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

Abstract

SUMMARY1. 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 0C.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 (A = 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.

Temperature dependence of gating current in myelinated nerve fibers

“…Howarth (1975) observed measurable heat production during an action potential and explained this phenomenon with large entropy changes of the membrane, too. Baumann (1978) reported ASa and AS values in the same order of magnitude for the conversion between metarhodopsin I and II (ASa = 230 J mo1-1 g -1 and AS = 142 J mol -j K-I). Considering the lower molecular weight of metarhodopsin, the entropy changes seem to be even larger in this molecule.…”

Effect of pH on the formation and decay of the metarhodopsins of the frog.

“…MR I and MR III, appreciably absorb light of 475 nm wavelength. The formation of MR III is very slow, however, and, immediately after the flash, the concentration of MR III is negligible, so that the following equation holds (Baumann, 1978): Fig. 2, the relative concentration of MR I amounts to 0-42/1-24 = 0-34.…”

“…In addition, it must be compatible with the observed kinetics of the MR I-MR II transition. This reaction was previously treated as a reversible first-order process (Baumann, 1976(Baumann, , 1978. The experiments at different pH.…”

“…on a minute time scale). In a recent publication the two types of analysis were combined and led to evidence of the equilibrium between MR I and MR II in the frog retina (Baumann, 1978).…”

“…2. Normalization was carried out again with respect to the pre-flash level as 1 and the final base line as zero, and the function A475 = (6MR I/6R) [MR I]+ (6MR III/6R) [MR III] (13) was used to fit the data (Baumann, 1978). The ratio CMR I/CR is the same as in eqn.…”