High molecular weight Alzheimer's disease amyloid peptide immunoreactivity in human serum and CSF is an immunoglobulin G

Abstract

A radioimmunoassay (RIA) was developed to detect the 4200 Dalton amyloid (A 4) peptide or it's precursor (A 4P) in human serum or cerebrospinal fluid (CSF). A synthetic peptide containing the first 28 amino acids of the 43 amino acid A 4 peptide was covalently coupled to bovine thyroglobulin and a polyclonal antiserum in rabbits was prepared. This antiserum was specific for vascular amyloid and neuritic plagues in Alzheimer's disease brain as detected by immunoperoxidase. The synthetic peptide, which has a tyrosine at residue 10, was iodinated with chloramine T and [ 125I]iodine and was purified to homogeneity by C 4 reverse phase high performance liquid chromatography (HPLC). Extraction of human serum over a C 18 Sep Pak cartridge indicated immunoreactive A 4 peptide was not detectable in human serum. Conversely, high molecular weight A 4 peptide immunoreactivity was detectable in human serum, at a concentration of 8.9 ± 1.2 pmol-eq./ml, and in human CSF, at a concentration of 0.25 ± 0.01 pmol-eq./ml, giving a CSF/serum ratio of 3.2%. The immunoreactivity in human serum was nearly completely removed by affinity deletion of serum immunoglobulin G (IgG), but not by affinity removal of IgA or IgM. Serum immunoreactivity was decreased 90% in hypogammaglobulinemia, and was increased 83% in human cord serum. There was no statistical difference in serum A 4 immunoreactivity in Alzheimer's serum or CSF. Serum immunoreactivity in Down's syndrome was increased 50%. These studies indicate the high molecular weight A 4P immunoreactivity in human serum or CSF is an IgG. Whether the A 4 precursor in Alzheimer's disease is, in fact, an IgG, or whether there is an antibody in human serum and CSF that cross reacts with the A 4 precursor cannot be determined until the serum immunoreactivity is purified and structurally characterized.