Appearance of sodium dodecyl sulfate-stable amyloid beta-protein (A beta) dimer in the cortex during aging

We previously noted that some aged human cortical specimens containing very low or negligible levels of amyloid beta-protein (A beta) by enzyme immuno assay (EIA) provided prominent signals at 6 similar to 8 kd on the Western blot, probably representing sodium dodecyl sulfate (SDS)-stable A beta dime r, Re-examination of the specificity of the EIA revealed that BAN50- and BN T77-based EIA, most commonly used for the quantitation of A beta, capture S DS-dissociable A beta but not SDS-stable A beta dimer. Thus, all cortical s pecimens in which the levels of A beta were below the detection limits of E IA were subjected to Western blot analysis. A fraction of such specimens co ntained SDS-stable dimer at 6 similar to 8 kd, but not SDS-dissociable A be ta monomer at similar to 4 kd, as judged from the blot. This A beta dimer i s unlikely to be generated after death, because (i) specimens with very sho rt postmortem delay contained the A beta dimer, and (ii) until 12 hours pos tmortem, such SDS-stable A beta dimer is detected only faintly in PDAPP tra nsgenic mice. The presence of A beta dimer in the cortex may characterize t he accumulation of A beta in the human brain, which takes much longer than that in PDAPP transgenic mice.