Overlapping drug interaction sites of human butyrylcholinesterase dissected by site-directed mutagenesis. Enzyme-linked immunosorbent assays for acetylcholinesterase (AChE) and for butyrylcholinesterase (BCHE) were markedly more specific than conventional assays using selective enzyme inhibitors. Butyrylcholinesterase in human brain and acetylcholinesterase in human plasma: trace enzymes measured by two-site immunoassay.