Studies on the apoproteins of rat lymph chylomicrons: characterization and metabolism of a new chylomicron-associated apoprotein.

The apoprotein composition of rat lymph chylomicrons and very low density lipoprotein (VLDL) was investigated following isolation and purification by preparative gel electrophoresis. The medium molecular weight apoproteins in the 27,000-60,000 range were characterized by electrophoretic mobility, immunochemical identification, amino acid composition, and molecular weight determination. In addition to three previously identified apoproteins, A-I, E, and A-IV, present in both rat serum high density lipoproteins (HDL) and lymph chylomicrons, a fourth peptide of molecular weight 59,000 was always a consistent major component of lymph VLDL washed free of serum protein contaminants by repeated ultracentrifugation. The protein formed complexes with phosphatidylcholine vesicles, was amphipathic in nature, and, when injected into rats, became associated with serum HDL and lymph chylomicrons. It was removed from chylomicrons after gel filtration through agarose columns but not after repeated ultracentrifugation; it differed slightly in amino acid composition and did not show immunochemical identity against antisera to any other apoprotein. The results of in vivo studies suggest that the protein preferentially associates with lymph chylomicrons and thus may play some important role in the metabolism of exogenous triglyceride transport. Since A-I, A-II, and A-IV apoproteins are also integral components of lymph chylomicrons, we suggest that the protein be designated A-V apoprotein. The distribution of A-I, E, A-IV, and A-V in serum HDL and lymph VLDL was approximately 54, 10, 12, and 0%, and 22, 3, 19, and 22%, prospectively.