Dark-processes following photoconversion of butterfly rhodopsins

Abstract

Photoconversion of rhodopsin to metarhodopsin by a short actinic flash creates photochemical changes in the absorbance spectrum of the butterfly rhabdom, which are measurable as changes in the reflectance spectrum of the intact eye. The difference spectrum relaxes in the dark, but changes considerably in shape when doing so. The positive peak caused by the accumulation of metarhodopsin relaxes to zero much faster than the negative peak caused by the loss of rhodopsin. The positive peak actually undershoots zero absorbance-difference before its final asymptotic approach to zero, whereas the negative peak approaches zero monotonically. The entire temporal evolution of difference spectra can be quantitatively reproduced by only assuming different kinetics for the dark-processes of metarhodopsin's decay and of rhodopsin's recovery. A consequence of this analysis is that no long-lived, coloured intermediates can be detected in the rhabdom other than metarhodopsin. Metarhodopsin's decay is well approximated by a first-order process, but has a time-constant that depends strongly on temperature. Examples are 71 min at 12.5‡ C, 18 min at 23‡ C, and 4 min at 26.5‡ C. Rhodopsin's recovery is kinetically complex. The rate of recovery shortly after a small photoconversion is somewhat slower than the rate for metarhodopsin's decay. At later times, or for a large photoconversion, rhodopsin's recovery is very much slower than metarhodopsin's decay.