Catalytic properties of the inorganic pyrophosphatase in rat liver mitochondria

Human mitochondrial pyrophosphatase: cDNA cloning and analysis of the gene in patients with mtDNA depletion syndromes

Pyrophosphatases (PPases) catalyze the hydrolysis of inorganic pyrophosphate generated in several cellular enzymatic reactions. A novel human pyrophosphatase cDNA encoding a 334-amino-acid protein approximately 60% identical to the previously identified human cytosolic PPase was cloned and characterized. The novel enzyme, named PPase-2, was enzymatically active and catalyzed hydrolysis of pyrophosphate at a rate similar to that of the previously identified PPase-1. A functional mitochondrial import signal sequence was identified in the N-terminus of PPase-2, which targeted the enzyme to the mitochondrial matrix. The human pyrophosphatase 2 gene (PPase-2) was mapped to chromosome 4q25 and the 1.4-kb mRNA was ubiquitously expressed in human tissues, with highest levels in muscle, liver, and kidney. The yeast homologue of the mitochondrial PPase-2 is required for mitochondrial DNA maintenance and yeast cells lacking the enzyme exhibit mitochondrial DNA depletion. We sequenced the PPA2 gene in 13 patients with mitochondrial DNA depletion syndromes (MDS) of unknown cause to determine if mutations in the PPA2 gene of these patients were associated with this disease. No pathogenic mutations were identified in the PPA2 gene of these patients and we found no evidence that PPA2 gene mutations are a common cause of MDS in humans.

Method for real-time detection of inorganic pyrophosphatase activity

A sensitive and simple method for real-time detection of inorganic pyrophosphatase (PPase) (EC 3.6.1.1) activity has been developed. The method is based on PPase-induced activation of the firefly luciferase activity in the presence of inorganic pyrophosphate (PPi). PPi inhibits the luciferase activity, but in the presence of PPase the luciferase activity is restored and the luminescence output increases. The assay yields linear responses between 8 and 500 mU. The detection limit was found to be 8 mU PPase. The method was used to detect the hydrolytic activity of PPases from Saccharomyces cerevisiae, Escherichia coli, and Bacillus stearothermophilus. As substrate for the luciferase, adenosine 5'-phosphosulfate can replace ATP, which is an advantage for detection of PPase activity in crude extracts containing ATP-hydrolyzing activities. The method can be used for kinetic and inhibition studies as well as for detection of PPase activity during different purification procedures.

Allosteric regulation by Mg2+ of the vacuolar H(+)-PPase from Acer pseudoplatanus cells. Ca2+/Mg2+ interactions

The tonoplast H(+)-PPase was previously characterized in Acer pseudoplatanus cells (Pugin et al (1991) Plant Sci 73, 23-34; Fraichard et al (1993) Plant Physiol Biochem 31, 349-359). Tonoplast vesicles were obtained from vacuoles isolated from protoplasts of A pseudoplatanus suspension cultures and used to study kinetic effects of Mg2+ and Ca2+ on PPi hydrolysis. The concentrations of ionic species (free Mg2+, free PPi, and MgPPi complexes) were calculated with apparent dissociation constants of 55.3 microM for MgPPi and 59.6 microM for CaPPi. Our results indicated that the substrate of the tonoplast PPase was a MgPPi complex and that free Mg2+ was essential for PPi hydrolysis. With fixed free Mg2+ concentrations, PPase activity showed Michaelis-Menten kinetics with respect to MgPPi. Moreover, free Mg2+ acted as an allosteric activator with a Hill coefficient of 2.4, indicating at least two Mg2+ binding sites on the enzyme. The Mg-imidodiphosphate complex was a competitive inhibitor of the substrate MgPPi but did not change significantly the allosteric activation by free Mg2+. This result confirmed the presence of Mg2+ regulatory sites. Ca2+ acted as an uncompetitive inhibitor of MgPPi hydrolysis. Furthermore, the sensitivity of the H(+)-PPase to Ca2+ increased with decrease in free Mg2+ concentration. Therefore, Ca2+ and Mg2+ may compete for a common binding site. Taken together, our results confirm that activation by free Mg2+ and inhibition by Ca2+ could be involved in the regulation of the PPase activity in vivo.

Pyrophosphate metabolism in the perfused heart and isolated heart mitochondria and its role in regulation of mitochondrial function by calcium

1. Langendorff-perfused rat hearts were treated, either alone or in combination, with acetate, adrenaline or supraphysiological [Ca2+] to mimic increased workload. No significant increases in the PPi content of the freeze-clamped tissue were observed. 2. In contrast with liver mitochondria, isolated rat heart mitochondria incubated at a free [Mg2+] of 1.5 mM showed little or no increase in matrix PPi content or volume in the presence of 0.75 microM Ca2+; neither did 5 mM acetate cause heart mitochondrial PPi to increase. 3. At 0.1 mM Mg2+ some increase in matrix PPi content of heart mitochondria was observed, and increases in liver mitochondria were significantly greater. 4. Substantial increases in heart mitochondrial-matrix PPi content were observed at 1.5 mM Mg2+ when [Ca2+] > 1 microM, especially in the presence of acetate, or at 0.75 microM Ca2+ in the presence of bongkrekic acid to inhibit the adenine nucleotide translocase. 5. Total pyrophosphatase activity was similar in heart and liver mitochondrial matrix extract and toluene-permeabilized mitochondria. 6. These data suggest that under physiological conditions Ca2+ does not regulate heart mitochondrial [PPi], because higher matrix [Mg2+] overcomes the inhibition of pyrophosphatase by Ca2+, and also significant loss of PPi from the matrix on the adenine nucleotide translocase may occur. 7. It is concluded that, in contrast with the situation in liver [Halestrap (1989) Biochim. Biophys. Acta 973, 355-382], Ca(2+)-induced PPi-mediated increases in mitochondrial volume are unlikely to play a physiological role in the heart. 8. The substantial swelling of liver and kidney mitochondria caused by addition of supraphysiological [Ca2+] (20 nmol/mg of protein) was associated with significant increases in PPi content, but was not observed in heart mitochondria. 9. This substantiates the role that we have proposed for PPi in the opening of a non-specific pore by Ca2+ overload of mitochondria [Halestrap and Davidson (1990) Biochem. J. 268, 153-160].