Isoproteins of human apolipoprotein A-II: isolation and characterization

In human serum, polymorphism of apoA-II predominantly in HDL3 could be demonstrated. HDL3-apoA-II was composed of four isoproteins, each with a molecular weight of 8600 (reduced form) and identical immunological properties. The isoproteins are designated apoA-II-1 (pI 5.16), apoA-II-2 (pI 4.89) corresponding to the already known apoA-II monomer band, apoA-II-3 (pI 4.58), and apoA-II-4 (pI 4.31). The amino acid compositions of the A-II isoproteins were virtually identical with the published data for apoA-II. Treatment with acid phosphatase, alkaline phosphatase, or neuraminidase before electrophoresis did not alter the apoA-II pattern. The apoA-II isoprotein pattern was studied in ten male and ten female normolipidemic volunteers, in two patients with Tangier disease, and in three patients with abetalipoproteinemia. The isoelectric focusing patterns of apoA-II appeared virtually identical in all subjects. However, in Tangier disease, due to the low apo-A-II concentration, only apoA-II-1 and apoA-II-2 were detectable, and in abetalipoproteinemia a different relative distribution pattern of the individual isoforms was found as compared to normal HDL3. Our studies indicate that apoA-II, similar to apoA-I, exists in several isoforms. The relationship of these isoforms to each other is at present unclear. They may originate from relatively basic isoproteins that are modified in charge by post-translational processes such as proteolytic cleavage, sequential deamidation, or other mechanisms.