Methionine is an essential amino acid in humans and our fur kids. As the substrate for other amino acids such as cysteine and taurine, versatile compounds such as SAM-e, and the important antioxidant glutathione, methionine plays a critical role in the metabolism and health of many species. Methionine is an amino acid found in many proteins, including the proteins in foods and those found in the tissues and organs of your body. In addition to being a building block for proteins, it has several other unique features. It also plays a critical role in starting the process of making new proteins inside your cells, something that is continuously occurring as older proteins break down (see: Wikipedia).