Regulation of intestinal mucosa guanylate cyclase by hemin, heme and protoporphyrin IX

Mg2+-dependent activity of intestinal brush border guanylate cyclase was stimulated 4-5-fold by 50-100 microM hemin. Higher concentrations were inhibitory. In the presence of 25% dimethyl sulfoxide, which stimulated activity 9-times, 50 microM hemin further increased activity 1.7-fold. However, when activity was stimulated 32-fold by the Escherichia coli heat-stable enterotoxin, or 26-fold by Lubrol PX, hemin produced only concentration-dependent inhibition. The first type of activation was more sensitive to hemin than the second. Reduction of hemin by dithiothreitol eliminated stimulation of basal activity, while inhibition of Lubrol PX-stimulated activity remained. Protoporphyrin IX also had no effect on basal activity, however, it inhibited enterotoxin- and Lubrol PX-stimulated activities similarly, but only to half the extent of hemin. Substitution of Mn2+ for Mg2+ elevated basal activity 15-fold, and this Mn2+-dependent activity was inhibited by hemin. Mn2+-dependent activity was stimulated (43%) by enterotoxin, however, the stimulated activity was more sensitive to hemin inhibition than the basal Mn2+-dependent activity and both inhibition curves were congruent above 50 microM hemin. Hemin inhibition of Lubrol PX-stimulated activity was much less with Mn2+ than with Mg2+. These results were interpreted as suggesting two sites of hemin inhibition; on an inhibitory regulator and on the enzyme. We also found that the secretory effect of enterotoxin in the suckling mouse bioassay was reduced 56% by the oral administration of hemin.