Selective inhibition of multimolecular forms of human serum and placental oxytocinase activity by prostaglandins and cyclic GMP.

Ultrogel acrylamide-agarose chromatography was employed for fractionation of oxytocinase isoenzymes from serum of pregnant women and from human placenta. Using S-benzyl-L-cysteine-p-nitroanilide ( BCN ) and L-leucine-p-nitroanilide (LN) as substrates, three activity peaks (PI, PII, PIII) from placenta, and one peak (SI) from serum were identified. SI coincided with PII, and with all isoenzymes the hydrolysis of LN was greater than that of BCN . Prostaglandins E1, E2 and F2 alpha inhibited all oxytocinases , more potently the hydrolysis of LN than BCN and at pH 6.2 than at pH 6.8. Although cyclic GMP and its 8-bromo derivative similarly inhibited these isoenzymes except PIII, they were considerably more effective against the hydrolysis of BCN than LN.