COMPARISON OF THE BIOCHEMICAL-PROPERTIES, REGULATION AND FUNCTION OF ATP-DIPHOSPHOHYDROLASE FROM HUMAN PLACENTA AND RAT-KIDNEY

ATP-diphosphohydrolase (apyrase, EC 3.6.1.5) has both ATPase and ADPas e activity that are stimulated by bivalent metals, with Ca2+ being the most effective. The possible physiological function of this enzyme, a ssociated with placental and renal microvilli, is related to the extra cellular metabolism of nucleotides. A comparison of the biochemical pr operties of human placenta and rat kidney apyrase is presented, showin g similarities in Mr, bivalent metal stimulation, nucleotide nonspecif icity, insensitivity towards specific ATPase inhibitors, and lack of e ssential sulfhydryl and aliphatic hydroxyl groups. We describe the tre atment of membrane preparations from both tissues with different deter gents and the isoelectric focusing of the solubilized proteins to part ially purify apyrase. An ectoenzyme localization is assigned both in m icrovillus membranes and in the vasculature on the basis of organ perf usion experiments with nucleotides in the presence of antibodies. Plac ental and kidney microvillus membranes inhibited ADP-induced platelet aggregation, in agreement with an extracellular role. Initial studies on enzyme regulation suggested the existence of at least two types of modulatory proteins: an activating protein in the cytosol of both tiss ues, and an inhibitory protein associated with placental microsomes. P ossible hormonal regulation was investigated in kidneys using in vivo estradiol treatment, but only slight changes in total apyrase activity were observed.