title: Ionic regulation of adenylate cyclase from the cilia of Paramecium tetraurelia.
abstract: The kinetics of the ionic regulation of an adenylate cyclase associated with the excitable ciliary membrane from Paramecium tetraurelia was examined. Glycerol (30%, v/v) stabilized the enzyme, and activated by an increase in Vmax. (3-fold) and a decrease in the apparent Km for MgATP (6-fold). Kinetic analysis of Mg2+ effects showed a stimulation via a single metal-binding site separate from the substrate site, with a dissociation constant, Ks, of 0.27 mM. Analysis of Ca2+ effects showed (i) an uncompetitive inhibition with respect to substrate MgATP, and (ii) dependence of the extent of inhibition on the free Mg2+ concentration. Ki values ranged from 4 to 130 microM-Ca2+ in the presence of 0.55-2 mM-Mg2+ respectively. This indicates competition between Mg2+ and Ca2+ at the metal-binding site. The Ca2+ effect was specific; Sr2+ and Ba2+ were almost without effect, and 100 microM-Ba2+ did not interfere with the Ca2+ inhibition. The actions of Ca2+ were readily reversible after addition of EGTA. K+ activated the adenylate cyclase at concentrations around 20 mM. The stimulatory potency of K+ was dependent on the free Mg2+ concentration. At 1 mM free Mg2+, 20 mM-K+ doubled the adenylate cyclase activity. The inhibitory Ca2+ and stimulatory K+ inputs were independent of each other.
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000262:Adenylyl Cyclases / Q000378:metabolism*; D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000494:pharmacology; D002412:Cations / Q000494:pharmacology*; D002923:Cilia / Q000201:enzymology*; D007700:Kinetics; D008274:Magnesium / Q000378:metabolism / Q000494:pharmacology; D010247:Paramecium / Q000201:enzymology*; D011188:Potassium / Q000494:pharmacology
chemical_list: D002412:Cations; D000255:Adenosine Triphosphate; D000262:Adenylyl Cyclases; D008274:Magnesium; D011188:Potassium; D002118:Calcium