Proline was chemically synthesized in 1900 and was isolated from the milk protein casein in 1901. Proline is not essential for humans as it can be formed in the body from glutamic acid. Conversely, arginine can also be formed from proline with the help of ornithine. Proline plays an important role in the formation of collagen, the protein in connective tissue and bones. This consists of 21 percent proline and its derivative hydroxyproline and is also found in other tissue fibers. During the formation of collagen, proline is partly converted into hydroxyproline; vitamin C is required for this process.
