Inorganic pyrophosphate (PPi ) has long been known as a by-product of many intracellular biosynthetic reactions. PPi plays the regulatory role in living systems, such as activities of enzymes, fidelity of syntheses of macromolecules, and proliferation of cells. PPi is used as a biochemical energy source instead of ATP especially in bacteria, protists, and plants. PPi may also regulate the formation and dissolution of bone as well as pathologic calcification of soft tissues and the formation of urinary stones. The formation of calcium pyrophosphate dihydrate crystals in the extracellular fluids of joints cause the disease called pseudogout. Sodium, potassium and calcium pyrophosphates (E450) are used as food additives as buffers and emulsifiers.