Creatine (Methylguanidoacetic acid) is an amino acid that occurs in vertebrate tissues and in urine. In muscle tissue, creatine generally occurs as phosphocreatine. Creatine is excreted as creatinine in the urine. Creatine functions as part of the cell's energy shuttle. The high energy phosphate group of ATP is transferred to creatine to form phosphocreatine in the following reaction: Cr + ATP <-> PCr + ADP. This reaction is reversibly catalyzed by creatine kinase. In the human body, creatine is synthesized mainly in the liver by the use of parts from three different amino acids: arginine, glycine, and methionine. 95% of it is later stored in the skeletal muscles and the rest is stored in the brain, heart, and testes.