT TRUNCATED AT 400 WORDS)