Isolation and characterization of human apolipoprotein A-IV from lipoprotein-depleted serum

Human apolipoprotein A-IV is an acidic polypeptide of molecular weight 46,000 that is secreted into lymph on the surface of nascent chylomicrons, but which exists in circulation unassociated with lipoproteins. Previous studies of this protein have utilized material isolated from a d < 1.006 g/ml fraction of human serum and from human lymph. Although it has been suggested that apoA-IV circulating in plasma is the product of dissociation from the surface of nascent chylomicrons, it has been characterized by immunological techniques only. We have isolated human apoA-IV on a preparative scale from lipoprotein-depleted serum using a technique of adsorption to a phospholipid-triglyceride emulsion, followed by delipidation and preparative gel electrophoresis. The molecular weight, pI, and amino acid composition of material thus prepared agree with literature values for apoA-IV derived from chylomicrons. We have determined that apoA-IV is a glycoprotein containing 6% carbohydrate by weight (mannose 1.8%, galactose 1.55%, N-acetyl glucosamine 1.55%, sialic acid 1.1%). Electroimmunoassay of human serum using a monospecific rabbit antibody to serum-derived apoA-IV found 13.1 +/- 1.8 mg/dl, a value in agreement with determinations using antibodies to chylomicron-derived apoA-IV. We conclude that apoA-IV may be easily purified from normal human serum, and that the material thus isolated has identical chemical, physical, and immunological properties to apoA-IV obtained from human lymph. It is therefore likely that the dissociation of apoA-IV from the surface of nascent chylomicrons following their entry into circulation is not attended by changes in the structure or composition of this apoprotein.-Weinberg, R. B., and A. M. Scanu. Isolation and characterization of human apolipoprotein A-IV from lipoprotein-depleted serum.