The anion  is abbreviated PP, standing for inorganic pyrophosphate. It is formed by the hydrolysis of ATP into AMP in cells.
:ATP → AMP + PP
For example, when a nucleotide is incorporated into a growing DNA or RNA strand by a polymerase, pyrophosphate (PP) is released. Pyrophosphorolysis is the reverse of the polymerization reaction in which pyrophosphate reacts with the 3′-nucleosidemonophosphate (NMP or dNMP), which is removed from the oligonucleotide to release the corresponding triphosphate (dNTP from DNA, or NTP from RNA).
The pyrophosphate anion has the structure , and is an acid anhydride of phosphate. It is unstable in aqueous solution and hydrolyzes into inorganic phosphate:
or in biologists' shorthand notation:
In the absence of enzymic catalysis, hydrolysis reactions of simple polyphosphates such as pyrophosphate, linear triphosphate, ADP, and ATP normally proceed extremely slowly in all but highly acidic media.
(The reverse of this reaction is a method of preparing pyrophosphates by heating phosphates.)
This hydrolysis to inorganic phosphate effectively renders the cleavage of ATP to AMP and PP irreversible, and biochemical reactions coupled to this hydrolysis are irreversible as well.
PP occurs in synovial fluid, blood plasma, and urine at levels sufficient to block calcification and may be a natural inhibitor of hydroxyapatite formation in extracellular fluid (ECF). Cells may channel intracellular PP into ECF. ANK is a nonenzymatic plasma-membrane PP channel that supports extracellular PP levels. Defective function of the membrane PP channel ANK is associated with low extracellular PP and elevated intracellular PP. Ectonucleotide pyrophosphatase/phosphodiesterase (ENPP) may function to raise extracellular PP.
From the standpoint of high energy phosphate accounting, the hydrolysis of ATP to AMP and PP requires two high-energy phosphates, as to reconstitute AMP into ATP requires two phosphorylation reactions.
:AMP + ATP → 2 ADP
:2 ADP + 2 P → 2 ATP
The plasma concentration of inorganic pyrophosphate has a reference range of 0.58–3.78 µM (95% prediction interval).