Characterization of apolipoprotein A-IV complexes and A-IV isoforms in human lymph and plasma lipoproteins

We have isolated and characterised A-IV apolipoprotein (apo-A-IV) from human lymph and plasma by immunoabsorbance chromatography and two-dimensional electrophoresis. Two different apo-A-IV-containing lipoproteins were isolated from four different sources, human lymph triglyceride-rich fraction (TRL), lymph lipoprotein-deficient fraction (LDF), plasma high-density lipoprotein (HDL), and plasma lipoprotein-deficient fraction (LDF). The lipoprotein complexes obtained from lymph TRL and plasma HDL were similar and contained apo-A-IV, apo-A-I, and small molecular weight peptides (apo-C or -A-II). The second lipoprotein complex was isolated from lymph LDF and plasma LDF, and contained apo-A-IV, apo-A-I, and a peptide of Mr = 59,000. The lipid composition of the lipoprotein complexes varied according to the source: triglyceride predominating in lymph TRL and phospholipid and cholesteryl ester from the other sources. Free cholesterol was conspicuously present in very small amounts. Using two-dimensional electrophoresis and immunoblotting techniques, eleven isoproteins of apo-A-IV were identified (pI-4.98, 5.06, 5.10, 5.15, 5.20, 5.22, 5.25, 5.30, 5.34, 5.42, and 5.48). The isoprotein pattern of lymph TRL and plasma HDL was similar, but that of lymph and plasma LDF were different patterns. These results suggest that apo-A-IV associated with d less than 1.21 lipoproteins and apo-A-IV present in LDF may be in metabolically separate lipoproteins and may have different physiological roles.