| Abstract: | Rod outer segments (ROS) of the frog retina are shown to contain high affinity binding sites to guanylic nucleotides. Concentration of the binding sites comprises several per cent of rhodopsin concentration in our ROS preparations. These sites possess high affinity to GDP (Kd less than 10(-6) M) in dark-adapted preparations, and in the presence of bleached rhodopsin they effectively bind the non-hydrolizable analog of GTP--GPP (NH) P (Kd less than 10(-6) M). It is shown that one bleached rhodopsin molecule can induce the binding of up to 100 molecules of GPP (NH) P at low rhodopsin photolysis. Qur experimental results raise serious doubts as to the applicability of nucleotide exchange scheme by Fung and Stryer (1980). |
