Isolation and subunit composition of membrane inorganic pyrophosphatase from rat-liver mitochondria

The activity of inorganic pyrophosphatase (pyrophosphate phosphohydrolase, EC 3.6.1.1) extracted from rat-liver mitochondria with sodium cholate is distributed between three enzyme forms (I-III) which originally reside in the matrix (I) or the inner membrane (II and III). The three forms of pyrophosphatase were separated by means of hydroxyapatite chromatography. The Mr values for enzymes I-III, were 60 000, 120 000 and 210 000, respectively. Membrane pyrophosphatase II, which predominates, was purified to homogeneity. Its molecule consists of two subunits with Mr of 28 000 and 35 000 giving different peptide maps upon BrCN cleavage and polyacrylamide gel electrophoresis. The results suggest that pyrophosphatase II may represent a catalytic part of the membrane pyrophosphatase of these mitochondria.