The essential activated carboxyl group of inorganic pyrophosphatase.

1. A carboxyl group of high reactivity has been found in inorganic pyrophosphatase (pyrophosphate phosphohydrolase, EC 3.6.1.1) from yeast. This group interacts with agents which react neither with carboxyl groups of low molecular weight compounds nor with other carboxyl groups of the protein. 2. The reaction of this activated carboxyl group with inorganic phosphate, hydroxylamine, N-methyl- and O-methylhydroxylamines, and glycine methyl ester has been studied. 3. Homoserine and homoserine lactone were found in the hydrolyzate of phosphorylated and NaBH4-reduced pyrophosphatase, indicating that an aspartyl residue is phosphorylated. 4. Hydroxylamine and other nucleophilic agents cause inactivation of pyrophosphatase as a result of interaction with a carboxyl group. Both diaminobutyric and diaminopropionic acids were seen in the acid hydrolyzate of the protein treated with hydroxylamine and subjected to rearrangement in the presence of carbodiimide. 5. The ways in which the activation of a carboxyl group in the enzyme is achieved and the presumed mechanism of action of inorganic pyrophosphatase are discussed.