ATPase-ADPase activities of rat placental tissue.

1. Calcium-stimulated ATPase-ADPase activities were studied in a microsomal fraction of rat placental tissue. 2. The kinetic characteristics correspond to those of ATP-diphosphohydrolase, also known as apyrase (E.C. 3.6.1.5). 3. These characteristics include the lack of specificity towards nucleoside di- and triphosphates, activation by Ca2+, Mg2+ or Mn2+, insensitivity to specific inhibitors of some ATPase and absence of an effect of sulphydryl reagents. 4. Chemical modification of tyrosine, tryptophan, arginine and carboxylic residues decreases both ATPase and ADPase activities. 5. The substrate analogue, 5'-(beta, gamma-methylene)triphosphate, protected both enzyme activities against all the modifying amino acid reagents tested. 6. Placental fractions (homogenate and microsomes) inhibit ADP-dependent platelet aggregation. 7. The solubilized microsomal enzyme has a molecular mass of 67 kDa by size-exclusion chromatography; the pI is 9.36. 8. A differential effect is observed on the activation produced by Concanavalin A on microsomal and solubilized fractions when treated in the presence and absence of alpha-methylmannoside.