The properties of a highly purified inorganic pyrophosphatase (pyrophosphate phosphohydrolase; EC 3.6.1.1) from pig scapula cartilage were studied. The enzyme had a molecular weight of 66 000 and a pH optimum of 7-8. It was markedly activated by magnesium, but not, or only to a much smaller degree, by other metal ions. PP1 was the only substrate found and had a Km value of 11 muM. The enzyme was not inhibited by phosphate and other inhibitors of alkaline phosphatase such as CN- minus, amino acids and theophylline; it was slightly inhibited by tartrate, formaldehyde and ammonium molybdate and strongly inhibited by F- minus, Ca2+ and other metal ions. The properties of the enzyme in the presence of concentrations of PP1 present in plasma (3.5 muM) were similar to those found at higher (2 mM) concentrations of PP1. The diphosphonates ethane-1-hydroxy-1,1-diphosphonate and dichloromethylenediphosphonate inhibited the enzyme in the presence of low PP1 concentrations. The characteristics of this enzyme are therefore similar to pyrophosphatases from other sources, such as from yeast and erythrocytes, and do not support a specific role of this enzyme in the calcification process.