HUMAN PLACENTAL ATP-DIPHOSPHOHYDROLASE - BIOCHEMICAL-CHARACTERIZATION, REGULATION AND FUNCTION

1. Kinetic and physico-chemical studies on human placental microsomal fraction confirmed that the ATPase and ADPase activities detected in t his fraction correspond to the enzyme ATP-diphosphohydrolase or apyras e (EC 3.6.1.5). These include substrate specificity, and coincident M( r) and pI values of both ATPase-ADPase activities. 2. This enzyme hydr olyses both the free unprotonated and cation-nucleotide complex, the c atalytic efficiency for the latter being considerably higher. 3. Micro somal apyrase is insensitive to ouabain and Ap5A. The highly purified enzyme was only inhibited by o-vanadate, DES and slightly by DCCD. 4. Apyrase seems to be a glycoprotein from its interaction with Concanava lin-A. 5. Preliminary studies on the essential amino acid residues sug gest the participation of Arg, Lys and His residues, and discard the r equirement of -SH, COO-, -OH, and probably also Tyr and Trp. 6. Two ki netic modulatory proteins of apyrase were detected in placental tissue . An activating protein was found in the soluble fraction and an inhib itory protein was loosely bound to the membranes. 7. The proposed in v ivo function for apyrase is related to the inhibition of platelet aggr egation due to its ADPase activity, which is supported by the direct e ffect on washed platelets and by its plasma membrane localization.