oxidizes phenolic compounds in the presence of oxygen, turning apples and potatoes brown

The protein, PpO (also known as polyphenol oxidase), is an enzyme which oxidizes phenol-containing compounds into o-quinones, which are rapidly converted to brown-colored products. PpO is found naturally in apples and potatoes and other plants and results in the rapid brown discoloration after the interiors are exposed to oxygen.

The USDA has approved sales of apples with reduced PpO expression developed by the company Arctic Apples.1