Partial characterization of guanylate cyclase activity of the human placenta

An optimal assay system for the measurement of GC activity in subcellular fractions of human placenta homogenates was developed. MN2+ was more effective than Mg2+ as a cofactor and, in addition, it markedly potentiated the Mg2+-dependent GC activity. The apparent optimal temperature for GC activity was 45 degrees C and the pH optimum was in the range of 7.5 to 8.3. The apparent Km of Mn2+-dependent GC for GTP was 0.1 and that of MG2+-dependent GC for GTP was 0.4 mm. More than 95 per cent of GC activity was associated with the 100 000 X g supernatant fraction. ATP at a concentration of 0.5 mm inhibited GC activity by 50 per cent. The Mn2+-dependent GC activity was inhibited by p-hydroxymercuriphenyl sulphonate at concentrations between 10 and 50 micrometers; this inhibition was reversed, in part, by dithiothreitol at concentrations of 150 to 500 micrometers to approximately one-fifth the activity of control values. In addition, we found that dithiothreitol was an inhibitor of GC activity. Studies on the responsiveness of placental GC to nucleophilic compounds indicated that phenylhydrazine was the most potent stimulus, while hydroxylamine, hydrazine, sodium azide, sodium nitrite and sodium nitroprusside were less potent. Placental GC was unaffected by alpha- and beta-adrenergic agonists or by cholinergic agonist, as well as by steroids produced by the placenta.