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

Abstract

Rapid and slow changes in the absorbance of isolated frog [Rana esculenta] retinae produced by exposure to brief flashes were studied at temperatures between 5-30.degree. C. Rapid changes observed at 475 nm consisted of a transient increase of absorbance followed by an exponential decay to a new level of absorbance which was lower than before the flash exposure. The new level of absorbance determined the initial conditions of slow changes following the rapid changes. At higher temperatures, the loss of absorbance during the rapid changes was greater than at lower temperatures. Accordingly, the slow reactions started at lower levels of absorbance when the temperature was high. Quantitative analysis showed that the rapid reactions could be described in terms of 2 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. The slow absorbance changes observed in the visible (.lambda. = 480 nm) were due to metarhodopsin I and to metarhodopsin III. Metarhodopsin I decayed during the early phase of slow reactions but could noticeably influence the kinetics at lower temperatures. The activation energy of the lumirhodopsin decay was 22.5 kcal/mol, that of the conversion of metarhodopsin I into metarhodopsin II was 30.1 kcal/mol. The entropy change associated with the metarhodopsin I-II equilibrium was +34 cal/mol .cntdot. K.