Purification and characterization of the plastidial inorganic pyrophosphatase from Dunaliella salina

Chloroplastic inorganic pyrophosphatase (PPase, EC 3.6.1.1) has been isolated from the unicellular green alga Dunaliella salina. Immunofluorescence, used for the first time to localize PPase, showed that this enzyme is plastidial and is found specially around the pyrenoid. The enzyme was purified to electrophoretic homogeneity by combining non-denaturing electrophoresis and electroelution. It is probably a monomeric protein with an apparent molecular mass of about 60 kDa. This soluble PPase shows an absolute requirement for Mg2+ and a high specificity for PPi. The maximal rate of PPi hydrolysis occurs for Mg2+ concentration of 4 mM and pH of 7.5-8.5, values corresponding to those found in the chloroplast of various species. All these results indicate that the PPase is perfectly efficient in vivo , allowing the immediate hydrolysis of PPi, even when released in high amounts. The possibility of a light-dependent regulation of the PPase activity via intraplastidial changes in the pH or in the Mg2+ concentration is considered.