A Mg2+ dependent inorganic pyrophosphatase from chloroplasts of Sorghum vulgare has been purified 275-fold to electrophoretic purity with an overall recovery of about 25% activity. Estimations of native and monomeric relative molecular weights by size exclusion chromatography and denaturing electrophoresis suggest that the holoenzyme is a monomer of 42 +/- 1.5 kDa. A high specificity for tetrasodium pyrophosphate (PPi) as substrate has been observed, as the other phosphoesters tested were virtually unaffected. The Mg2+:PPi ratio of 5:1 at pH 8.0 shifts to 2.5:1.0 at pH 9.0 and 10:1 at pH 7.0. None of the divalent cations tested could substitute for Mg2+. Further, in the presence of Mg2+, these divalent cations inhibit the catalytic hydrolysis of PPi. EDTA rapidly and irreversibly inactivates the purified enzyme in a biphasic manner. Of the metabolites tested, Pi and L-malate significantly inhibited the catalytic activity of the enzyme. Malate inhibits the enzyme through an allosteric mechanism. A Hill plot of this inhibition shows that at least two molecules of malate bind to each molecule of the purified enzyme. The likely physiological significance of this result is discussed.