What is the name of the enzyme that causes browning in cut apples?

The enzyme that causes browning in cut apples is polyphenol oxidase, commonly abbreviated PPO.

Cutting an apple breaks cellular compartments and exposes PPO and its phenolic substrates to oxygen. PPO catalyzes oxidation reactions that produce o-quinones; those reactive compounds then combine and polymerize into the brown pigments visible on the cut surface.

PPO is a copper-containing enzyme found in the chloroplasts and mitochondria of apples. It is not unique to apples: related enzymes contribute to browning in avocados, bananas, potatoes, mushrooms, and lettuce. Browning is usually a quality problem in prepared fruit, although similar reactions are deliberately used in products such as black tea and some dried fruits.

Acidity, antioxidants, refrigeration, and limiting oxygen can slow the process. Lemon juice works partly because its acidity inhibits PPO and its vitamin C reduces oxidized compounds. Catalase, amylase, and lipase perform different biological jobs and are not the principal enzyme responsible for this familiar browning reaction.