GUANYLATE-CYCLASE OF ISOLATED BOVINE RETINAL ROD AXONEMES

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 9.5ËŸ10-4 and 1.1ËŸ10-4 M, respectively. The apparent dissociation constants for binding of MgGTP and MnGTP to the complex of the enzyme with the same metal are 7.9ËŸ10-4 and 1.4ËŸ10-4 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 1.4ËŸ10-6 M2. The axoneme-basal apparatus preparations contain adenylate cyclase activity whose magnitude is 1-10% that of the guanylate cyclase activity. © 1979, American Chemical Society. All rights reserved.