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Serum amyloid P component (SAP), a normal glycoprotein, is universally found in amyloid deposits, including cerebrovascular amyloid of Alzheimer's disease. This paper describes the Ca(2+)-dependent binding of human SAP to Alzheimer's beta-amyloid peptide (A beta). 125I-SAP binds to synthetic human A beta-(1-40) immobilized on microtiter plates at a dissociation constant of 6.0 x 10(-9) M in 0.01 M Tris-HCl, 0.15 M NaCl, pH 7.5, containing 2 mM Ca2+, 1% bovine serum albumin, and 0.05% Tween 20. Binding inhibition assay has shown that soluble A beta-(1-40) and A beta-(1-28) also bind to SAP. Since SAP is resistant to proteases in the presence of calcium, the Ca(2+)-dependent binding of SAP to soluble A beta and to beta-amyloid fibrils would give pathological effects on fibril formation and persistence of beta-amyloid in Alzheimer's disease.

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