title: Guanylate cyclase of isolated bovine retinal rod axonemes.
abstract: The guanylate cyclase activity of axoneme--basal apparatus complexes isolated from bovine retinal rods has been investigated. The Mg2+ and Mn2+ complexes of GTP4- serve as substrates. Binding of an additional mole of Mg2+ or Mn2+ per mole of enzyme is required. Among cations which are ineffective are Ca2+, Ni2+, Fe2+, Fe3+, Zn2+, and Co2+. The kinetics are consistent with a mechanism in which binding of Mg2+ or Mn2+ to the enzyme must precede binding of MgGTP or MnGTP. The apparent dissociation constants of the Mg--enzyme complex and the Mn--enzyme complex are 9.5 x 10(-4) and 1.1 x 10(-4) M, respectively. The apparent dissociation constants for binding of MgGTP and MnGTP to the complex of the enzyme with the same metal are 7.9 x 10(-4) and 1.4 x 10(-4) M, respectively. The cyclase activity is maximal and independent of pH between pH 7 and 9. KCl and NaCl are stimulatory, especially at suboptimal concentrations of Mg2+ or Mn2+. Ca2+ and high concentrations of Mg2+ and Mn2+ are inhibitory. Ca2+ inhibition appears to require the binding of 2 mol of Ca2+ per mol of enzyme. The dissociation constant of the Ca2--enzyme complex is estimated to be 1.4 x 10(-6) M2. The axoneme--basal apparatus preparations contain adenylate cyclase activity whose magnitude is 1--10% that of the guanylate cyclase activity.
mesh_terms: D000818:Animals; D002118:Calcium / Q000494:pharmacology; D002417:Cattle; D006162:Guanylate Cyclase / Q000302:isolation & purification / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D008345:Manganese / Q000494:pharmacology; D010786:Photoreceptor Cells / Q000201:enzymology*; D011092:Polyethylene Glycols / Q000494:pharmacology; D011189:Potassium Chloride / Q000494:pharmacology
chemical_list: D011092:Polyethylene Glycols; D008345:Manganese; D011189:Potassium Chloride; D006162:Guanylate Cyclase; D008274:Magnesium; D002118:Calcium