Misshapen proteins called prions lie at the root of mad cow disease and similar brain ailments, but the role of these molecules in their normal form remains unclear. In humans, normal prion proteins may generally protect against Alzheimer's disease. In Alzheimer's, abnormally folded beta-amyloid protein accumulates in the brain. Biochemist Nigel Hooper of the University of Leeds in England and his colleagues found that high levels of normal prion proteins in human cells prevent beta-amyloid formation by inhibiting an enzyme called beta-secretase. The brains of mice genetically modified to lack normal prion proteins had significantly higher beta-amyloid levels. Hooper says that prion proteins might prevent Alzheimer's from occurring earlier than it usually does in life or that perhaps they guard against oxidative stress, which has been linked to Alzheimer's and other neurodegenerative diseases. Further research could lead to new drugs that target beta-secretase, he adds. The team reports its findings in the June 26 Proceedings of the National Academy of Sciences USA.