Thus, phosphocreatine breaks down to creatine, giving its inorganic phosphate for ATP formation. The functions of these isoenzymes are to convert creatine, using ATP, into phosphocreatine expelling ADP. The quick energy sources consist of the phosphocreatine (PCr) system, fast glycolysis, and adenylate kinase. This makes conversion of creatine to phosphocreatine a highly favored reaction. Possible physiological advantages from RSH include compensatory vasodilation and regeneration of phosphocreatine (PCr). At the onset of exercise phosphocreatine is broken down to provide ATP for muscle contraction. This is part of phosphocreatine metabolism. The discovery of phosphocreatine was reported in 1927. Another was Otto Folin, an American chemist who discovered Phosphocreatine. Phosphocreatine is a very-high-energy compound.