title: Regulation of intestinal mucosa guanylate cyclase by hemin, heme and protoporphyrin IX.
abstract: 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.
mesh_terms: D000818:Animals; D001427:Bacterial Toxins / Q000494:pharmacology; D004121:Dimethyl Sulfoxide / Q000494:pharmacology; D004229:Dithiothreitol / Q000494:pharmacology; D004768:Enterotoxins / Q000494:pharmacology; D029968:Escherichia coli Proteins; D006162:Guanylate Cyclase / Q000037:antagonists & inhibitors / Q000378:metabolism*; D006418:Heme / Q000031:analogs & derivatives* / Q000502:physiology*; D006427:Hemin / Q000502:physiology*; D007413:Intestinal Mucosa / Q000201:enzymology*; D008345:Manganese / Q000494:pharmacology; D008871:Microvilli / Q000201:enzymology; D009569:Nitric Oxide / Q000494:pharmacology; D011092:Polyethylene Glycols / Q000494:pharmacology; D011166:Porphyrins / Q000502:physiology*; D011524:Protoporphyrins / Q000502:physiology*; D013552:Swine
chemical_list: D001427:Bacterial Toxins; D004768:Enterotoxins; D029968:Escherichia coli Proteins; D011166:Porphyrins; D011524:Protoporphyrins; C034151:heat stable toxin (E coli); D009569:Nitric Oxide; D011092:Polyethylene Glycols; D006418:Heme; D008345:Manganese; C005092:lubrol; D006427:Hemin; C028025:protoporphyrin IX; D006162:Guanylate Cyclase; D004229:Dithiothreitol; D004121:Dimethyl Sulfoxide