The equilibrium between metarhodopsin I and metarhodopsin II in the isolated frog retina.

1. Rapid and slow changes in the absorbance of isolated frog retinae produced by exposure to brief flashes were studied at temperatures between 5 and 30 degrees C.2. Rapid changes observed at 475 nm consist of a transient increase of absorbance followed by an exponential decay to a new level of absorbance which is lower than before the flash exposure.3. The new level of absorbance determines the initial conditions of slow changes following the rapid ones. At higher temperatures, the loss of absorbance during the rapid changes is greater than at lower temperatures. Accordingly, the slow reactions start at lower levels of absorbance when the temperature is high.4. Quantitative analysis showed that the rapid reactions can be described in terms of two consecutive reactions followed by an equilibrium reaction: the light-controlled formation of lumirhodopsin, decay of lumirhodopsin to metarhodopsin I, and the equilibrium reaction between the metarhodopsins I and II.5. The slow absorbance changes observed in the visible (lambda = 480 nm) are due to metarhodopsin I and to metarhodopsin III. Metarhodopsin I decays during the early phase of slow reactions but can noticeably influence the kinetics at lower temperatures.6. The activation energy of the lumirhodopsin decay is 22.5 kcal/mole, that of the conversion of metarhodopsin I into metarhodopsin II is 30.1 kcal/mole. The entropy change associated with the metarhodopsin I-II equilibrium amounts to +34 cal/mole. K.