Rank-order of potencies for inhibition of the secretion of A beta 40 and Abeta 42 suggests that both are generated by a single gamma-secretase

The Alzheimer's disease amyloid peptide A beta has a heterogeneous COOH ter minus, as variants 40 and 42 residues long are found in neuritic plaques an d are secreted constitutively by cultured cells, The proteolytic activity t hat liberates the A beta COOH terminus from the beta-amyloid precursor prot ein is called gamma-secretase. It could be one protease with dual specifici ty or two distinct enzymes. By using enzyme-linked immunosorbent assays sel ective for A beta 40 or A beta 42, we have measured A beta secretion by a H eLa cell line, and we have examined the dose responses for a panel of five structurally diverse gamma-secretase inhibitors. The inhibitors lowered A b eta and p3 secretion and increased levels of the COOH-terminal 99-residue b eta-amyloid precursor protein derivative that is the precursor for A beta b ut did not alter secretion of beta-amyloid precursor protein derivatives ge nerated by other secretases, indicating that the inhibitors blocked the gam ma-secretase processing step. The dose dependent inhibition of A beta 42 wa s unusual, as the compounds elevated A beta 42 secretion at sub-inhibitory doses and then inhibited secretion at higher doses. A compound was identifi ed that elevated A beta 42 secretion at a low concentration without inhibit ing A beta 42 or A beta 40 at high concentrations, demonstrating that these phenomena are separable pharmacologically, Using either of two methods, IC 50 values for inhibition of A beta 42 and A beta 40 were found to have the same rank order and fall on a trend line with near-unit slope. These result s favor the hypothesis that A beta variants ending at residue 40 or 42 are generated by a single gamma-seeretase.