All proteins in mammalian cells are continually being degraded and replaced. In the cytosol, nucleus, and organelles, individual proteins are degraded at widely differing rates; some cytosolic enzymes have half-lives as short as 10 minutes, whereas others last for days. The average rate of protein turnover also varies among tissues; the majority of proteins in rat hepatocytes are replaced every few days, whereas those in muscle or brain cells are replaced every one to two weeks. The amount of intracellular protein turned over each day is quite large. In a normal 70-kg adult, about 280 g of protein is synthesized . . .