Calorimetric study of thermal denaturation of vertebrate visual pigments

The thermostability of visual pigments in bovine, rat and frog rod outer segments (ROS) has been studied by means of differential scanning calorimetry and thermal gel analysis methods. Use of the two different methods has allowed to assign calorimetric peaks to rhodopsin and opsin denaturation. The denaturation enthalpy changes for rhodopsin in bovine, rat and frog ROS are 630 kJ/mole (Td = 347 degrees C), 416 kJ/mole (Td = 340 degrees K) and 410 kJ/mole (Td = 340 degrees K), respectively. Corresponding values for opsins are 490 kJ/mole (Td = 332 degrees K), 269 kJ/mole (Td = 320 degrees K) and 158 kJ/mole (Td = 319 degrees K). The free energy of stabilization of the rhodopsin native structure is not very large and practically similar to that for native water soluble globular proteins.