Calcium inhibits ST-stimulated guanylyl cyclase C by competing with free Mg2+ for binding to an allosteric site
Maximum ST-induced guanylyl cyclase C activities and Ka (concentration of Mg2+ yielding 50% of the maximum activity) values were extracted from nonlinear regression analyses of the sigmoidal concentration-response curves of Mg2+-dependent guanylyl cyclase C activity in the absence or presence of 0.25 mM Ca2+. Values are means ± S.E.M. of three experiments performed in duplicate, with each sample analyzed in triplicate.
[Ca2+] | Maximum ST-Induced Guanylyl Cyclase C Activity | Ka |
|---|---|---|
| pmol cGMP/min/mg protein | ||
| 0 mM | 155.43 ± 33.73 | 1.29 ± 0.32 |
| 0.25 mM | 151.10 ± 27.78 | 2.83 ± 0.38* |
↵* p < 0.05 with respect to control (no added Ca2+) employing paired Student's t test (two-tailed).