Many biosynthetic reactions are coupled to the cleavage of ATP to yield AMP and pyrophosphate. These reactions are typically freely reversible when carried out with purified substrates and enzymes in vitro. In vivo, however, the pyrophosphate is rapdily and essentially irreversibly hydrolyzed by a ubiquitous inorganic pyrophosphatase. This hydrolysis has the effect of pulling the first reaction strongly in the direction of biosynthesis, at the expense of two high-energy phosphate bonds. Studies of human cells have identified two forms of the enzyme, one localized to the cytosol and the other to the mitochondrial matrix (Raja et al. 1981).