Cytoplasmic inorganic pyrophosphatase of rat liver can be phosphorylated by cAMP-dependent protein kinase on a serine residue with a concomitant increase in enzymic activity. Phosphorylation is also observed in the absence of protein kinase, but in this case much higher concentrations of ATP are required and the stability characteristics of the phosphoenzyme resemble those of an acyi phosphate. Kinase-free phosphorylation of the animal inorganic pyrophosphatase, unlike that of microbial pyrophosphatases, does not activate the enzyme. Pyrophosphatase may thus provide a new example of an enzyme whose evolution involves convergence of regulatory phosphorylation mechanisms.
