Beta-secretase: An enzyme that appears to be directly involved in the early development of Alzheimer's disease. Beta-secretase is a protease (an enzyme that catalyses the splitting of interior peptide bonds in a protein). Beta-secretase acts by trimming off a protein protruding from a brain cell. This small snip is thought to be the first step in the buildup of microscopic balls of debris known as amyloid that are toxic to brain cells. The brain cells (neurons) gradually die and with them slowly go the person's memory and other mental faculties.