Human proapolipoprotein A-I: development of an antibody to the propeptide as a probe of apolipoprotein A-I biosynthesis and processing.

In human plasma, apolipoprotein A-I is present as pro and mature isoproteins. The development of a highly specific antibody to the pro isoprotein of apoA-I has been difficult due to the close structural similarity between the pro and mature isoforms of apoA-I. To sermount this difficulty, a peptide was synthesized by the solid phase method which corresponded to the amino acid sequence present in the pro region of apoA-I. The synthetic peptide was coupled to serum albumin and the conjugate utilized to immunize rabbits for antibody production. Immunoblot analysis of purified proapoA-I and mature apoA-I revealed that the antibody was specific for the propeptide of apoA-I. Analysis of apoA-I in the plasma from a Tangier disease patient and newly secreted apoA-I from HepG2 cells clearly demonstrated the isoforms which contained the proisoprotein. The proapoA-I specific antibody should prove to be a useful tool in developing a radioimmunoassay for quantitation of the proisoprotein in plasma, isolation of proapoA-I from normal and dyslipoproteinemic subjects by immunoaffinity chromatography and in studies related to the synthesis and processing of apoA-I.