Inhibition of serum oxytocinase activity by prostaglandins

The effect of prostaglandins (PGs) and other compounds on human serum oxytocinase (EC 3.4.11.3) activity in vitro was studied by a sensitive assay using S-berizyl-l-cysteine-p-nitroanilide as substrate. PGE₁, PGE₂ and PGF_2α significantly inhibited serum oxytocinase activity in a dose-related manner and in decreasing order of potency. cGMP, 8-bromo-cGMP, indomethacin, polyphloretin phosphate, hypertonic saline and urea were also active. cAMP, db-cAMP, 8-bromo-cAMP, db-cCMP, 5′-AMP, 5′-ADP, 5′-ATP, 5′-GDP, 5′-GTP, aspirin, sodium salicylate, paracetamol, theophylline and IBMX did not inhibit the enzyme activity. The results suggest that the oxytocic action of prostaglandins may be due, at least in part, to an inhibition of oxytocinase activity.